base on List of top 500 ReactJS Interview Questions & Answers....Coding exercise questions are coming soon!! # React Interview Questions & Answers
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**Note:** This repository is specific to ReactJS. Please check [Javascript Interview questions](https://github.com/sudheerj/javascript-interview-questions) for core javascript questions and [DataStructures and Algorithms](https://github.com/sudheerj/datastructures-algorithms) for DSA related questions or problems.
### Table of Contents
<details open>
<summary>
Hide/Show table of contents
</summary>
| No. | Questions |
| --- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| | **Core React** |
| 1 | [What is React?](#what-is-react) |
| 2 | [What is the history behind React evolution?](#What-is-the-history-behind-React-evolution) |
| 3 | [What are the major features of React?](#what-are-the-major-features-of-react) |
| 4 | [What is JSX?](#what-is-jsx) |
| 5 | [What is the difference between Element and Component?](#what-is-the-difference-between-element-and-component) |
| 6 | [How to create components in React?](#how-to-create-components-in-react) |
| 7 | [When to use a Class Component over a Function Component?](#when-to-use-a-class-component-over-a-function-component) |
| 8 | [What are Pure Components?](#what-are-pure-components) |
| 9 | [What is state in React?](#what-is-state-in-react) |
| 10 | [What are props in React?](#what-are-props-in-react) |
| 11 | [What is the difference between state and props?](#what-is-the-difference-between-state-and-props) |
| 12 | [What is the difference between HTML and React event handling?](#what-is-the-difference-between-html-and-react-event-handling) |
| 13 | [What are synthetic events in React?](#what-are-synthetic-events-in-react) |
| 14 | [What are inline conditional expressions?](#what-are-inline-conditional-expressions) |
| 15 | [What is "key" prop and what is the benefit of using it in arrays of elements?](#what-is-key-prop-and-what-is-the-benefit-of-using-it-in-arrays-of-elements) |
| 16 | [What is Virtual DOM?](#what-is-virtual-dom) |
| 17 | [How Virtual DOM works?](#how-virtual-dom-works) |
| 18 | [What is the difference between Shadow DOM and Virtual DOM?](#what-is-the-difference-between-shadow-dom-and-virtual-dom) |
| 19 | [What is React Fiber?](#what-is-react-fiber) |
| 20 | [What is the main goal of React Fiber?](#what-is-the-main-goal-of-react-fiber) |
| 21 | [What are controlled components?](#what-are-controlled-components) |
| 22 | [What are uncontrolled components?](#what-are-uncontrolled-components) |
| 23 | [What is the difference between createElement and cloneElement?](#what-is-the-difference-between-createelement-and-cloneelement) |
| 24 | [What is Lifting State Up in React?](#what-is-lifting-state-up-in-react) |
| 25 | [What are Higher-Order components?](#what-are-higher-order-components) |
| 26 | [What is children prop?](#what-is-children-prop) |
| 27 | [How to write comments in React?](#how-to-write-comments-in-react) |
| 28 | [What is reconciliation?](#what-is-reconciliation) |
| 29 | [Does the lazy function support named exports?](#does-the-lazy-function-support-named-exports) |
| 30 | [Why React uses className over class attribute?](#why-react-uses-classname-over-class-attribute) |
| 31 | [What are fragments?](#what-are-fragments) |
| 32 | [Why fragments are better than container divs?](#why-fragments-are-better-than-container-divs) |
| 33 | [What are portals in React?](#what-are-portals-in-react) |
| 34 | [What are stateless components?](#what-are-stateless-components) |
| 35 | [What are stateful components?](#what-are-stateful-components) |
| 36 | [How to apply validation on props in React?](#how-to-apply-validation-on-props-in-react) |
| 37 | [What are the advantages of React?](#what-are-the-advantages-of-react) |
| 38 | [What are the limitations of React?](#what-are-the-limitations-of-react) |
| 39 | [What are the recommended ways for static type checking?](#what-are-the-recommended-ways-for-static-type-checking) |
| 40 | [What is the use of react-dom package?](#what-is-the-use-of-react-dom-package) |
| 41 | [What is ReactDOMServer?](#what-is-reactdomserver) |
| 42 | [How to use InnerHtml in React?](#how-to-use-innerhtml-in-react) |
| 43 | [How to use styles in React?](#how-to-use-styles-in-react) |
| 44 | [How events are different in React?](#how-events-are-different-in-react) |
| 45 | [What is the impact of indexes as keys?](#what-is-the-impact-of-indexes-as-keys) |
| 46 | [How do you conditionally render components?](#how-do-you-conditionally-render-components) |
| 47 | [Why we need to be careful when spreading props on DOM elements??](#why-we-need-to-be-careful-when-spreading-props-on-dom-elements) |
| 48 | [How do you memoize a component?](#how-do-you-memoize-a-component) |
| 49 | [How you implement Server-Side Rendering or SSR?](#how-you-implement-server-side-rendering-or-ssr) |
| 50 | [How to enable production mode in React?](#how-to-enable-production-mode-in-react) |
| 51 | [Do Hooks replace render props and higher order components?](#do-hooks-replace-render-props-and-higher-order-components) |
| 52 | [What is a switching component?](#what-is-a-switching-component) |
| 53 | [What are React Mixins?](#what-are-react-mixins) |
| 54 | [What are the Pointer Events supported in React?](#what-are-the-pointer-events-supported-in-react) |
| 55 | [Why should component names start with capital letter?](#why-should-component-names-start-with-capital-letter) |
| 56 | [Are custom DOM attributes supported in React v16?](#are-custom-dom-attributes-supported-in-react-v16) |
| 57 | [How to loop inside JSX?](#how-to-loop-inside-jsx) |
| 58 | [How do you access props in attribute quotes?](#how-do-you-access-props-in-attribute-quotes) |
| 59 | [What is React PropType array with shape?](#what-is-react-proptype-array-with-shape) |
| 60 | [How to conditionally apply class attributes?](#how-to-conditionally-apply-class-attributes) |
| 61 | [What is the difference between React and ReactDOM?](#what-is-the-difference-between-react-and-reactdom) |
| 62 | [Why ReactDOM is separated from React?](#why-reactdom-is-separated-from-react) |
| 63 | [How to use React label element?](#how-to-use-react-label-element) |
| 64 | [How to combine multiple inline style objects?](#how-to-combine-multiple-inline-style-objects) |
| 65 | [How to re-render the view when the browser is resized?](#how-to-re-render-the-view-when-the-browser-is-resized) |
| 66 | [How to pretty print JSON with React?](#how-to-pretty-print-json-with-react) |
| 67 | [Why you can't update props in React?](#why-you-cant-update-props-in-react) |
| 68 | [How to focus an input element on page load?](#how-to-focus-an-input-element-on-page-load) |
| 69 | [How can we find the version of React at runtime in the browser?](#how-can-we-find-the-version-of-react-at-runtime-in-the-browser) |
| 70 | [How to add Google Analytics for react-router?](#how-to-add-google-analytics-for-react-router) |
| 71 | [How do you apply vendor prefixes to inline styles in React?](#how-do-you-apply-vendor-prefixes-to-inline-styles-in-react) |
| 72 | [How to import and export components using react and ES6?](#how-to-import-and-export-components-using-react-and-es6) |
| 73 | [What are the exceptions on React component naming?](#what-are-the-exceptions-on-react-component-naming) |
| 74 | [Is it possible to use async/await in plain React?](#is-it-possible-to-use-asyncawait-in-plain-react) |
| 75 | [What are the common folder structures for React?](#what-are-the-common-folder-structures-for-react) |
| 76 | [What are the popular packages for animation?](#what-are-the-popular-packages-for-animation) |
| 77 | [What is the benefit of styles modules?](#what-is-the-benefit-of-styles-modules) |
| 78 | [What are the popular React-specific linters?](#what-are-the-popular-react-specific-linters) |
| | **React Router** |
| 79 | [What is React Router?](#what-is-react-router) |
| 80 | [How React Router is different from history library?](#how-react-router-is-different-from-history-library) |
| 81 | [What are the \<Router> components of React Router v6?](#what-are-the-router-components-of-react-router-v6) |
| 82 | [What is the purpose of push and replace methods of history?](#what-is-the-purpose-of-push-and-replace-methods-of-history) |
| 83 | [How do you programmatically navigate using React router v4?](#how-do-you-programmatically-navigate-using-react-router-v4) |
| 84 | [How to get query parameters in React Router v4](#how-to-get-query-parameters-in-react-router-v4) |
| 85 | [Why you get "Router may have only one child element" warning?](#why-you-get-router-may-have-only-one-child-element-warning) |
| 86 | [How to pass params to history.push method in React Router v4?](#how-to-pass-params-to-historypush-method-in-react-router-v4) |
| 87 | [How to implement default or NotFound page?](#how-to-implement-default-or-notfound-page) |
| 88 | [How to get history on React Router v4?](#how-to-get-history-on-react-router-v4) |
| 89 | [How to perform automatic redirect after login?](#how-to-perform-automatic-redirect-after-login) |
| | **React Internationalization** |
| 90 | [What is React Intl?](#what-is-react-intl) |
| 91 | [What are the main features of React Intl?](#what-are-the-main-features-of-react-intl) |
| 92 | [What are the two ways of formatting in React Intl?](#what-are-the-two-ways-of-formatting-in-react-intl) |
| 93 | [How to use FormattedMessage as placeholder using React Intl?](#how-to-use-formattedmessage-as-placeholder-using-react-intl) |
| 94 | [How to access current locale with React Intl](#how-to-access-current-locale-with-react-intl) |
| 95 | [How to format date using React Intl?](#how-to-format-date-using-react-intl) |
| | **React Testing** |
| 96 | [What is Shallow Renderer in React testing?](#what-is-shallow-renderer-in-react-testing) |
| 97 | [What is TestRenderer package in React?](#what-is-testrenderer-package-in-react) |
| 98 | [What is the purpose of ReactTestUtils package?](#what-is-the-purpose-of-reacttestutils-package) |
| 99 | [What is Jest?](#what-is-jest) |
| 100 | [What are the advantages of Jest over Jasmine?](#what-are-the-advantages-of-jest-over-jasmine) |
| 101 | [Give a simple example of Jest test case](#give-a-simple-example-of-jest-test-case) |
| | **React Redux** |
| 102 | [What is Flux?](#what-is-flux) |
| 103 | [What is Redux?](#what-is-redux) |
| 104 | [What are the core principles of Redux?](#what-are-the-core-principles-of-redux) |
| 105 | [What are the downsides of Redux compared to Flux?](#what-are-the-downsides-of-redux-compared-to-flux) |
| 106 | [What is the difference between mapStateToProps() and mapDispatchToProps()?](#what-is-the-difference-between-mapstatetoprops-and-mapdispatchtoprops) |
| 107 | [Can I dispatch an action in reducer?](#can-i-dispatch-an-action-in-reducer) |
| 108 | [How to access Redux store outside a component?](#how-to-access-redux-store-outside-a-component) |
| 109 | [What are the drawbacks of MVW pattern](#what-are-the-drawbacks-of-mvw-pattern) |
| 110 | [Are there any similarities between Redux and RxJS?](#are-there-any-similarities-between-redux-and-rxjs) |
| 111 | [How to reset state in Redux?](#how-to-reset-state-in-redux) |
| 112 | [What is the difference between React context and React Redux?](#what-is-the-difference-between-react-context-and-react-redux) |
| 113 | [Why are Redux state functions called reducers?](#why-are-redux-state-functions-called-reducers) |
| 114 | [How to make AJAX request in Redux?](#how-to-make-ajax-request-in-redux) |
| 115 | [Should I keep all component's state in Redux store?](#should-i-keep-all-components-state-in-redux-store) |
| 116 | [What is the proper way to access Redux store?](#what-is-the-proper-way-to-access-redux-store) |
| 117 | [What is the difference between component and container in React Redux?](#what-is-the-difference-between-component-and-container-in-react-redux) |
| 118 | [What is the purpose of the constants in Redux? ](#what-is-the-purpose-of-the-constants-in-redux) |
| 119 | [What are the different ways to write mapDispatchToProps()?](#what-are-the-different-ways-to-write-mapdispatchtoprops) |
| 120 | [What is the use of the ownProps parameter in mapStateToProps() and mapDispatchToProps()?](#what-is-the-use-of-the-ownprops-parameter-in-mapstatetoprops-and-mapdispatchtoprops) |
| 121 | [How to structure Redux top level directories?](#how-to-structure-redux-top-level-directories) |
| 122 | [What is redux-saga?](#what-is-redux-saga) |
| 123 | [What is the mental model of redux-saga?](#what-is-the-mental-model-of-redux-saga) |
| 124 | [What are the differences between call and put in redux-saga](#what-are-the-differences-between-call-and-put-in-redux-saga) |
| 125 | [What is Redux Thunk?](#what-is-redux-thunk) |
| 126 | [What are the differences between redux-saga and redux-thunk](#what-are-the-differences-between-redux-saga-and-redux-thunk) |
| 127 | [What is Redux DevTools?](#what-is-redux-devtools) |
| 128 | [What are the features of Redux DevTools?](#what-are-the-features-of-redux-devtools) |
| 129 | [What are Redux selectors and Why use them?](#what-are-redux-selectors-and-why-use-them) |
| 130 | [What is Redux Form?](#what-is-redux-form) |
| 131 | [What are the main features of Redux Form?](#what-are-the-main-features-of-redux-form) |
| 132 | [How to add multiple middlewares to Redux?](#how-to-add-multiple-middlewares-to-redux) |
| 133 | [How to set initial state in Redux?](#how-to-set-initial-state-in-redux) |
| 134 | [How Relay is different from Redux?](#how-relay-is-different-from-redux) |
| 135 | [What is an action in Redux?](#what-is-an-action-in-redux) |
| | **React Native** |
| 136 | [What is the difference between React Native and React?](#what-is-the-difference-between-react-native-and-react) |
| 137 | [How to test React Native apps?](#how-to-test-react-native-apps) |
| 138 | [How to do logging in React Native?](#how-to-do-logging-in-react-native) |
| 139 | [How to debug your React Native?](#how-to-debug-your-react-native) |
| | **React supported libraries and Integration** |
| 140 | [What is reselect and how it works?](#what-is-reselect-and-how-it-works) |
| 141 | [What is Flow?](#what-is-flow) |
| 142 | [What is the difference between Flow and PropTypes?](#what-is-the-difference-between-flow-and-proptypes) |
| 143 | [How to use font-awesome icons in React?](#how-to-use-font-awesome-icons-in-react) |
| 144 | [What is React Dev Tools?](#what-is-react-dev-tools) |
| 145 | [Why is DevTools not loading in Chrome for local files?](#why-is-devtools-not-loading-in-chrome-for-local-files) |
| 146 | [How to use Polymer in React?](#how-to-use-polymer-in-react) |
| 147 | [What are the advantages of React over Vue.js?](#what-are-the-advantages-of-react-over-vuejs) |
| 148 | [What is the difference between React and Angular?](#what-is-the-difference-between-react-and-angular) |
| 149 | [Why React tab is not showing up in DevTools?](#why-react-tab-is-not-showing-up-in-devtools) |
| 150 | [What are styled components?](#what-are-styled-components) |
| 151 | [Give an example of Styled Components?](#give-an-example-of-styled-components) |
| 152 | [What is Relay?](#what-is-relay) |
| | **Miscellaneous** |
| 153 | [What are the main features of reselect library?](#what-are-the-main-features-of-reselect-library) |
| 154 | [Give an example of reselect usage?](#give-an-example-of-reselect-usage) |
| 155 | [Can Redux only be used with React?](#can-redux-only-be-used-with-react) |
| 156 | [Do you need to have a particular build tool to use Redux?](#do-you-need-to-have-a-particular-build-tool-to-use-redux) |
| 157 | [How Redux Form initialValues get updated from state?](#how-redux-form-initialvalues-get-updated-from-state) |
| 158 | [How React PropTypes allow different type for one prop?](#how-react-proptypes-allow-different-types-for-one-prop) |
| 159 | [Can I import an SVG file as react component?](#can-i-import-an-svg-file-as-react-component) |
| 160 | [What is render hijacking in React?](#what-is-render-hijacking-in-react) |
| 161 | [How to pass numbers to React component?](#how-to-pass-numbers-to-react-component) |
| 162 | [Do I need to keep all my state into Redux? Should I ever use react internal state?](#do-i-need-to-keep-all-my-state-into-redux-should-i-ever-use-react-internal-state) |
| 163 | [What is the purpose of registerServiceWorker in React?](#what-is-the-purpose-of-registerserviceworker-in-react) |
| 164 | [What is React memo function?](#what-is-react-memo-function) |
| 165 | [What is React lazy function?](#what-is-react-lazy-function) |
| 166 | [How to prevent unnecessary updates using setState?](#how-to-prevent-unnecessary-updates-using-setstate) |
| 167 | [How do you render Array, Strings and Numbers in React 16 Version?](#how-do-you-render-array-strings-and-numbers-in-react-16-version) |
| 168 | [What are hooks?](#what-are-hooks) |
| 169 | [What rules need to be followed for hooks?](#what-rules-need-to-be-followed-for-hooks) |
| 170 | [How to ensure hooks followed the rules in your project?](#how-to-ensure-hooks-followed-the-rules-in-your-project) |
| 171 | [What are the differences between Flux and Redux?](#what-are-the-differences-between-flux-and-redux) |
| 172 | [What are the benefits of React Router V4?](#what-are-the-benefits-of-react-router-v4) |
| 173 | [Can you describe about componentDidCatch lifecycle method signature?](#can-you-describe-about-componentdidcatch-lifecycle-method-signature) |
| 174 | [In which scenarios do error boundaries not catch errors?](#in-which-scenarios-do-error-boundaries-not-catch-errors) |
| 175 | [What is the behavior of uncaught errors in react 16?](#what-is-the-behavior-of-uncaught-errors-in-react-16) |
| 176 | [What is the proper placement for error boundaries?](#what-is-the-proper-placement-for-error-boundaries) |
| 177 | [What is the benefit of component stack trace from error boundary?](#what-is-the-benefit-of-component-stack-trace-from-error-boundary) |
| 178 | [What are default props?](#what-are-default-props) |
| 179 | [What is the purpose of displayName class property?](#what-is-the-purpose-of-displayname-class-property) |
| 180 | [What is the browser support for react applications?](#what-is-the-browser-support-for-react-applications) |
| 181 | [What is code-splitting?](#what-is-code-splitting) |
| 182 | [What are Keyed Fragments?](#what-are-keyed-fragments) |
| 183 | [Does React support all HTML attributes?](#does-react-support-all-html-attributes) |
| 184 | [When component props defaults to true?](#when-component-props-defaults-to-true) |
| 185 | [What is NextJS and major features of it?](#what-is-nextjs-and-major-features-of-it) |
| 186 | [How do you pass an event handler to a component?](#how-do-you-pass-an-event-handler-to-a-component) |
| 187 | [How to prevent a function from being called multiple times?](#how-to-prevent-a-function-from-being-called-multiple-times) |
| 188 | [How JSX prevents Injection Attacks?](#how-jsx-prevents-injection-attacks) |
| 189 | [How do you update rendered elements?](#how-do-you-update-rendered-elements) |
| 190 | [How do you say that props are read only?](#how-do-you-say-that-props-are-read-only) |
| 191 | [What are the conditions to safely use the index as a key?](#what-are-the-conditions-to-safely-use-the-index-as-a-key) |
| 192 | [Is it keys should be globally unique?](#is-it-keys-should-be-globally-unique) |
| 193 | [What is the popular choice for form handling?](#what-is-the-popular-choice-for-form-handling) |
| 194 | [What are the advantages of formik over redux form library?](#what-are-the-advantages-of-formik-over-redux-form-library) |
| 195 | [Why do you not required to use inheritance?](#why-do-you-not-required-to-use-inheritance) |
| 196 | [Can I use web components in react application?](#can-i-use-web-components-in-react-application) |
| 197 | [What is dynamic import?](#what-is-dynamic-import) |
| 198 | [What are loadable components?](#what-are-loadable-components) |
| 199 | [What is suspense component?](#what-is-suspense-component) |
| 200 | [What is route based code splitting?](#what-is-route-based-code-splitting) |
| 201 | [What is the purpose of default value in context?](#what-is-the-purpose-of-default-value-in-context) |
| 202 | [What is diffing algorithm?](#what-is-diffing-algorithm) |
| 203 | [What are the rules covered by diffing algorithm?](#what-are-the-rules-covered-by-diffing-algorithm) |
| 204 | [When do you need to use refs?](#when-do-you-need-to-use-refs) |
| 205 | [Is it prop must be named as render for render props?](#is-it-prop-must-be-named-as-render-for-render-props) |
| 206 | [What are the problems of using render props with pure components?](#what-are-the-problems-of-using-render-props-with-pure-components) |
| 207 | [What is windowing technique?](#what-is-windowing-technique) |
| 208 | [How do you print falsy values in JSX?](#how-do-you-print-falsy-values-in-jsx) |
| 209 | [What is the typical use case of portals?](#what-is-the-typical-use-case-of-portals) |
| 210 | [How do you set default value for uncontrolled component?](#how-do-you-set-default-value-for-uncontrolled-component) |
| 211 | [What is your favorite React stack?](#what-is-your-favorite-react-stack) |
| 212 | [What is the difference between Real DOM and Virtual DOM?](#what-is-the-difference-between-real-dom-and-virtual-dom) |
| 213 | [How to add Bootstrap to a react application?](#how-to-add-bootstrap-to-a-react-application) |
| 214 | [Can you list down top websites or applications using react as front end framework?](#can-you-list-down-top-websites-or-applications-using-react-as-front-end-framework) |
| 215 | [Is it recommended to use CSS In JS technique in React?](#is-it-recommended-to-use-css-in-js-technique-in-react) |
| 216 | [Do I need to rewrite all my class components with hooks?](#do-i-need-to-rewrite-all-my-class-components-with-hooks) |
| 217 | [How to fetch data with React Hooks?](#how-to-fetch-data-with-react-hooks) |
| 218 | [Is Hooks cover all use cases for classes?](#is-hooks-cover-all-use-cases-for-classes) |
| 219 | [What is the stable release for hooks support?](#what-is-the-stable-release-for-hooks-support) |
| 220 | [Why do we use array destructuring (square brackets notation) in useState?](#why-do-we-use-array-destructuring-square-brackets-notation-in-usestate) |
| 221 | [What are the sources used for introducing hooks?](#what-are-the-sources-used-for-introducing-hooks) |
| 222 | [How do you access imperative API of web components?](#how-do-you-access-imperative-api-of-web-components) |
| 223 | [What is formik?](#what-is-formik) |
| 224 | [What are typical middleware choices for handling asynchronous calls in Redux?](#what-are-typical-middleware-choices-for-handling-asynchronous-calls-in-redux) |
| 225 | [Do browsers understand JSX code?](#do-browsers-understand-jsx-code) |
| 226 | [Describe about data flow in react?](#describe-about-data-flow-in-react) |
| 227 | [What is MobX?](#what-is-mobx) |
| 228 | [What are the differences between Redux and MobX?](#what-are-the-differences-between-redux-and-mobx) |
| 229 | [Should I learn ES6 before learning ReactJS?](#should-i-learn-es6-before-learning-reactjs) |
| 230 | [What is Concurrent Rendering?](#what-is-concurrent-rendering) |
| 231 | [What is the difference between async mode and concurrent mode?](#what-is-the-difference-between-async-mode-and-concurrent-mode) |
| 232 | [Can I use javascript urls in react16.9?](#can-i-use-javascript-urls-in-react169) |
| 233 | [What is the purpose of eslint plugin for hooks?](#what-is-the-purpose-of-eslint-plugin-for-hooks) |
| 234 | [What is the difference between Imperative and Declarative in React?](#what-is-the-difference-between-imperative-and-declarative-in-react) |
| 235 | [What are the benefits of using typescript with reactjs?](#what-are-the-benefits-of-using-typescript-with-reactjs) |
| 236 | [How do you make sure that user remains authenticated on page refresh while using Context API State Management?](#how-do-you-make-sure-that-user-remains-authenticated-on-page-refresh-while-using-context-api-state-management) |
| 237 | [What are the benefits of new JSX transform?](#what-are-the-benefits-of-new-jsx-transform) |
| 238 | [How is the new JSX transform different from old transform?](#how-is-the-new-jsx-transform-different-from-old-transform) |
| 239 | [What are React Server components?](#what-are-react-server-components) |
| 240 | [What is prop drilling?](#what-is-prop-drilling) |
| 241 | [What is the difference between useState and useRef hook?](#what-is-the-difference-between-usestate-and-useref-hook) |
| 242 | [What is a wrapper component ](#what-is-a-wrapper-component) |
| 243 | [What are the differences between useEffect and useLayoutEffect hooks](#what-are-the-differences-between-useEffect-and-useLayoutEffect-hooks) |
| 244 | [What are the differences between Functional and Class Components ](#what-are-the-differences-between-functional-and-class-components) |
| 245 | [What is strict mode in React?](#what-is-strict-mode-in-react) |
| 246 | [What is the benefit of strict mode?](#what-is-the-benefit-of-strict-mode) |
| 247 | [Why does strict mode render twice in React?](#why-does-strict-mode-render-twice-in-react) |
| 248 | [What are the rules of JSX?](#what-are-the-rules-of-JSX) |
| 249 | [What is the reason behind multiple JSX tags to be wrapped?](#what-is-the-reason-behind-multiple-JSX-tags-to-be-wrapped) |
| 250 | [How do you prevent mutating array variables?](#how-do-you-prevent-mutating-array-variables) |
| 251 | [What are capture phase events?](#what-are-capture-phase-events) |
| 252 | [How does React updates screen in an application?](#how-does-react-updates-screen-in-an-application) |
| 253 | [How does React batch multiple state updates?](#how-does-react-batch-multiple-state-updates) |
| 254 | [Is it possible to prevent automatic batching?](#is-it-possible-to-prevent-automatic-batching) |
| 255 | [What is React hydration?](#what-is-react-hydration) |
| 256 | [How do you update objects inside state?](#how-do-you-update-objects-inside-state) |
| 257 | [How do you update nested objects inside state?](#How-do-you-update-nested-objects-inside-state) |
| 258 | [How do you update arrays inside state?](#how-do-you-update-arrays-inside-state) |
| 259 | [How do you use immer library for state updates?](#how-do-you-use-immer-library-for-state-updates) |
| 260 | [What are the benefits of preventing the direct state mutations?](#what-are-the-benefits-of-preventing-the-direct-state-mutations) |
| 261 | [What are the preferred and non-preferred array operations for updating the state?](#what-are-the-preferred-and-non-preferred-array-operations-for-updating-the-state) |
| 262 | [What will happen by defining nested function components?](#what-will-happen-by-defining-nested-function-components) |
| 263 | [Can I use keys for non-list items?](#can-i-use-keys-for-non-list-items) |
| 264 | [What are the guidelines to be followed for writing reducers?](#what-are-the-guidelines-to-be-followed-for-writing-reducers) |
| 265 | [What is useReducer hook? Can you describe its usage?](#what-is-use-reducer-hook-Can-you-describe-its-usage) |
| 266 | [How do you compare useState and useReducer?](#how-do-you-compare-use-state-and-use-reducer) |
| 267 | [How does context works using useContext hook?](#how-does-context-works-using-use-context-hook) |
| 268 | [What are the use cases of useContext hook?](#what-are-the-use-cases-of-use-context-hook) |
| 269 | [When to use client and server components?](#when-to-use-client-and-server-components) |
| 270 | [What are the differences between page router and app router in nextjs?](#what-are-the-differences-between-page-router-and-app-router-in-nextjs) |
</details>
### Table of Contents
<details open>
<summary>
Hide/Show table of contents
</summary>
| No. | Questions |
| --- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| | **Old Q&A** |
| 1 | [Why should we not update the state directly?](#why-should-we-not-update-the-state-directly) |
| 2 | [What is the purpose of callback function as an argument of setState()?](#what-is-the-purpose-of-callback-function-as-an-argument-of-setstate) |
| 3 | [How to bind methods or event handlers in JSX callbacks?](#how-to-bind-methods-or-event-handlers-in-jsx-callbacks) |
| 4 | [How to pass a parameter to an event handler or callback?](#how-to-pass-a-parameter-to-an-event-handler-or-callback) |
| 5 | [What is the use of refs?](#what-is-the-use-of-refs) |
| 6 | [How to create refs?](#how-to-create-refs) |
| 7 | [What are forward refs?](#what-are-forward-refs) |
| 8 | [Which is preferred option with in callback refs and findDOMNode()?](#which-is-preferred-option-with-in-callback-refs-and-finddomnode) |
| 9 | [Why are String Refs legacy?](#why-are-string-refs-legacy) |
| 10 | [What are the different phases of component lifecycle?](#what-are-the-different-phases-of-component-lifecycle) |
| 11 | [What are the lifecycle methods of React?](#what-are-the-lifecycle-methods-of-react) |
| 12 | [How to create props proxy for HOC component?](#how-to-create-props-proxy-for-hoc-component) |
| 13 | [What is context?](#what-is-context) |
| 14 | [What is the purpose of using super constructor with props argument?](#what-is-the-purpose-of-using-super-constructor-with-props-argument) |
| 15 | [How to set state with a dynamic key name?](#how-to-set-state-with-a-dynamic-key-name) |
| 16 | [What would be the common mistake of function being called every time the component renders?](#what-would-be-the-common-mistake-of-function-being-called-every-time-the-component-renders) |
| 17 | [What are error boundaries in React v16](#what-are-error-boundaries-in-react-v16) |
| 18 | [How are error boundaries handled in React v15?](#how-are-error-boundaries-handled-in-react-v15) |
| 19 | [What is the purpose of render method of react-dom?](#what-is-the-purpose-of-render-method-of-react-dom) |
| 20 | [What will happen if you use setState in constructor?](#what-will-happen-if-you-use-setstate-in-constructor) |
| 21 | [Is it good to use setState() in componentWillMount() method?](#is-it-good-to-use-setstate-in-componentwillmount-method) |
| 22 | [What will happen if you use props in initial state?](#what-will-happen-if-you-use-props-in-initial-state) |
| 23 | [How you use decorators in React?](#how-you-use-decorators-in-react) |
| 24 | [What is CRA and its benefits?](#what-is-cra-and-its-benefits) |
| 25 | [What is the lifecycle methods order in mounting?](#what-is-the-lifecycle-methods-order-in-mounting) |
| 26 | [What are the lifecycle methods going to be deprecated in React v16?](#what-are-the-lifecycle-methods-going-to-be-deprecated-in-react-v16) |
| 27 | [What is the purpose of getDerivedStateFromProps() lifecycle method?](#what-is-the-purpose-of-getderivedstatefromprops-lifecycle-method) |
| 28 | [What is the purpose of getSnapshotBeforeUpdate() lifecycle method?](#what-is-the-purpose-of-getsnapshotbeforeupdate-lifecycle-method) |
| 29 | [What is the recommended way for naming components?](#what-is-the-recommended-way-for-naming-components) |
| 30 | [What is the recommended ordering of methods in component class?](#what-is-the-recommended-ordering-of-methods-in-component-class) |
| 31 | [Why we need to pass a function to setState()?](#why-we-need-to-pass-a-function-to-setstate) |
| 32 | [Why is isMounted() an anti-pattern and what is the proper solution?](#why-is-ismounted-an-anti-pattern-and-what-is-the-proper-solution) |
| 33 | [What is the difference between constructor and getInitialState?](#what-is-the-difference-between-constructor-and-getinitialstate) |
| 34 | [Can you force a component to re-render without calling setState?](#can-you-force-a-component-to-re-render-without-calling-setstate) |
| 35 | [What is the difference between super() and super(props) in React using ES6 classes?](#what-is-the-difference-between-super-and-superprops-in-react-using-es6-classes) |
| 36 | [What is the difference between setState and replaceState methods?](#what-is-the-difference-between-setstate-and-replacestate-methods) |
| 37 | [How to listen to state changes?](#how-to-listen-to-state-changes) |
| 38 | [What is the recommended approach of removing an array element in react state?](#what-is-the-recommended-approach-of-removing-an-array-element-in-react-state) |
| 39 | [Is it possible to use React without rendering HTML?](#is-it-possible-to-use-react-without-rendering-html) |
| 40 | [What are the possible ways of updating objects in state?](#what-are-the-possible-ways-of-updating-objects-in-state) |
| 41 | [What are the approaches to include polyfills in your create-react-app?](#what-are-the-approaches-to-include-polyfills-in-your-create-react-app) |
| 42 | [How to use https instead of http in create-react-app?](#how-to-use-https-instead-of-http-in-create-react-app) |
| 43 | [How to avoid using relative path imports in create-react-app?](#how-to-avoid-using-relative-path-imports-in-create-react-app) |
| 44 | [How to update a component every second?](#how-to-update-a-component-every-second) |
| 45 | [Why is a component constructor called only once?](#why-is-a-component-constructor-called-only-once) |
| 46 | [How to define constants in React?](#how-to-define-constants-in-react) |
| 47 | [How to programmatically trigger click event in React?](#how-to-programmatically-trigger-click-event-in-react) |
| 48 | [How to make AJAX call and In which component lifecycle methods should I make an AJAX call?](#how-to-make-ajax-call-and-in-which-component-lifecycle-methods-should-i-make-an-ajax-call) |
| 49 | [What are render props?](#what-are-render-props) |
| 50 | [How to dispatch an action on load?](#how-to-dispatch-an-action-on-load) |
| 51 | [How to use connect from React Redux?](#how-to-use-connect-from-react-redux) |
| 52 | [Whats the purpose of at symbol in the redux connect decorator?](#whats-the-purpose-of-at-symbol-in-the-redux-connect-decorator) |
| 53 | [How to use TypeScript in create-react-app application?](#how-to-use-typescript-in-create-react-app-application) |
| 54 | [Does the statics object work with ES6 classes in React?](#does-the-statics-object-work-with-es6-classes-in-react) |
| 55 | [Why are inline ref callbacks or functions not recommended?](#why-are-inline-ref-callbacks-or-functions-not-recommended) |
| 56 | [What are HOC factory implementations?](#what-are-hoc-factory-implementations) |
| 57 | [How to use class field declarations syntax in React classes?](#how-to-use-class-field-declarations-syntax-in-react-classes) |
| 58 | [Why do you not need error boundaries for event handlers?](#why-do-you-not-need-error-boundaries-for-event-handlers) |
| 59 | [What is the difference between try catch block and error boundaries?](#what-is-the-difference-between-try-catch-block-and-error-boundaries) |
| 60 | [What is the required method to be defined for a class component?](#what-is-the-required-method-to-be-defined-for-a-class-component) |
| 61 | [What are the possible return types of render method?](#what-are-the-possible-return-types-of-render-method) |
| 62 | [What is the main purpose of constructor?](#what-is-the-main-purpose-of-constructor) |
| 63 | [Is it mandatory to define constructor for React component?](#is-it-mandatory-to-define-constructor-for-react-component) |
| 64 | [Why should not call setState in componentWillUnmount?](#why-should-not-call-setstate-in-componentwillunmount) |
| 65 | [What is the purpose of getDerivedStateFromError?](#what-is-the-purpose-of-getderivedstatefromerror) |
| 66 | [What is the methods order when component re-rendered?](#what-is-the-methods-order-when-component-re-rendered) |
| 67 | [What are the methods invoked during error handling?](#what-are-the-methods-invoked-during-error-handling) |
| 68 | [What is the purpose of unmountComponentAtNode method?](#what-is-the-purpose-of-unmountcomponentatnode-method) |
| 69 | [What are the limitations with HOCs?](#what-are-the-limitations-with-hocs) |
| 70 | [How to debug forwardRefs in DevTools?](#how-to-debug-forwardrefs-in-devtools) |
| 71 | [Is it good to use arrow functions in render methods?](#is-it-good-to-use-arrow-functions-in-render-methods) |
| 72 | [How do you say that state updates are merged?](#how-do-you-say-that-state-updates-are-merged) |
| 73 | [How do you pass arguments to an event handler?](#how-do-you-pass-arguments-to-an-event-handler) |
| 74 | [How to prevent component from rendering?](#how-to-prevent-component-from-rendering) |
| 75 | [Give an example on How to use context?](#give-an-example-on-how-to-use-context) |
| 76 | [How do you use contextType?](#how-do-you-use-contexttype) |
| 77 | [What is a consumer?](#what-is-a-consumer) |
| 78 | [How do you solve performance corner cases while using context?](#how-do-you-solve-performance-corner-cases-while-using-context) |
| 79 | [What is the purpose of forward ref in HOCs?](#what-is-the-purpose-of-forward-ref-in-hocs) |
| 80 | [Is it ref argument available for all functions or class components?](#is-it-ref-argument-available-for-all-functions-or-class-components) |
| 81 | [Why do you need additional care for component libraries while using forward refs?](#why-do-you-need-additional-care-for-component-libraries-while-using-forward-refs) |
| 82 | [How to create react class components without ES6?](#how-to-create-react-class-components-without-es6) |
| 83 | [Is it possible to use react without JSX?](#is-it-possible-to-use-react-without-jsx) |
| 84 | [How do you create HOC using render props?](#how-do-you-create-hoc-using-render-props) |
| 85 | [What is react scripts?](#what-is-react-scripts) |
| 86 | [What are the features of create react app?](#what-are-the-features-of-create-react-app) |
| 87 | [What is the purpose of renderToNodeStream method?](#what-is-the-purpose-of-rendertonodestream-method) |
| 88 | [How do you get redux scaffolding using create-react-app?](#how-do-you-get-redux-scaffolding-using-create-react-app) |
| 89 | [What is state mutation and how to prevent it?](#what-is-state-mutation-and-how-to-prevent-it) |
</details>
## Core React
1. ### What is React?
React (aka React.js or ReactJS) is an **open-source front-end JavaScript library** that is used for building composable user interfaces, especially for single-page applications. It is used for handling view layer for web and mobile apps based on components in a declarative approach.
React was created by [Jordan Walke](https://github.com/jordwalke), a software engineer working for Facebook. React was first deployed on Facebook's News Feed in 2011 and on Instagram in 2012.
**[⬆ Back to Top](#table-of-contents)**
2. ### What is the history behind React evolution?
The history of ReactJS started in 2010 with the creation of **XHP**. XHP is a PHP extension which improved the syntax of the language such that XML document fragments become valid PHP expressions and the primary purpose was used to create custom and reusable HTML elements.
The main principle of this extension was to make front-end code easier to understand and to help avoid cross-site scripting attacks. The project was successful to prevent the malicious content submitted by the scrubbing user.
But there was a different problem with XHP in which dynamic web applications require many roundtrips to the server, and XHP did not solve this problem. Also, the whole UI was re-rendered for small change in the application. Later, the initial prototype of React is created with the name **FaxJ** by Jordan inspired from XHP. Finally after sometime React has been introduced as a new library into JavaScript world.
**Note:** JSX comes from the idea of XHP
**[⬆ Back to Top](#table-of-contents)**
3. ### What are the major features of React?
The major features of React are:
- Uses **JSX** syntax, a syntax extension of JS that allows developers to write HTML in their JS code.
- It uses **Virtual DOM** instead of Real DOM considering that Real DOM manipulations are expensive.
- Supports **server-side rendering** which is useful for Search Engine Optimizations(SEO).
- Follows **Unidirectional or one-way** data flow or data binding.
- Uses **reusable/composable** UI components to develop the view.
**[⬆ Back to Top](#table-of-contents)**
4. ### What is JSX?
_JSX_ stands for _JavaScript XML_ and it is an XML-like syntax extension to ECMAScript. Basically it just provides the syntactic sugar for the `React.createElement(type, props, ...children)` function, giving us expressiveness of JavaScript along with HTML like template syntax.
In the example below, the text inside `<h1>` tag is returned as JavaScript function to the render function.
```jsx harmony
export default function App() {
return <h1 className="greeting">{"Hello, this is a JSX Code!"}</h1>;
}
```
If you don't use JSX syntax then the respective JavaScript code should be written as below,
```javascript
import { createElement } from "react";
export default function App() {
return createElement(
"h1",
{ className: "greeting" },
"Hello, this is a JSX Code!"
);
}
```
<details><summary><b>See Class</b></summary>
<p>
```jsx harmony
class App extends React.Component {
render() {
return <h1 className="greeting">{"Hello, this is a JSX Code!"}</h1>;
}
}
```
</p>
</details>
**Note:** JSX is stricter than HTML
**[⬆ Back to Top](#table-of-contents)**
5. ### What is the difference between Element and Component?
An _Element_ is a plain object describing what you want to appear on the screen in terms of the DOM nodes or other components. _Elements_ can contain other _Elements_ in their props. Creating a React element is cheap. Once an element is created, it cannot be mutated.
The JavaScript representation(Without JSX) of React Element would be as follows:
```javascript
const element = React.createElement("div", { id: "login-btn" }, "Login");
```
and this element can be simiplified using JSX
```html
<div id="login-btn">Login</div>
```
The above `React.createElement()` function returns an object as below:
```javascript
{
type: 'div',
props: {
children: 'Login',
id: 'login-btn'
}
}
```
Finally, this element renders to the DOM using `ReactDOM.render()`.
Whereas a **component** can be declared in several different ways. It can be a class with a `render()` method or it can be defined as a function. In either case, it takes props as an input, and returns a JSX tree as the output:
```javascript
const Button = ({ handleLogin }) => (
<div id={"login-btn"} onClick={handleLogin}>
Login
</div>
);
```
Then JSX gets transpiled to a `React.createElement()` function tree:
```javascript
const Button = ({ handleLogin }) =>
React.createElement(
"div",
{ id: "login-btn", onClick: handleLogin },
"Login"
);
```
**[⬆ Back to Top](#table-of-contents)**
6. ### How to create components in React?
Components are the building blocks of creating User Interfaces(UI) in React. There are two possible ways to create a component.
1. **Function Components:** This is the simplest way to create a component. Those are pure JavaScript functions that accept props object as the one and only one parameter and return React elements to render the output:
```jsx harmony
function Greeting({ message }) {
return <h1>{`Hello, ${message}`}</h1>;
}
```
2. **Class Components:** You can also use ES6 class to define a component. The above function component can be written as a class component:
```jsx harmony
class Greeting extends React.Component {
render() {
return <h1>{`Hello, ${this.props.message}`}</h1>;
}
}
```
**[⬆ Back to Top](#table-of-contents)**
7. ### When to use a Class Component over a Function Component?
After the addition of Hooks(i.e. React 16.8 onwards) it is always recommended to use Function components over Class components in React. Because you could use state, lifecycle methods and other features that were only available in class component present in function component too.
But even there are two reasons to use Class components over Function components.
1. If you need a React functionality whose Function component equivalent is not present yet, like Error Boundaries.
2. In older versions, If the component needs _state or lifecycle methods_ then you need to use class component.
So the summary to this question is as follows:
**Use Function Components:**
- If you don't need state or lifecycle methods, and your component is purely presentational.
- For simplicity, readability, and modern code practices, especially with the use of React Hooks for state and side effects.
**Use Class Components:**
- If you need to manage state or use lifecycle methods.
- In scenarios where backward compatibility or integration with older code is necessary.
**Note:** You can also use reusable [react error boundary](https://github.com/bvaughn/react-error-boundary) third-party component without writing any class. i.e, No need to use class components for Error boundaries.
The usage of Error boundaries from the above library is quite straight forward.
> **_Note when using react-error-boundary:_** ErrorBoundary is a client component. You can only pass props to it that are serializable or use it in files that have a `"use client";` directive.
```jsx
"use client";
import { ErrorBoundary } from "react-error-boundary";
<ErrorBoundary fallback={<div>Something went wrong</div>}>
<ExampleApplication />
</ErrorBoundary>;
```
**[⬆ Back to Top](#table-of-contents)**
8. ### What are Pure Components?
Pure components are the components which render the same output for the same state and props. In function components, you can achieve these pure components through memoized `React.memo()` API wrapping around the component. This API prevents unnecessary re-renders by comparing the previous props and new props using shallow comparison. So it will be helpful for performance optimizations.
But at the same time, it won't compare the previous state with the current state because function component itself prevents the unnecessary rendering by default when you set the same state again.
The syntactic representation of memoized components looks like below,
```jsx
const MemoizedComponent = memo(SomeComponent, arePropsEqual?);
```
Below is the example of how child component(i.e., EmployeeProfile) prevents re-renders for the same props passed by parent component(i.e.,EmployeeRegForm).
```jsx
import { memo, useState } from "react";
const EmployeeProfile = memo(function EmployeeProfile({ name, email }) {
return (
<>
<p>Name:{name}</p>
<p>Email: {email}</p>
</>
);
});
export default function EmployeeRegForm() {
const [name, setName] = useState("");
const [email, setEmail] = useState("");
return (
<>
<label>
Name:{" "}
<input value={name} onChange={(e) => setName(e.target.value)} />
</label>
<label>
Email:{" "}
<input value={email} onChange={(e) => setEmail(e.target.value)} />
</label>
<hr />
<EmployeeProfile name={name} />
</>
);
}
```
In the above code, the email prop has not been passed to child component. So there won't be any re-renders for email prop change.
In class components, the components extending _`React.PureComponent`_ instead of _`React.Component`_ become the pure components. When props or state changes, _PureComponent_ will do a shallow comparison on both props and state by invoking `shouldComponentUpdate()` lifecycle method.
**Note:** `React.memo()` is a higher-order component.
**[⬆ Back to Top](#table-of-contents)**
9. ### What is state in React?
_State_ of a component is an object that holds some information that may change over the lifetime of the component. The important point is whenever the state object changes, the component re-renders. It is always recommended to make our state as simple as possible and minimize the number of stateful components.
![state](images/state.jpg)
Let's take an example of **User** component with `message` state. Here, **useState** hook has been used to add state to the User component and it returns an array with current state and function to update it.
```jsx harmony
import { useState } from "react";
function User() {
const [message, setMessage] = useState("Welcome to React world");
return (
<div>
<h1>{message}</h1>
</div>
);
}
```
Whenever React calls your component or access `useState` hook, it gives you a snapshot of the state for that particular render.
<details><summary><b>See Class</b></summary>
<p>
```jsx harmony
import React from "react";
class User extends React.Component {
constructor(props) {
super(props);
this.state = {
message: "Welcome to React world",
};
}
render() {
return (
<div>
<h1>{this.state.message}</h1>
</div>
);
}
}
```
</p>
</details>
State is similar to props, but it is private and fully controlled by the component ,i.e., it is not accessible to any other component till the owner component decides to pass it.
**[⬆ Back to Top](#table-of-contents)**
10. ### What are props in React?
_Props_ are inputs to components. They are single values or objects containing a set of values that are passed to components on creation similar to HTML-tag attributes. Here, the data is passed down from a parent component to a child component.
The primary purpose of props in React is to provide following component functionality:
1. Pass custom data to your component.
2. Trigger state changes.
3. Use via `this.props.reactProp` inside component's `render()` method.
For example, let us create an element with `reactProp` property:
```jsx harmony
<Element reactProp={"1"} />
```
This `reactProp` (or whatever you came up with) attribute name then becomes a property attached to React's native props object which originally already exists on all components created using React library.
```jsx harmony
props.reactProp;
```
For example, the usage of props in function component looks like below:
```jsx
import React from "react";
import ReactDOM from "react-dom";
const ChildComponent = (props) => {
return (
<div>
<p>{props.name}</p>
<p>{props.age}</p>
<p>{props.gender}</p>
</div>
);
};
const ParentComponent = () => {
return (
<div>
<ChildComponent name="John" age="30" gender="male" />
<ChildComponent name="Mary" age="25" geneder="female" />
</div>
);
};
```
The properties from props object can be accessed directly using destructing feature from ES6 (ECMAScript 2015). It is also possible to fallback to default value when the prop value is not specified. The above child component can be simplified like below.
```jsx harmony
const ChildComponent = ({ name, age, gender = "male" }) => {
return (
<div>
<p>{name}</p>
<p>{age}</p>
<p>{gender}</p>
</div>
);
};
```
**Note:** The default value won't be used if you pass `null` or `0` value. i.e, default value is only used if the prop value is missed or `undefined` value has been passed.
<details><summary><b>See Class</b></summary>
The Props accessed in Class Based Component as below
```jsx
import React from "react";
import ReactDOM from "react-dom";
class ChildComponent extends React.Component {
render() {
return (
<div>
<p>{this.props.name}</p>
<p>{this.props.age}</p>
<p>{this.props.gender}</p>
</div>
);
}
}
class ParentComponent extends React.Component {
render() {
return (
<div>
<ChildComponent name="John" age="30" gender="male" />
<ChildComponent name="Mary" age="25" gender="female" />
</div>
);
}
}
```
</details>
**[⬆ Back to Top](#table-of-contents)**
11. ### What is the difference between state and props?
In React, both `state` and `props` are plain JavaScript objects and used to manage the data of a component, but they are used in different ways and have different characteristics.
The `state` entity is managed by the component itself and can be updated using the setter(`setState()` for class components) function. Unlike props, state can be modified by the component and is used to manage the internal state of the component. i.e, state acts as a component's memory. Moreover, changes in the state trigger a re-render of the component and its children. The components cannot become reusable with the usage of state alone.
On the otherhand, `props` (short for "properties") are passed to a component by its parent component and are `read-only`, meaning that they cannot be modified by the own component itself. i.e, props acts as arguments for a function. Also, props can be used to configure the behavior of a component and to pass data between components. The components become reusable with the usage of props.
**[⬆ Back to Top](#table-of-contents)**
12. ### What is the difference between HTML and React event handling?
Below are some of the main differences between HTML and React event handling,
1. In HTML, the event name usually represents in _lowercase_ as a convention:
```html
<button onclick="activateLasers()"></button>
```
Whereas in React it follows _camelCase_ convention:
```jsx harmony
<button onClick={activateLasers}>
```
2. In HTML, you can return `false` to prevent default behavior:
```html
<a
href="#"
onclick='console.log("The link was clicked."); return false;'
/>
```
Whereas in React you must call `preventDefault()` explicitly:
```javascript
function handleClick(event) {
event.preventDefault();
console.log("The link was clicked.");
}
```
3. In HTML, you need to invoke the function by appending `()`
Whereas in react you should not append `()` with the function name. (refer "activateLasers" function in the first point for example)
**[⬆ Back to Top](#table-of-contents)**
13. ### What are synthetic events in React?
`SyntheticEvent` is a cross-browser wrapper around the browser's native event. Its API is same as the browser's native event, including `stopPropagation()` and `preventDefault()`, except the events work identically across all browsers. The native events can be accessed directly from synthetic events using `nativeEvent` attribute.
Let's take an example of BookStore title search component with the ability to get all native event properties
```js
function BookStore() {
function handleTitleChange(e) {
console.log("The new title is:", e.target.value);
// 'e' represents synthetic event
const nativeEvent = e.nativeEvent;
console.log(nativeEvent);
e.stopPropagation();
e.preventDefault();
}
return <input name="title" onChange={handleTitleChange} />;
}
```
**[⬆ Back to Top](#table-of-contents)**
14. ### What are inline conditional expressions?
You can use either _if statements_ or _ternary expressions_ which are available from JS to conditionally render expressions. Apart from these approaches, you can also embed any expressions in JSX by wrapping them in curly braces and then followed by JS logical operator `&&`.
```jsx harmony
<h1>Hello!</h1>;
{
messages.length > 0 && !isLogin ? (
<h2>You have {messages.length} unread messages.</h2>
) : (
<h2>You don't have unread messages.</h2>
);
}
```
**[⬆ Back to Top](#table-of-contents)**
15. ### What is "key" prop and what is the benefit of using it in arrays of elements?
A `key` is a special attribute you **should** include when mapping over arrays to render data. _Key_ prop helps React identify which items have changed, are added, or are removed.
Keys should be unique among its siblings. Most often we use ID from our data as _key_:
```jsx harmony
const todoItems = todos.map((todo) => <li key={todo.id}>{todo.text}</li>);
```
When you don't have stable IDs for rendered items, you may use the item _index_ as a _key_ as a last resort:
```jsx harmony
const todoItems = todos.map((todo, index) => (
<li key={index}>{todo.text}</li>
));
```
**Note:**
1. Using _indexes_ for _keys_ is **not recommended** if the order of items may change. This can negatively impact performance and may cause issues with component state.
2. If you extract list item as separate component then apply _keys_ on list component instead of `li` tag.
3. There will be a warning message in the console if the `key` prop is not present on list items.
4. The key attribute accepts either string or number and internally convert it as string type.
5. Don't generate the key on the fly something like `key={Math.random()}`. Because the keys will never match up between re-renders and DOM created everytime.
**[⬆ Back to Top](#table-of-contents)**
16. ### What is Virtual DOM?
The _Virtual DOM_ (VDOM) is an in-memory representation of _Real DOM_. The representation of a UI is kept in memory and synced with the "real" DOM. It's a step that happens between the render function being called and the displaying of elements on the screen. This entire process is called _reconciliation_.
**[⬆ Back to Top](#table-of-contents)**
17. ### How Virtual DOM works?
The _Virtual DOM_ works in three simple steps.
1. Whenever any underlying data changes, the entire UI is re-rendered in Virtual DOM representation.
![vdom](images/vdom1.png)
2. Then the difference between the previous DOM representation and the new one is calculated.
![vdom2](images/vdom2.png)
3. Once the calculations are done, the real DOM will be updated with only the things that have actually changed.
![vdom3](images/vdom3.png)
**[⬆ Back to Top](#table-of-contents)**
18. ### What is the difference between Shadow DOM and Virtual DOM?
The _Shadow DOM_ is a browser technology designed primarily for scoping variables and CSS in _web components_. The _Virtual DOM_ is a concept implemented by libraries in JavaScript on top of browser APIs.
**[⬆ Back to Top](#table-of-contents)**
19. ### What is React Fiber?
Fiber is the new _reconciliation_ engine or reimplementation of core algorithm in React v16. The goal of React Fiber is to increase its suitability for areas like animation, layout, gestures, ability to pause, abort, or reuse work and assign priority to different types of updates; and new concurrency primitives.
**[⬆ Back to Top](#table-of-contents)**
20. ### What is the main goal of React Fiber?
The goal of _React Fiber_ is to increase its suitability for areas like animation, layout, and gestures. Its headline feature is **incremental rendering**: the ability to split rendering work into chunks and spread it out over multiple frames.
_from documentation_
Its main goals are:
1. Ability to split interruptible work in chunks.
2. Ability to prioritize, rebase and reuse work in progress.
3. Ability to yield back and forth between parents and children to support layout in React.
4. Ability to return multiple elements from render().
5. Better support for error boundaries.
**[⬆ Back to Top](#table-of-contents)**
21. ### What are controlled components?
A component that controls the input elements within the forms on subsequent user input is called **Controlled Component**, i.e, every state mutation will have an associated handler function. That means, the displayed data is always in sync with the state of the component.
The controlled components will be implemented using the below steps,
1. Initialize the state using use state hooks in function components or inside constructor for class components.
2. Set the value of the form element to the respective state variable.
3. Create an event handler to handle the user input changes through useState updater function or setState from class component.
4. Attach the above event handler to form elements change or click events
For example, the name input field updates the user name using `handleChange` event handler as below,
```javascript
import React, { useState } from "react";
function UserProfile() {
const [username, setUsername] = useState("");
const handleChange = (e) => {
setUsername(e.target.value);
};
return (
<form>
<label>
Name:
<input type="text" value={username} onChange={handleChange} />
</label>
</form>
);
}
```
**[⬆ Back to Top](#table-of-contents)**
22. ### What are uncontrolled components?
The **Uncontrolled Components** are the ones that store their own state internally, and you query the DOM using a ref to find its current value when you need it. This is a bit more like traditional HTML.
The uncontrolled components will be implemented using the below steps,
1. Create a ref using useRef react hook in function component or `React.createRef()` in class based component.
2. Attach this ref to the form element.
3. The form element value can be accessed directly through `ref` in event handlers or `componentDidMount` for class components
In the below UserProfile component, the `username` input is accessed using ref.
```jsx harmony
import React, { useRef } from "react";
function UserProfile() {
const usernameRef = useRef(null);
const handleSubmit = (event) => {
event.preventDefault();
console.log("The submitted username is: " + usernameRef.current.value);
};
return (
<form onSubmit={handleSubmit}>
<label>
Username:
<input type="text" ref={usernameRef} />
</label>
<button type="submit">Submit</button>
</form>
);
}
```
In most cases, it's recommend to use controlled components to implement forms. In a controlled component, form data is handled by a React component. The alternative is uncontrolled components, where form data is handled by the DOM itself.
<details><summary><b>See Class</b></summary>
<p>
```jsx harmony
class UserProfile extends React.Component {
constructor(props) {
super(props);
this.handleSubmit = this.handleSubmit.bind(this);
this.input = React.createRef();
}
handleSubmit(event) {
alert("A name was submitted: " + this.input.current.value);
event.preventDefault();
}
render() {
return (
<form onSubmit={this.handleSubmit}>
<label>
{"Name:"}
<input type="text" ref={this.input} />
</label>
<input type="submit" value="Submit" />
</form>
);
}
}
```
</p>
</details>
**[⬆ Back to Top](#table-of-contents)**
23. ### What is the difference between createElement and cloneElement?
JSX elements will be transpiled to `React.createElement()` functions to create React elements which are going to be used for the object representation of UI. Whereas `cloneElement` is used to clone an element and pass it new props.
**[⬆ Back to Top](#table-of-contents)**
24. ### What is Lifting State Up in React?
When several components need to share the same changing data then it is recommended to _lift the shared state up_ to their closest common ancestor. That means if two child components share the same data from its parent, then move the state to parent instead of maintaining local state in both of the child components.
**[⬆ Back to Top](#table-of-contents)**
25. ### What are Higher-Order Components?
A _higher-order component_ (_HOC_) is a function that takes a component and returns a new component. Basically, it's a pattern that is derived from React's compositional nature.
We call them **pure components** because they can accept any dynamically provided child component but they won't modify or copy any behavior from their input components.
```javascript
const EnhancedComponent = higherOrderComponent(WrappedComponent);
```
HOC can be used for many use cases:
1. Code reuse, logic and bootstrap abstraction.
2. Render hijacking.
3. State abstraction and manipulation.
4. Props manipulation.
**[⬆ Back to Top](#table-of-contents)**
26. ### What is children prop?
_Children_ is a prop that allows you to pass components as data to other components, just like any other prop you use. Component tree put between component's opening and closing tag will be passed to that component as `children` prop.
A simple usage of children prop looks as below,
```jsx harmony
function MyDiv({ children }){
return (
<div>
{children}
</div>;
);
}
export default function Greeting() {
return (
<MyDiv>
<span>{"Hello"}</span>
<span>{"World"}</span>
</MyDiv>
);
}
```
<details><summary><b>See Class</b></summary>
<p>
```jsx harmony
const MyDiv = React.createClass({
render: function () {
return <div>{this.props.children}</div>;
},
});
ReactDOM.render(
<MyDiv>
<span>{"Hello"}</span>
<span>{"World"}</span>
</MyDiv>,
node
);
```
</p>
</details>
**Note:** There are several methods available in the legacy React API to work with this prop. These include `React.Children.map`, `React.Children.forEach`, `React.Children.count`, `React.Children.only`, `React.Children.toArray`.
**[⬆ Back to Top](#table-of-contents)**
27. ### How to write comments in React?
The comments in React/JSX are similar to JavaScript Multiline comments but are wrapped in curly braces.
**Single-line comments:**
```jsx harmony
<div>
{/* Single-line comments(In vanilla JavaScript, the single-line comments are represented by double slash(//)) */}
{`Welcome ${user}, let's play React`}
</div>
```
**Multi-line comments:**
```jsx harmony
<div>
{/* Multi-line comments for more than
one line */}
{`Welcome ${user}, let's play React`}
</div>
```
**[⬆ Back to Top](#table-of-contents)**
28. ### What is reconciliation?
`Reconciliation` is the process through which React updates the Browser DOM and makes React work faster. React use a `diffing algorithm` so that component updates are predictable and faster. React would first calculate the difference between the `real DOM` and the copy of DOM `(Virtual DOM)` when there's an update of components.
React stores a copy of Browser DOM which is called `Virtual DOM`. When we make changes or add data, React creates a new Virtual DOM and compares it with the previous one. This comparison is done by `Diffing Algorithm`.
Now React compares the Virtual DOM with Real DOM. It finds out the changed nodes and updates only the changed nodes in Real DOM leaving the rest nodes as it is. This process is called _Reconciliation_.
**[⬆ Back to Top](#table-of-contents)**
29. ### Does the lazy function support named exports?
No, currently `React.lazy` function supports default exports only. If you would like to import modules which are named exports, you can create an intermediate module that reexports it as the default. It also ensures that tree shaking keeps working and don’t pull unused components.
Let's take a component file which exports multiple named components,
```javascript
// MoreComponents.js
export const SomeComponent = /* ... */;
export const UnusedComponent = /* ... */;
```
and reexport `MoreComponents.js` components in an intermediate file `IntermediateComponent.js`
```javascript
// IntermediateComponent.js
export { SomeComponent as default } from "./MoreComponents.js";
```
Now you can import the module using lazy function as below,
```javascript
import React, { lazy } from "react";
const SomeComponent = lazy(() => import("./IntermediateComponent.js"));
```
**[⬆ Back to Top](#table-of-contents)**
30. ### Why React uses `className` over `class` attribute?
The attribute names written in JSX turned into keys of JavaScript objects and the JavaScript names cannot contain dashes or reserved words, it is recommended to use camelCase wherever applicable in JSX code. The attribute `class` is a keyword in JavaScript, and JSX is an extension of JavaScript. That's the principle reason why React uses `className` instead of `class`. Pass a string as the `className` prop.
```jsx harmony
render() {
return <span className="menu navigation-menu">{'Menu'}</span>
}
```
**[⬆ Back to Top](#table-of-contents)**
31. ### What are fragments?
It's a common pattern or practice in React for a component to return multiple elements. _Fragments_ let you group a list of children without adding extra nodes to the DOM.
You need to use either `<Fragment>` or a shorter syntax having empty tag (`<></>`).
Below is the example of how to use fragment inside _Story_ component.
```jsx harmony
function Story({ title, description, date }) {
return (
<Fragment>
<h2>{title}</h2>
<p>{description}</p>
<p>{date}</p>
</Fragment>
);
}
```
It is also possible to render list of fragments inside a loop with the mandatory **key** attribute supplied.
```jsx harmony
function StoryBook() {
return stories.map((story) => (
<Fragment key={story.id}>
<h2>{story.title}</h2>
<p>{story.description}</p>
<p>{story.date}</p>
</Fragment>
));
}
```
Usually, you don't need to use `<Fragment>` until there is a need of _key_ attribute. The usage of shorter syntax looks like below.
```jsx harmony
function Story({ title, description, date }) {
return (
<>
<h2>{title}</h2>
<p>{description}</p>
<p>{date}</p>
</>
);
}
```
**[⬆ Back to Top](#table-of-contents)**
32. ### Why fragments are better than container divs?
Below are the list of reasons to prefer fragments over container DOM elements,
1. Fragments are a bit faster and use less memory by not creating an extra DOM node. This only has a real benefit on very large and deep trees.
2. Some CSS mechanisms like _Flexbox_ and _CSS Grid_ have a special parent-child relationships, and adding divs in the middle makes it hard to keep the desired layout.
3. The DOM Inspector is less cluttered.
**[⬆ Back to Top](#table-of-contents)**
33. ### What are portals in React?
_Portal_ is a recommended way to render children into a DOM node that exists outside the DOM hierarchy of the parent component. When using
CSS transform in a component, its descendant elements should not use fixed positioning, otherwise the layout will blow up.
```javascript
ReactDOM.createPortal(child, container);
```
The first argument is any render-able React child, such as an element, string, or fragment. The second argument is a DOM element.
**[⬆ Back to Top](#table-of-contents)**
34. ### What are stateless components?
If the behaviour of a component is independent of its state then it can be a stateless component. You can use either a function or a class for creating stateless components. But unless you need to use a lifecycle hook in your components, you should go for function components. There are a lot of benefits if you decide to use function components here; they are easy to write, understand, and test, a little faster, and you can avoid the `this` keyword altogether.
**[⬆ Back to Top](#table-of-contents)**
35. ### What are stateful components?
If the behaviour of a component is dependent on the _state_ of the component then it can be termed as stateful component. These _stateful components_ are either function components with hooks or _class components_.
Let's take an example of function stateful component which update the state based on click event,
```javascript
import React, {useState} from 'react';
const App = (props) => {
const [count, setCount] = useState(0);
handleIncrement() {
setCount(count+1);
}
return (
<>
<button onClick={handleIncrement}>Increment</button>
<span>Counter: {count}</span>
</>
)
}
```
<details><summary><b>See Class</b></summary>
<p>
The equivalent class stateful component with a state that gets initialized in the `constructor`.
```jsx harmony
class App extends Component {
constructor(props) {
super(props);
this.state = { count: 0 };
}
handleIncrement() {
setState({ count: this.state.count + 1 });
}
render() {
<>
<button onClick={() => this.handleIncrement}>Increment</button>
<span>Count: {count}</span>
</>;
}
}
```
</p>
</details>
**[⬆ Back to Top](#table-of-contents)**
36. ### How to apply validation on props in React?
When the application is running in _development mode_, React will automatically check all props that we set on components to make sure they have _correct type_. If the type is incorrect, React will generate warning messages in the console. It's disabled in _production mode_ due to performance impact. The mandatory props are defined with `isRequired`.
The set of predefined prop types:
1. `PropTypes.number`
2. `PropTypes.string`
3. `PropTypes.array`
4. `PropTypes.object`
5. `PropTypes.func`
6. `PropTypes.node`
7. `PropTypes.element`
8. `PropTypes.bool`
9. `PropTypes.symbol`
10. `PropTypes.any`
We can define `propTypes` for `User` component as below:
```jsx harmony
import React from "react";
import PropTypes from "prop-types";
class User extends React.Component {
static propTypes = {
name: PropTypes.string.isRequired,
age: PropTypes.number.isRequired,
};
render() {
return (
<>
<h1>{`Welcome, ${this.props.name}`}</h1>
<h2>{`Age, ${this.props.age}`}</h2>
</>
);
}
}
```
**Note:** In React v15.5 _PropTypes_ were moved from `React.PropTypes` to `prop-types` library.
_The Equivalent Functional Component_
```jsx harmony
import React from "react";
import PropTypes from "prop-types";
function User({ name, age }) {
return (
<>
<h1>{`Welcome, ${name}`}</h1>
<h2>{`Age, ${age}`}</h2>
</>
);
}
User.propTypes = {
name: PropTypes.string.isRequired,
age: PropTypes.number.isRequired,
};
```
**[⬆ Back to Top](#table-of-contents)**
37. ### What are the advantages of React?
Below are the list of main advantages of React,
1. Increases the application's performance with _Virtual DOM_.
2. JSX makes code easy to read and write.
3. It renders both on client and server side (_SSR_).
4. Easy to integrate with frameworks (Angular, Backbone) since it is only a view library.
5. Easy to write unit and integration tests with tools such as Jest.
**[⬆ Back to Top](#table-of-contents)**
38. ### What are the limitations of React?
Apart from the advantages, there are few limitations of React too,
1. React is just a view library, not a full framework.
2. There is a learning curve for beginners who are new to web development.
3. Integrating React into a traditional MVC framework requires some additional configuration.
4. The code complexity increases with inline templating and JSX.
5. Too many smaller components leading to over engineering or boilerplate.
**[⬆ Back to Top](#table-of-contents)**
39. ### What are the recommended ways for static type checking?
Normally we use _PropTypes library_ (`React.PropTypes` moved to a `prop-types` package since React v15.5) for _type checking_ in the React applications. For large code bases, it is recommended to use _static type checkers_ such as Flow or TypeScript, that perform type checking at compile time and provide auto-completion features.
**[⬆ Back to Top](#table-of-contents)**
40. ### What is the use of `react-dom` package?
The `react-dom` package provides _DOM-specific methods_ that can be used at the top level of your app. Most of the components are not required to use this module. Some of the methods of this package are:
1. `render()`
2. `hydrate()`
3. `unmountComponentAtNode()`
4. `findDOMNode()`
5. `createPortal()`
**[⬆ Back to Top](#table-of-contents)**
41. ### What is ReactDOMServer?
The `ReactDOMServer` object enables you to render components to static markup (typically used on node server). This object is mainly used for _server-side rendering_ (SSR). The following methods can be used in both the server and browser environments:
1. `renderToString()`
2. `renderToStaticMarkup()`
For example, you generally run a Node-based web server like Express, Hapi, or Koa, and you call `renderToString` to render your root component to a string, which you then send as response.
```javascript
// using Express
import { renderToString } from "react-dom/server";
import MyPage from "./MyPage";
app.get("/", (req, res) => {
res.write(
"<!DOCTYPE html><html><head><title>My Page</title></head><body>"
);
res.write('<div id="content">');
res.write(renderToString(<MyPage />));
res.write("</div></body></html>");
res.end();
});
```
**[⬆ Back to Top](#table-of-contents)**
42. ### How to use innerHTML in React?
The `dangerouslySetInnerHTML` attribute is React's replacement for using `innerHTML` in the browser DOM. Just like `innerHTML`, it is risky to use this attribute considering cross-site scripting (XSS) attacks. You just need to pass a `__html` object as key and HTML text as value.
In this example MyComponent uses `dangerouslySetInnerHTML` attribute for setting HTML markup:
```jsx harmony
function createMarkup() {
return { __html: "First · Second" };
}
function MyComponent() {
return <div dangerouslySetInnerHTML={createMarkup()} />;
}
```
**[⬆ Back to Top](#table-of-contents)**
43. ### How to use styles in React?
The `style` attribute accepts a JavaScript object with camelCased properties rather than a CSS string. This is consistent with the DOM style JavaScript property, is more efficient, and prevents XSS security holes.
```jsx harmony
const divStyle = {
color: "blue",
backgroundImage: "url(" + imgUrl + ")",
};
function HelloWorldComponent() {
return <div style={divStyle}>Hello World!</div>;
}
```
Style keys are camelCased in order to be consistent with accessing the properties on DOM nodes in JavaScript (e.g. `node.style.backgroundImage`).
**[⬆ Back to Top](#table-of-contents)**
44. ### How events are different in React?
Handling events in React elements has some syntactic differences:
1. React event handlers are named using camelCase, rather than lowercase.
2. With JSX you pass a function as the event handler, rather than a string.
**[⬆ Back to Top](#table-of-contents)**
45. ### What is the impact of indexes as keys?
Keys should be stable, predictable, and unique so that React can keep track of elements.
In the below code snippet each element's key will be based on ordering, rather than tied to the data that is being represented. This limits the optimizations that React can do and creates confusing bugs in the application.
```jsx harmony
{
todos.map((todo, index) => <Todo {...todo} key={index} />);
}
```
If you use element data for unique key, assuming `todo.id` is unique to this list and stable, React would be able to reorder elements without needing to reevaluate them as much.
```jsx harmony
{
todos.map((todo) => <Todo {...todo} key={todo.id} />);
}
```
**Note:** If you don't specify `key` prop at all, React will use index as a key's value while iterating over an array of data.
**[⬆ Back to Top](#table-of-contents)**
46. ### How do you conditionally render components?
In some cases you want to render different components depending on some state. JSX does not render `false` or `undefined`, so you can use conditional _short-circuiting_ to render a given part of your component only if a certain condition is true.
```jsx harmony
const MyComponent = ({ name, address }) => (
<div>
<h2>{name}</h2>
{address && <p>{address}</p>}
</div>
);
```
If you need an `if-else` condition then use _ternary operator_.
```jsx harmony
const MyComponent = ({ name, address }) => (
<div>
<h2>{name}</h2>
{address ? <p>{address}</p> : <p>{"Address is not available"}</p>}
</div>
);
```
**[⬆ Back to Top](#table-of-contents)**
47. ### Why we need to be careful when spreading props on DOM elements?
When we _spread props_ we run into the risk of adding unknown HTML attributes, which is a bad practice. Instead we can use prop destructuring with `...rest` operator, so it will add only required props.
For example,
```jsx harmony
const ComponentA = () => (
<ComponentB isDisplay={true} className={"componentStyle"} />
);
const ComponentB = ({ isDisplay, ...domProps }) => (
<div {...domProps}>{"ComponentB"}</div>
);
```
**[⬆ Back to Top](#table-of-contents)**
48. ### How do you memoize a component?
There are memoize libraries available which can be used on function components.
For example `moize` library can memoize the component in another component.
```jsx harmony
import moize from "moize";
import Component from "./components/Component"; // this module exports a non-memoized component
const MemoizedFoo = moize.react(Component);
const Consumer = () => {
<div>
{"I will memoize the following entry:"}
<MemoizedFoo />
</div>;
};
```
**Update:** Since React v16.6.0, we have a `React.memo`. It provides a higher order component which memoizes component unless the props change. To use it, simply wrap the component using React.memo before you use it.
```js
const MemoComponent = React.memo(function MemoComponent(props) {
/* render using props */
});
OR;
export default React.memo(MyFunctionComponent);
```
**[⬆ Back to Top](#table-of-contents)**
49. ### How you implement Server Side Rendering or SSR?
React is already equipped to handle rendering on Node servers. A special version of the DOM renderer is available, which follows the same pattern as on the client side.
```jsx harmony
import ReactDOMServer from "react-dom/server";
import App from "./App";
ReactDOMServer.renderToString(<App />);
```
This method will output the regular HTML as a string, which can be then placed inside a page body as part of the server response. On the client side, React detects the pre-rendered content and seamlessly picks up where it left off.
**[⬆ Back to Top](#table-of-contents)**
50. ### How to enable production mode in React?
You should use Webpack's `DefinePlugin` method to set `NODE_ENV` to `production`, by which it strip out things like propType validation and extra warnings. Apart from this, if you minify the code, for example, Uglify's dead-code elimination to strip out development only code and comments, it will drastically reduce the size of your bundle.
**[⬆ Back to Top](#table-of-contents)**
51. ### Do Hooks replace render props and higher order components?
Both render props and higher-order components render only a single child but in most of the cases Hooks are a simpler way to serve this by reducing nesting in your tree.
**[⬆ Back to Top](#table-of-contents)**
52. ### What is a switching component?
A _switching component_ is a component that renders one of many components. We need to use object to map prop values to components.
For example, a switching component to display different pages based on `page` prop:
```jsx harmony
import HomePage from "./HomePage";
import AboutPage from "./AboutPage";
import ServicesPage from "./ServicesPage";
import ContactPage from "./ContactPage";
const PAGES = {
home: HomePage,
about: AboutPage,
services: ServicesPage,
contact: ContactPage,
};
const Page = (props) => {
const Handler = PAGES[props.page] || ContactPage;
return <Handler {...props} />;
};
// The keys of the PAGES object can be used in the prop types to catch dev-time errors.
Page.propTypes = {
page: PropTypes.oneOf(Object.keys(PAGES)).isRequired,
};
```
**[⬆ Back to Top](#table-of-contents)**
53. ### What are React Mixins?
_Mixins_ are a way to totally separate components to have a common functionality. Mixins **should not be used** and can be replaced with _higher-order components_ or _decorators_.
One of the most commonly used mixins is `PureRenderMixin`. You might be using it in some components to prevent unnecessary re-renders when the props and state are shallowly equal to the previous props and state:
```javascript
const PureRenderMixin = require("react-addons-pure-render-mixin");
const Button = React.createClass({
mixins: [PureRenderMixin],
// ...
});
```
<!-- TODO: mixins are deprecated -->
**[⬆ Back to Top](#table-of-contents)**
54. ### What are the Pointer Events supported in React?
_Pointer Events_ provide a unified way of handling all input events. In the old days we had a mouse and respective event listeners to handle them but nowadays we have many devices which don't correlate to having a mouse, like phones with touch surface or pens. We need to remember that these events will only work in browsers that support the _Pointer Events_ specification.
The following event types are now available in _React DOM_:
1. `onPointerDown`
2. `onPointerMove`
3. `onPointerUp`
4. `onPointerCancel`
5. `onGotPointerCapture`
6. `onLostPointerCapture`
7. `onPointerEnter`
8. `onPointerLeave`
9. `onPointerOver`
10. `onPointerOut`
**[⬆ Back to Top](#table-of-contents)**
55. ### Why should component names start with capital letter?
If you are rendering your component using JSX, the name of that component has to begin with a capital letter otherwise React will throw an error as an unrecognized tag. This convention is because only HTML elements and SVG tags can begin with a lowercase letter.
```jsx harmony
function SomeComponent {
// Code goes here
}
```
You can define function component whose name starts with lowercase letter, but when it's imported it should have a capital letter. Here lowercase is fine:
```jsx harmony
function myComponent {
render() {
return <div />;
}
}
export default myComponent;
```
While when imported in another file it should start with capital letter:
```jsx harmony
import MyComponent from "./myComponent";
```
**[⬆ Back to Top](#table-of-contents)**
56. ### Are custom DOM attributes supported in React v16?
Yes. In the past, React used to ignore unknown DOM attributes. If you wrote JSX with an attribute that React doesn't recognize, React would just skip it.
For example, let's take a look at the below attribute:
```jsx harmony
<div mycustomattribute={"something"} />
```
Would render an empty div to the DOM with React v15:
```html
<div />
```
In React v16 any unknown attributes will end up in the DOM:
```html
<div mycustomattribute="something" />
```
This is useful for supplying browser-specific non-standard attributes, trying new DOM APIs, and integrating with opinionated third-party libraries.
**[⬆ Back to Top](#table-of-contents)**
57. ### How to loop inside JSX?
You can simply use `Array.prototype.map` with ES6 _arrow function_ syntax.
For example, the `items` array of objects is mapped into an array of components:
```jsx harmony
<tbody>
{items.map((item) => (
<SomeComponent key={item.id} name={item.name} />
))}
</tbody>
```
But you can't iterate using `for` loop:
```jsx harmony
<tbody>
for (let i = 0; i < items.length; i++) {
<SomeComponent key={items[i].id} name={items[i].name} />
}
</tbody>
```
This is because JSX tags are transpiled into _function calls_, and you can't use statements inside expressions. This may change thanks to `do` expressions which are _stage 1 proposal_.
**[⬆ Back to Top](#table-of-contents)**
58. ### How do you access props in attribute quotes?
React (or JSX) doesn't support variable interpolation inside an attribute value. The below representation won't work:
```jsx harmony
<img className="image" src="images/{this.props.image}" />
```
But you can put any JS expression inside curly braces as the entire attribute value. So the below expression works:
```jsx harmony
<img className="image" src={"images/" + this.props.image} />
```
Using _template strings_ will also work:
```jsx harmony
<img className="image" src={`images/${this.props.image}`} />
```
**[⬆ Back to Top](#table-of-contents)**
59. ### What is React proptype array with shape?
If you want to pass an array of objects to a component with a particular shape then use `React.PropTypes.shape()` as an argument to `React.PropTypes.arrayOf()`.
```javascript
ReactComponent.propTypes = {
arrayWithShape: React.PropTypes.arrayOf(
React.PropTypes.shape({
color: React.PropTypes.string.isRequired,
fontSize: React.PropTypes.number.isRequired,
})
).isRequired,
};
```
**[⬆ Back to Top](#table-of-contents)**
60. ### How to conditionally apply class attributes?
You shouldn't use curly braces inside quotes because it is going to be evaluated as a string.
```jsx harmony
<div className="btn-panel {this.props.visible ? 'show' : 'hidden'}">
```
Instead you need to move curly braces outside (don't forget to include spaces between class names):
```jsx harmony
<div className={'btn-panel ' + (this.props.visible ? 'show' : 'hidden')}>
```
_Template strings_ will also work:
```jsx harmony
<div className={`btn-panel ${this.props.visible ? 'show' : 'hidden'}`}>
```
**[⬆ Back to Top](#table-of-contents)**
61. ### What is the difference between React and ReactDOM?
The `react` package contains `React.createElement()`, `React.Component`, `React.Children`, and other helpers related to elements and component classes. You can think of these as the isomorphic or universal helpers that you need to build components. The `react-dom` package contains `ReactDOM.render()`, and in `react-dom/server` we have _server-side rendering_ support with `ReactDOMServer.renderToString()` and `ReactDOMServer.renderToStaticMarkup()`.
**[⬆ Back to Top](#table-of-contents)**
62. ### Why ReactDOM is separated from React?
The React team worked on extracting all DOM-related features into a separate library called _ReactDOM_. React v0.14 is the first release in which the libraries are split. By looking at some of the packages, `react-native`, `react-art`, `react-canvas`, and `react-three`, it has become clear that the beauty and essence of React has nothing to do with browsers or the DOM.
To build more environments that React can render to, React team planned to split the main React package into two: `react` and `react-dom`. This paves the way to writing components that can be shared between the web version of React and React Native.
**[⬆ Back to Top](#table-of-contents)**
63. ### How to use React label element?
If you try to render a `<label>` element bound to a text input using the standard `for` attribute, then it produces HTML missing that attribute and prints a warning to the console.
```jsx harmony
<label for={'user'}>{'User'}</label>
<input type={'text'} id={'user'} />
```
Since `for` is a reserved keyword in JavaScript, use `htmlFor` instead.
```jsx harmony
<label htmlFor={'user'}>{'User'}</label>
<input type={'text'} id={'user'} />
```
**[⬆ Back to Top](#table-of-contents)**
64. ### How to combine multiple inline style objects?
You can use _spread operator_ in regular React:
```jsx harmony
<button style={{ ...styles.panel.button, ...styles.panel.submitButton }}>
{"Submit"}
</button>
```
If you're using React Native then you can use the array notation:
```jsx harmony
<button style={[styles.panel.button, styles.panel.submitButton]}>
{"Submit"}
</button>
```
**[⬆ Back to Top](#table-of-contents)**
65. ### How to re-render the view when the browser is resized?
You can use the `useState` hook to manage the width and height state variables, and the `useEffect` hook to add and remove the `resize` event listener. The `[]` dependency array passed to useEffect ensures that the effect only runs once (on mount) and not on every re-render.
```javascript
import React, { useState, useEffect } from "react";
function WindowDimensions() {
const [dimensions, setDimensions] = useState({
width: window.innerWidth,
height: window.innerHeight,
});
useEffect(() => {
function handleResize() {
setDimensions({
width: window.innerWidth,
height: window.innerHeight,
});
}
window.addEventListener("resize", handleResize);
return () => window.removeEventListener("resize", handleResize);
}, []);
return (
<span>
{dimensions.width} x {dimensions.height}
</span>
);
}
```
<details>
<summary><h4>Using Class Component</h4></summary>
You can listen to the `resize` event in `componentDidMount()` and then update the dimensions (`width` and `height`). You should remove the listener in `componentWillUnmount()` method.
```javascript
class WindowDimensions extends React.Component {
constructor(props) {
super(props);
this.updateDimensions = this.updateDimensions.bind(this);
}
componentWillMount() {
this.updateDimensions();
}
componentDidMount() {
window.addEventListener("resize", this.updateDimensions);
}
componentWillUnmount() {
window.removeEventListener("resize", this.updateDimensions);
}
updateDimensions() {
this.setState({
width: window.innerWidth,
height: window.innerHeight,
});
}
render() {
return (
<span>
{this.state.width} x {this.state.height}
</span>
);
}
}
```
</details>
**[⬆ Back to Top](#table-of-contents)**
66. ### How to pretty print JSON with React?
We can use `<pre>` tag so that the formatting of the `JSON.stringify()` is retained:
```jsx harmony
const data = { name: "John", age: 42 };
function User {
return <pre>{JSON.stringify(data, null, 2)}</pre>;
}
const container = createRoot(document.getElementById("container"));
container.render(<User />);
```
<details><summary><b>See Class</b></summary>
<p>
```jsx harmony
const data = { name: "John", age: 42 };
class User extends React.Component {
render() {
return <pre>{JSON.stringify(data, null, 2)}</pre>;
}
}
React.render(<User />, document.getElementById("container"));
```
</p>
</details>
**[⬆ Back to Top](#table-of-contents)**
67. ### Why you can't update props in React?
The React philosophy is that props should be _immutable_(read only) and _top-down_. This means that a parent can send any prop values to a child, but the child can't modify received props.
**[⬆ Back to Top](#table-of-contents)**
68. ### How to focus an input element on page load?
You need to use `useEffect` hook to set focus on input field during page load time for functional component.
```jsx harmony
import React, { useEffect, useRef } from "react";
const App = () => {
const inputElRef = useRef(null);
useEffect(() => {
inputElRef.current.focus();
}, []);
return (
<div>
<input defaultValue={"Won't focus"} />
<input ref={inputElRef} defaultValue={"Will focus"} />
</div>
);
};
ReactDOM.render(<App />, document.getElementById("app"));
```
<details><summary><b>See Class</b></summary>
<p>
You can do it by creating _ref_ for `input` element and using it in `componentDidMount()`:
```jsx harmony
class App extends React.Component {
componentDidMount() {
this.nameInput.focus();
}
render() {
return (
<div>
<input defaultValue={"Won't focus"} />
<input
ref={(input) => (this.nameInput = input)}
defaultValue={"Will focus"}
/>
</div>
);
}
}
ReactDOM.render(<App />, document.getElementById("app"));
```
</p>
</details>
**[⬆ Back to Top](#table-of-contents)**
69. ### How can we find the version of React at runtime in the browser?
You can use `React.version` to get the version.
```jsx harmony
const REACT_VERSION = React.version;
ReactDOM.render(
<div>{`React version: ${REACT_VERSION}`}</div>,
document.getElementById("app")
);
```
**[⬆ Back to Top](#table-of-contents)**
70. ### How to add Google Analytics for React Router?
Add a listener on the `history` object to record each page view:
```javascript
history.listen(function (location) {
window.ga("set", "page", location.pathname + location.search);
window.ga("send", "pageview", location.pathname + location.search);
});
```
**[⬆ Back to Top](#table-of-contents)**
71. ### How do you apply vendor prefixes to inline styles in React?
React _does not_ apply _vendor prefixes_ automatically. You need to add vendor prefixes manually.
```jsx harmony
<div
style={{
transform: "rotate(90deg)",
WebkitTransform: "rotate(90deg)", // note the capital 'W' here
msTransform: "rotate(90deg)", // 'ms' is the only lowercase vendor prefix
}}
/>
```
**[⬆ Back to Top](#table-of-contents)**
72. ### How to import and export components using React and ES6?
You should use default for exporting the components
```jsx harmony
import User from "user";
export default function MyProfile {
return <User type="customer">//...</User>;
}
```
<details><summary><b>See Class</b></summary>
<p>
```jsx harmony
import React from "react";
import User from "user";
export default class MyProfile extends React.Component {
render() {
return <User type="customer">//...</User>;
}
}
```
</p>
</details>
With the export specifier, the MyProfile is going to be the member and exported to this module and the same can be imported without mentioning the name in other components.
```
**[⬆ Back to Top](#table-of-contents)**
73. ### What are the exceptions on React component naming?
The component names should start with an uppercase letter but there are few exceptions to this convention. The lowercase tag names with a dot (property accessors) are still considered as valid component names.
For example, the below tag can be compiled to a valid component,
```jsx harmony
render() {
return (
<obj.component/> // `React.createElement(obj.component)`
)
}
```
**[⬆ Back to Top](#table-of-contents)**
74. ### Is it possible to use async/await in plain React?
If you want to use `async`/`await` in React, you will need _Babel_ and [transform-async-to-generator](https://babeljs.io/docs/en/babel-plugin-transform-async-to-generator) plugin. React Native ships with Babel and a set of transforms.
**[⬆ Back to Top](#table-of-contents)**
75. ### What are the common folder structures for React?
There are two common practices for React project file structure.
1. **Grouping by features or routes:**
One common way to structure projects is locate CSS, JS, and tests together, grouped by feature or route.
```
common/
├─ Avatar.js
├─ Avatar.css
├─ APIUtils.js
└─ APIUtils.test.js
feed/
├─ index.js
├─ Feed.js
├─ Feed.css
├─ FeedStory.js
├─ FeedStory.test.js
└─ FeedAPI.js
profile/
├─ index.js
├─ Profile.js
├─ ProfileHeader.js
├─ ProfileHeader.css
└─ ProfileAPI.js
```
2. **Grouping by file type:**
Another popular way to structure projects is to group similar files together.
```
api/
├─ APIUtils.js
├─ APIUtils.test.js
├─ ProfileAPI.js
└─ UserAPI.js
components/
├─ Avatar.js
├─ Avatar.css
├─ Feed.js
├─ Feed.css
├─ FeedStory.js
├─ FeedStory.test.js
├─ Profile.js
├─ ProfileHeader.js
└─ ProfileHeader.css
```
**[⬆ Back to Top](#table-of-contents)**
76. ### What are the popular packages for animation?
_React Transition Group_ and _React Motion_ are popular animation packages in React ecosystem.
**[⬆ Back to Top](#table-of-contents)**
77. ### What is the benefit of styles modules?
It is recommended to avoid hard coding style values in components. Any values that are likely to be used across different UI components should be extracted into their own modules.
For example, these styles could be extracted into a separate component:
```javascript
export const colors = {
white,
black,
blue,
};
export const space = [0, 8, 16, 32, 64];
```
And then imported individually in other components:
```javascript
import { space, colors } from "./styles";
```
**[⬆ Back to Top](#table-of-contents)**
78. ### What are the popular React-specific linters?
ESLint is a popular JavaScript linter. There are plugins available that analyse specific code styles. One of the most common for React is an npm package called `eslint-plugin-react`. By default, it will check a number of best practices, with rules checking things from keys in iterators to a complete set of prop types.
Another popular plugin is `eslint-plugin-jsx-a11y`, which will help fix common issues with accessibility. As JSX offers slightly different syntax to regular HTML, issues with `alt` text and `tabindex`, for example, will not be picked up by regular plugins.
**[⬆ Back to Top](#table-of-contents)**
## React Router
**[⬆ Back to Top](#table-of-contents)**
79. ### What is React Router?
React Router is a powerful routing library built on top of React that helps you add new screens and flows to your application incredibly quickly, all while keeping the URL in sync with what's being displayed on the page.
**[⬆ Back to Top](#table-of-contents)**
80. ### How React Router is different from history library?
React Router is a wrapper around the `history` library which handles interaction with the browser's `window.history` with its browser and hash histories. It also provides memory history which is useful for environments that don't have global history, such as mobile app development (React Native) and unit testing with Node.
**[⬆ Back to Top](#table-of-contents)**
81. ### What are the `<Router>` components of React Router v6?
React Router v6 provides below 4 `<Router>` components:
1. `<BrowserRouter>`:Uses the HTML5 history API for standard web apps.
2. `<HashRouter>`:Uses hash-based routing for static servers.
3. `<MemoryRouter>`:Uses in-memory routing for testing and non-browser environments.
4. `<StaticRouter>`:Provides static routing for server-side rendering (SSR).
The above components will create _browser_, _hash_, _memory_ and _static_ history instances. React Router v6 makes the properties and methods of the `history` instance associated with your router available through the context in the `router` object.
**[⬆ Back to Top](#table-of-contents)**
82. ### What is the purpose of `push()` and `replace()` methods of `history`?
A history instance has two methods for navigation purpose.
1. `push()`
2. `replace()`
If you think of the history as an array of visited locations, `push()` will add a new location to the array and `replace()` will replace the current location in the array with the new one.
**[⬆ Back to Top](#table-of-contents)**
83. ### How do you programmatically navigate using React Router v4?
There are three different ways to achieve programmatic routing/navigation within components.
1. **Using the `withRouter()` higher-order function:**
The `withRouter()` higher-order function will inject the history object as a prop of the component. This object provides `push()` and `replace()` methods to avoid the usage of context.
```jsx harmony
import { withRouter } from "react-router-dom"; // this also works with 'react-router-native'
const Button = withRouter(({ history }) => (
<button
type="button"
onClick={() => {
history.push("/new-location");
}}
>
{"Click Me!"}
</button>
));
```
2. **Using `<Route>` component and render props pattern:**
The `<Route>` component passes the same props as `withRouter()`, so you will be able to access the history methods through the history prop.
```jsx harmony
import { Route } from "react-router-dom";
const Button = () => (
<Route
render={({ history }) => (
<button
type="button"
onClick={() => {
history.push("/new-location");
}}
>
{"Click Me!"}
</button>
)}
/>
);
```
3. **Using context:**
This option is not recommended and treated as unstable API.
```jsx harmony
const Button = (props, context) => (
<button
type="button"
onClick={() => {
context.history.push("/new-location");
}}
>
{"Click Me!"}
</button>
);
Button.contextTypes = {
history: React.PropTypes.shape({
push: React.PropTypes.func.isRequired,
}),
};
```
**[⬆ Back to Top](#table-of-contents)**
84. ### How to get query parameters in React Router v4?
The ability to parse query strings was taken out of React Router v4 because there have been user requests over the years to support different implementation. So the decision has been given to users to choose the implementation they like. The recommended approach is to use query strings library.
```javascript
const queryString = require("query-string");
const parsed = queryString.parse(props.location.search);
```
You can also use `URLSearchParams` if you want something native:
```javascript
const params = new URLSearchParams(props.location.search);
const foo = params.get("name");
```
You should use a _polyfill_ for IE11.
**[⬆ Back to Top](#table-of-contents)**
85. ### Why you get "Router may have only one child element" warning?
You have to wrap your Route's in a `<Switch>` block because `<Switch>` is unique in that it renders a route exclusively.
At first you need to add `Switch` to your imports:
```javascript
import { Switch, Router, Route } from "react-router";
```
Then define the routes within `<Switch>` block:
```jsx harmony
<Router>
<Switch>
<Route {/* ... */} />
<Route {/* ... */} />
</Switch>
</Router>
```
**[⬆ Back to Top](#table-of-contents)**
86. ### How to pass params to `history.push` method in React Router v4?
While navigating you can pass props to the `history` object:
```javascript
this.props.history.push({
pathname: "/template",
search: "?name=sudheer",
state: { detail: response.data },
});
```
The `search` property is used to pass query params in `push()` method.
**[⬆ Back to Top](#table-of-contents)**
87. ### How to implement _default_ or _NotFound_ page?
A `<Switch>` renders the first child `<Route>` that matches. A `<Route>` with no path always matches. So you just need to simply drop path attribute as below
```jsx harmony
<Switch>
<Route exact path="/" component={Home} />
<Route path="/user" component={User} />
<Route component={NotFound} />
</Switch>
```
**[⬆ Back to Top](#table-of-contents)**
88. ### How to get history on React Router v4?
Below are the list of steps to get history object on React Router v4,
1. Create a module that exports a `history` object and import this module across the project.
For example, create `history.js` file:
```javascript
import { createBrowserHistory } from "history";
export default createBrowserHistory({
/* pass a configuration object here if needed */
});
```
2. You should use the `<Router>` component instead of built-in routers. Import the above `history.js` inside `index.js` file:
```jsx harmony
import { Router } from "react-router-dom";
import history from "./history";
import App from "./App";
ReactDOM.render(
<Router history={history}>
<App />
</Router>,
holder
);
```
3. You can also use push method of `history` object similar to built-in history object:
```javascript
// some-other-file.js
import history from "./history";
history.push("/go-here");
```
**[⬆ Back to Top](#table-of-contents)**
89. ### How to perform automatic redirect after login?
The `react-router` package provides `<Redirect>` component in React Router. Rendering a `<Redirect>` will navigate to a new location. Like server-side redirects, the new location will override the current location in the history stack.
```javascript
import { Redirect } from "react-router";
export default function Login {
if (this.state.isLoggedIn === true) {
return <Redirect to="/your/redirect/page" />;
} else {
return <div>{"Login Please"}</div>;
}
}
```
<details><summary><b>See Class</b></summary>
<p>
```jsx
import React, { Component } from "react";
import { Redirect } from "react-router";
export default class LoginComponent extends Component {
render() {
if (this.state.isLoggedIn === true) {
return <Redirect to="/your/redirect/page" />;
} else {
return <div>{"Login Please"}</div>;
}
}
}
```
</p>
</details>
**[⬆ Back to Top](#table-of-contents)**
## React Internationalization
90. ### What is React Intl?
The _React Intl_ library makes internationalization in React straightforward, with off-the-shelf components and an API that can handle everything from formatting strings, dates, and numbers, to pluralization. React Intl is part of _FormatJS_ which provides bindings to React via its components and API.
**[⬆ Back to Top](#table-of-contents)**
91. ### What are the main features of React Intl?
Below are the main features of React Intl,
1. Display numbers with separators.
2. Display dates and times correctly.
3. Display dates relative to "now".
4. Pluralize labels in strings.
5. Support for 150+ languages.
6. Runs in the browser and Node.
7. Built on standards.
**[⬆ Back to Top](#table-of-contents)**
92. ### What are the two ways of formatting in React Intl?
The library provides two ways to format strings, numbers, and dates:
1. **Using react components:**
```jsx harmony
<FormattedMessage
id={"account"}
defaultMessage={"The amount is less than minimum balance."}
/>
```
2. **Using an API:**
```javascript
const messages = defineMessages({
accountMessage: {
id: "account",
defaultMessage: "The amount is less than minimum balance.",
},
});
formatMessage(messages.accountMessage);
```
**[⬆ Back to Top](#table-of-contents)**
93. ### How to use `<FormattedMessage>` as placeholder using React Intl?
The `<Formatted... />` components from `react-intl` return elements, not plain text, so they can't be used for placeholders, alt text, etc. In that case, you should use lower level API `formatMessage()`. You can inject the `intl` object into your component using `injectIntl()` higher-order component and then format the message using `formatMessage()` available on that object.
```jsx harmony
import React from "react";
import { injectIntl, intlShape } from "react-intl";
const MyComponent = ({ intl }) => {
const placeholder = intl.formatMessage({ id: "messageId" });
return <input placeholder={placeholder} />;
};
MyComponent.propTypes = {
intl: intlShape.isRequired,
};
export default injectIntl(MyComponent);
```
**[⬆ Back to Top](#table-of-contents)**
94. ### How to access current locale with React Intl?
You can get the current locale in any component of your application using `injectIntl()`:
```jsx harmony
import { injectIntl, intlShape } from "react-intl";
const MyComponent = ({ intl }) => (
<div>{`The current locale is ${intl.locale}`}</div>
);
MyComponent.propTypes = {
intl: intlShape.isRequired,
};
export default injectIntl(MyComponent);
```
**[⬆ Back to Top](#table-of-contents)**
95. ### How to format date using React Intl?
The `injectIntl()` higher-order component will give you access to the `formatDate()` method via the props in your component. The method is used internally by instances of `FormattedDate` and it returns the string representation of the formatted date.
```jsx harmony
import { injectIntl, intlShape } from "react-intl";
const stringDate = this.props.intl.formatDate(date, {
year: "numeric",
month: "numeric",
day: "numeric",
});
const MyComponent = ({ intl }) => (
<div>{`The formatted date is ${stringDate}`}</div>
);
MyComponent.propTypes = {
intl: intlShape.isRequired,
};
export default injectIntl(MyComponent);
```
**[⬆ Back to Top](#table-of-contents)**
## React Testing
96. ### What is Shallow Renderer in React testing?
_Shallow rendering_ is useful for writing unit test cases in React. It lets you render a component _one level deep_ and assert facts about what its render method returns, without worrying about the behavior of child components, which are not instantiated or rendered.
For example, if you have the following component:
```javascript
function MyComponent() {
return (
<div>
<span className={"heading"}>{"Title"}</span>
<span className={"description"}>{"Description"}</span>
</div>
);
}
```
Then you can assert as follows:
```jsx harmony
import ShallowRenderer from "react-test-renderer/shallow";
// in your test
const renderer = new ShallowRenderer();
renderer.render(<MyComponent />);
const result = renderer.getRenderOutput();
expect(result.type).toBe("div");
expect(result.props.children).toEqual([
<span className={"heading"}>{"Title"}</span>,
<span className={"description"}>{"Description"}</span>,
]);
```
**[⬆ Back to Top](#table-of-contents)**
97. ### What is `TestRenderer` package in React?
This package provides a renderer that can be used to render components to pure JavaScript objects, without depending on the DOM or a native mobile environment. This package makes it easy to grab a snapshot of the platform view hierarchy (similar to a DOM tree) rendered by a ReactDOM or React Native without using a browser or `jsdom`.
```jsx harmony
import TestRenderer from "react-test-renderer";
const Link = ({ page, children }) => <a href={page}>{children}</a>;
const testRenderer = TestRenderer.create(
<Link page={"https://www.facebook.com/"}>{"Facebook"}</Link>
);
console.log(testRenderer.toJSON());
// {
// type: 'a',
// props: { href: 'https://www.facebook.com/' },
// children: [ 'Facebook' ]
// }
```
**[⬆ Back to Top](#table-of-contents)**
98. ### What is the purpose of ReactTestUtils package?
_ReactTestUtils_ are provided in the `with-addons` package and allow you to perform actions against a simulated DOM for the purpose of unit testing.
**[⬆ Back to Top](#table-of-contents)**
99. ### What is Jest?
_Jest_ is a JavaScript unit testing framework created by Facebook based on Jasmine and provides automated mock creation and a `jsdom` environment. It's often used for testing components.
**[⬆ Back to Top](#table-of-contents)**
100. ### What are the advantages of Jest over Jasmine?
There are couple of advantages compared to Jasmine:
- Automatically finds tests to execute in your source code.
- Automatically mocks dependencies when running your tests.
- Allows you to test asynchronous code synchronously.
- Runs your tests with a fake DOM implementation (via `jsdom`) so that your tests can be run on the command line.
- Runs tests in parallel processes so that they finish sooner.
**[⬆ Back to Top](#table-of-contents)**
101. ### Give a simple example of Jest test case
Let's write a test for a function that adds two numbers in `sum.js` file:
```javascript
const sum = (a, b) => a + b;
export default sum;
```
Create a file named `sum.test.js` which contains actual test:
```javascript
import sum from "./sum";
test("adds 1 + 2 to equal 3", () => {
expect(sum(1, 2)).toBe(3);
});
```
And then add the following section to your `package.json`:
```json
{
"scripts": {
"test": "jest"
}
}
```
Finally, run `yarn test` or `npm test` and Jest will print a result:
```console
$ yarn test
PASS ./sum.test.js
✓ adds 1 + 2 to equal 3 (2ms)
```
## React Redux
**[⬆ Back to Top](#table-of-contents)**
102. ### What is flux?
_Flux_ is an _application design paradigm_ used as a replacement for the more traditional MVC pattern. It is not a framework or a library but a new kind of architecture that complements React and the concept of Unidirectional Data Flow. Facebook uses this pattern internally when working with React.
The workflow between dispatcher, stores and views components with distinct inputs and outputs as follows:
![flux](images/flux.png)
**[⬆ Back to Top](#table-of-contents)**
103. ### What is Redux?
_Redux_ is a predictable state container for JavaScript apps based on the _Flux design pattern_. Redux can be used together with React, or with any other view library. It is tiny (about 2kB) and has no dependencies.
**[⬆ Back to Top](#table-of-contents)**
104. ### What are the core principles of Redux?
Redux follows three fundamental principles:
1. **Single source of truth:** The state of your whole application is stored in an object tree within a single store. The single state tree makes it easier to keep track of changes over time and debug or inspect the application.
2. **State is read-only:** The only way to change the state is to emit an action, an object describing what happened. This ensures that neither the views nor the network callbacks will ever write directly to the state.
3. **Changes are made with pure functions:** To specify how the state tree is transformed by actions, you write reducers. Reducers are just pure functions that take the previous state and an action as parameters, and return the next state.
**[⬆ Back to Top](#table-of-contents)**
105. ### What are the downsides of Redux compared to Flux?
Instead of saying downsides we can say that there are few compromises of using Redux over Flux. Those are as follows:
1. **You will need to learn to avoid mutations:** Flux is un-opinionated about mutating data, but Redux doesn't like mutations and many packages complementary to Redux assume you never mutate the state. You can enforce this with dev-only packages like `redux-immutable-state-invariant`, Immutable.js, or instructing your team to write non-mutating code.
2. **You're going to have to carefully pick your packages:** While Flux explicitly doesn't try to solve problems such as undo/redo, persistence, or forms, Redux has extension points such as middleware and store enhancers, and it has spawned a rich ecosystem.
3. **There is no nice Flow integration yet:** Flux currently lets you do very impressive static type checks which Redux doesn't support yet.
**[⬆ Back to Top](#table-of-contents)**
106. ### What is the difference between `mapStateToProps()` and `mapDispatchToProps()`?
`mapStateToProps()` is a utility which helps your component get updated state (which is updated by some other components):
```javascript
const mapStateToProps = (state) => {
return {
todos: getVisibleTodos(state.todos, state.visibilityFilter),
};
};
```
`mapDispatchToProps()` is a utility which will help your component to fire an action event (dispatching action which may cause change of application state):
```javascript
const mapDispatchToProps = (dispatch) => {
return {
onTodoClick: (id) => {
dispatch(toggleTodo(id));
},
};
};
```
It is recommended to always use the “object shorthand” form for the `mapDispatchToProps`.
Redux wraps it in another function that looks like (…args) => dispatch(onTodoClick(…args)), and pass that wrapper function as a prop to your component.
```javascript
const mapDispatchToProps = {
onTodoClick,
};
```
**[⬆ Back to Top](#table-of-contents)**
107. ### Can I dispatch an action in reducer?
Dispatching an action within a reducer is an **anti-pattern**. Your reducer should be _without side effects_, simply digesting the action payload and returning a new state object. Adding listeners and dispatching actions within the reducer can lead to chained actions and other side effects.
**[⬆ Back to Top](#table-of-contents)**
108. ### How to access Redux store outside a component?
You just need to export the store from the module where it created with `createStore()`. Also, it shouldn't pollute the global window object.
```javascript
store = createStore(myReducer);
export default store;
```
**[⬆ Back to Top](#table-of-contents)**
109. ### What are the drawbacks of MVW pattern?
1. DOM manipulation is very expensive which causes applications to behave slow and inefficient.
2. Due to circular dependencies, a complicated model was created around models and views.
3. Lot of data changes happens for collaborative applications(like Google Docs).
4. No way to do undo (travel back in time) easily without adding so much extra code.
**[⬆ Back to Top](#table-of-contents)**
110. ### Are there any similarities between Redux and RxJS?
These libraries are very different for very different purposes, but there are some vague similarities.
Redux is a tool for managing state throughout the application. It is usually used as an architecture for UIs. Think of it as an alternative to (half of) Angular. RxJS is a reactive programming library. It is usually used as a tool to accomplish asynchronous tasks in JavaScript. Think of it as an alternative to Promises. Redux uses the Reactive paradigm because the Store is reactive. The Store observes actions from a distance, and changes itself. RxJS also uses the Reactive paradigm, but instead of being an architecture, it gives you basic building blocks, Observables, to accomplish this pattern.
**[⬆ Back to Top](#table-of-contents)**
111. ### How to reset state in Redux?
You need to write a _root reducer_ in your application which delegate handling the action to the reducer generated by `combineReducers()`.
For example, let us take `rootReducer()` to return the initial state after `USER_LOGOUT` action. As we know, reducers are supposed to return the initial state when they are called with `undefined` as the first argument, no matter the action.
```javascript
const appReducer = combineReducers({
/* your app's top-level reducers */
});
const rootReducer = (state, action) => {
if (action.type === "USER_LOGOUT") {
state = undefined;
}
return appReducer(state, action);
};
```
In case of using `redux-persist`, you may also need to clean your storage. `redux-persist` keeps a copy of your state in a storage engine. First, you need to import the appropriate storage engine and then, to parse the state before setting it to undefined and clean each storage state key.
```javascript
const appReducer = combineReducers({
/* your app's top-level reducers */
});
const rootReducer = (state, action) => {
if (action.type === "USER_LOGOUT") {
Object.keys(state).forEach((key) => {
storage.removeItem(`persist:${key}`);
});
state = undefined;
}
return appReducer(state, action);
};
```
**[⬆ Back to Top](#table-of-contents)**
112. ### What is the difference between React context and React Redux?
You can use **Context** in your application directly and is going to be great for passing down data to deeply nested components which what it was designed for.
Whereas **Redux** is much more powerful and provides a large number of features that the Context API doesn't provide. Also, React Redux uses context internally but it doesn't expose this fact in the public API.
**[⬆ Back to Top](#table-of-contents)**
113. ### Why are Redux state functions called reducers?
Reducers always return the accumulation of the state (based on all previous and current actions). Therefore, they act as a reducer of state. Each time a Redux reducer is called, the state and action are passed as parameters. This state is then reduced (or accumulated) based on the action, and then the next state is returned. You could _reduce_ a collection of actions and an initial state (of the store) on which to perform these actions to get the resulting final state.
**[⬆ Back to Top](#table-of-contents)**
114. ### How to make AJAX request in Redux?
You can use `redux-thunk` middleware which allows you to define async actions.
Let's take an example of fetching specific account as an AJAX call using _fetch API_:
```javascript
export function fetchAccount(id) {
return (dispatch) => {
dispatch(setLoadingAccountState()); // Show a loading spinner
fetch(`/account/${id}`, (response) => {
dispatch(doneFetchingAccount()); // Hide loading spinner
if (response.status === 200) {
dispatch(setAccount(response.json)); // Use a normal function to set the received state
} else {
dispatch(someError);
}
});
};
}
function setAccount(data) {
return { type: "SET_Account", data: data };
}
```
**[⬆ Back to Top](#table-of-contents)**
115. ### Should I keep all component's state in Redux store?
Keep your data in the Redux store, and the UI related state internally in the component.
**[⬆ Back to Top](#table-of-contents)**
116. ### What is the proper way to access Redux store?
The best way to access your store in a component is to use the `connect()` function, that creates a new component that wraps around your existing one. This pattern is called _Higher-Order Components_, and is generally the preferred way of extending a component's functionality in React. This allows you to map state and action creators to your component, and have them passed in automatically as your store updates.
Let's take an example of `<FilterLink>` component using connect:
```javascript
import { connect } from "react-redux";
import { setVisibilityFilter } from "../actions";
import Link from "../components/Link";
const mapStateToProps = (state, ownProps) => ({
active: ownProps.filter === state.visibilityFilter,
});
const mapDispatchToProps = (dispatch, ownProps) => ({
onClick: () => dispatch(setVisibilityFilter(ownProps.filter)),
});
const FilterLink = connect(mapStateToProps, mapDispatchToProps)(Link);
export default FilterLink;
```
Due to it having quite a few performance optimizations and generally being less likely to cause bugs, the Redux developers almost always recommend using `connect()` over accessing the store directly (using context API).
```javascript
function MyComponent {
someMethod() {
doSomethingWith(this.context.store);
}
}
```
**[⬆ Back to Top](#table-of-contents)**
117. ### What is the difference between component and container in React Redux?
**Component** is a class or function component that describes the presentational part of your application.
**Container** is an informal term for a component that is connected to a Redux store. Containers _subscribe_ to Redux state updates and _dispatch_ actions, and they usually don't render DOM elements; they delegate rendering to presentational child components.
**[⬆ Back to Top](#table-of-contents)**
118. ### What is the purpose of the constants in Redux?
Constants allows you to easily find all usages of that specific functionality across the project when you use an IDE. It also prevents you from introducing silly bugs caused by typos – in which case, you will get a `ReferenceError` immediately.
Normally we will save them in a single file (`constants.js` or `actionTypes.js`).
```javascript
export const ADD_TODO = "ADD_TODO";
export const DELETE_TODO = "DELETE_TODO";
export const EDIT_TODO = "EDIT_TODO";
export const COMPLETE_TODO = "COMPLETE_TODO";
export const COMPLETE_ALL = "COMPLETE_ALL";
export const CLEAR_COMPLETED = "CLEAR_COMPLETED";
```
In Redux, you use them in two places:
1. **During action creation:**
Let's take `actions.js`:
```javascript
import { ADD_TODO } from "./actionTypes";
export function addTodo(text) {
return { type: ADD_TODO, text };
}
```
2. **In reducers:**
Let's create `reducer.js`:
```javascript
import { ADD_TODO } from "./actionTypes";
export default (state = [], action) => {
switch (action.type) {
case ADD_TODO:
return [
...state,
{
text: action.text,
completed: false,
},
];
default:
return state;
}
};
```
**[⬆ Back to Top](#table-of-contents)**
119. ### What are the different ways to write `mapDispatchToProps()`?
There are a few ways of binding _action creators_ to `dispatch()` in `mapDispatchToProps()`.
Below are the possible options:
```javascript
const mapDispatchToProps = (dispatch) => ({
action: () => dispatch(action()),
});
```
```javascript
const mapDispatchToProps = (dispatch) => ({
action: bindActionCreators(action, dispatch),
});
```
```javascript
const mapDispatchToProps = { action };
```
The third option is just a shorthand for the first one.
**[⬆ Back to Top](#table-of-contents)**
120. ### What is the use of the `ownProps` parameter in `mapStateToProps()` and `mapDispatchToProps()`?
If the `ownProps` parameter is specified, React Redux will pass the props that were passed to the component into your _connect_ functions. So, if you use a connected component:
```jsx harmony
import ConnectedComponent from "./containers/ConnectedComponent";
<ConnectedComponent user={"john"} />;
```
The `ownProps` inside your `mapStateToProps()` and `mapDispatchToProps()` functions will be an object:
```javascript
{
user: "john";
}
```
You can use this object to decide what to return from those functions.
**[⬆ Back to Top](#table-of-contents)**
121. ### How to structure Redux top level directories?
Most of the applications has several top-level directories as below:
1. **Components**: Used for _dumb_ components unaware of Redux.
2. **Containers**: Used for _smart_ components connected to Redux.
3. **Actions**: Used for all action creators, where file names correspond to part of the app.
4. **Reducers**: Used for all reducers, where files name correspond to state key.
5. **Store**: Used for store initialization.
This structure works well for small and medium size apps.
**[⬆ Back to Top](#table-of-contents)**
122. ### What is redux-saga?
`redux-saga` is a library that aims to make side effects (asynchronous things like data fetching and impure things like accessing the browser cache) in React/Redux applications easier and better.
It is available in NPM:
```console
$ npm install --save redux-saga
```
**[⬆ Back to Top](#table-of-contents)**
123. ### What is the mental model of redux-saga?
_Saga_ is like a separate thread in your application, that's solely responsible for side effects. `redux-saga` is a redux _middleware_, which means this thread can be started, paused and cancelled from the main application with normal Redux actions, it has access to the full Redux application state and it can dispatch Redux actions as well.
**[⬆ Back to Top](#table-of-contents)**
124. ### What are the differences between `call()` and `put()` in redux-saga?
Both `call()` and `put()` are effect creator functions. `call()` function is used to create effect description, which instructs middleware to call the promise. `put()` function creates an effect, which instructs middleware to dispatch an action to the store.
Let's take example of how these effects work for fetching particular user data.
```javascript
function* fetchUserSaga(action) {
// `call` function accepts rest arguments, which will be passed to `api.fetchUser` function.
// Instructing middleware to call promise, it resolved value will be assigned to `userData` variable
const userData = yield call(api.fetchUser, action.userId);
// Instructing middleware to dispatch corresponding action.
yield put({
type: "FETCH_USER_SUCCESS",
userData,
});
}
```
**[⬆ Back to Top](#table-of-contents)**
125. ### What is Redux Thunk?
_Redux Thunk_ middleware allows you to write action creators that return a function instead of an action. The thunk can be used to delay the dispatch of an action, or to dispatch only if a certain condition is met. The inner function receives the store methods `dispatch()` and `getState()` as parameters.
**[⬆ Back to Top](#table-of-contents)**
126. ### What are the differences between `redux-saga` and `redux-thunk`?
Both _Redux Thunk_ and _Redux Saga_ take care of dealing with side effects. In most of the scenarios, Thunk uses _Promises_ to deal with them, whereas Saga uses _Generators_. Thunk is simple to use and Promises are familiar to many developers, Sagas/Generators are more powerful but you will need to learn them. But both middleware can coexist, so you can start with Thunks and introduce Sagas when/if you need them.
**[⬆ Back to Top](#table-of-contents)**
127. ### What is Redux DevTools?
_Redux DevTools_ is a live-editing time travel environment for Redux with hot reloading, action replay, and customizable UI. If you don't want to bother with installing Redux DevTools and integrating it into your project, consider using Redux DevTools Extension for Chrome and Firefox.
**[⬆ Back to Top](#table-of-contents)**
128. ### What are the features of Redux DevTools?
Some of the main features of Redux DevTools are below,
1. Lets you inspect every state and action payload.
2. Lets you go back in time by _cancelling_ actions.
3. If you change the reducer code, each _staged_ action will be re-evaluated.
4. If the reducers throw, you will see during which action this happened, and what the error was.
5. With `persistState()` store enhancer, you can persist debug sessions across page reloads.
**[⬆ Back to Top](#table-of-contents)**
129. ### What are Redux selectors and why use them?
_Selectors_ are functions that take Redux state as an argument and return some data to pass to the component.
For example, to get user details from the state:
```javascript
const getUserData = (state) => state.user.data;
```
These selectors have two main benefits,
1. The selector can compute derived data, allowing Redux to store the minimal possible state
2. The selector is not recomputed unless one of its arguments changes
**[⬆ Back to Top](#table-of-contents)**
130. ### What is Redux Form?
_Redux Form_ works with React and Redux to enable a form in React to use Redux to store all of its state. Redux Form can be used with raw HTML5 inputs, but it also works very well with common UI frameworks like Material UI, React Widgets and React Bootstrap.
**[⬆ Back to Top](#table-of-contents)**
131. ### What are the main features of Redux Form?
Some of the main features of Redux Form are:
1. Field values persistence via Redux store.
2. Validation (sync/async) and submission.
3. Formatting, parsing and normalization of field values.
**[⬆ Back to Top](#table-of-contents)**
132. ### How to add multiple middlewares to Redux?
You can use `applyMiddleware()`.
For example, you can add `redux-thunk` and `logger` passing them as arguments to `applyMiddleware()`:
```javascript
import { createStore, applyMiddleware } from "redux";
const createStoreWithMiddleware = applyMiddleware(
ReduxThunk,
logger
)(createStore);
```
**[⬆ Back to Top](#table-of-contents)**
133. ### How to set initial state in Redux?
You need to pass initial state as second argument to createStore:
```javascript
const rootReducer = combineReducers({
todos: todos,
visibilityFilter: visibilityFilter,
});
const initialState = {
todos: [{ id: 123, name: "example", completed: false }],
};
const store = createStore(rootReducer, initialState);
```
**[⬆ Back to Top](#table-of-contents)**
134. ### How Relay is different from Redux?
Relay is similar to Redux in that they both use a single store. The main difference is that relay only manages state originated from the server, and all access to the state is used via _GraphQL_ queries (for reading data) and mutations (for changing data). Relay caches the data for you and optimizes data fetching for you, by fetching only changed data and nothing more.
135. ### What is an action in Redux?
_Actions_ are plain JavaScript objects or payloads of information that send data from your application to your store. They are the only source of information for the store. Actions must have a type property that indicates the type of action being performed.
For example, let's take an action which represents adding a new todo item:
```
{
type: ADD_TODO,
text: 'Add todo item'
}
```
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## React Native
**[⬆ Back to Top](#table-of-contents)**
136. ### What is the difference between React Native and React?
**React** is a JavaScript library, supporting both front end web and being run on the server, for building user interfaces and web applications.
**React Native** is a mobile framework that compiles to native app components, allowing you to build native mobile applications (iOS, Android, and Windows) in JavaScript that allows you to use React to build your components, and implements React under the hood.
**[⬆ Back to Top](#table-of-contents)**
137. ### How to test React Native apps?
React Native can be tested only in mobile simulators like iOS and Android. You can run the app in your mobile using expo app (https://expo.io) Where it syncs using QR code, your mobile and computer should be in same wireless network.
**[⬆ Back to Top](#table-of-contents)**
138. ### How to do logging in React Native?
You can use `console.log`, `console.warn`, etc. As of React Native v0.29 you can simply run the following to see logs in the console:
```
$ react-native log-ios
$ react-native log-android
```
**[⬆ Back to Top](#table-of-contents)**
139. ### How to debug your React Native?
Follow the below steps to debug React Native app:
1. Run your application in the iOS simulator.
2. Press `Command + D` and a webpage should open up at `http://localhost:8081/debugger-ui`.
3. Enable _Pause On Caught Exceptions_ for a better debugging experience.
4. Press `Command + Option + I` to open the Chrome Developer tools, or open it via `View` -> `Developer` -> `Developer Tools`.
5. You should now be able to debug as you normally would.
## React supported libraries & Integration
**[⬆ Back to Top](#table-of-contents)**
140. ### What is reselect and how it works?
_Reselect_ is a **selector library** (for Redux) which uses _memoization_ concept. It was originally written to compute derived data from Redux-like applications state, but it can't be tied to any architecture or library.
Reselect keeps a copy of the last inputs/outputs of the last call, and recomputes the result only if one of the inputs changes. If the same inputs are provided twice in a row, Reselect returns the cached output. It's memoization and cache are fully customizable.
**[⬆ Back to Top](#table-of-contents)**
141. ### What is Flow?
_Flow_ is a _static type checker_ designed to find type errors in JavaScript. Flow types can express much more fine-grained distinctions than traditional type systems. For example, Flow helps you catch errors involving `null`, unlike most type systems.
**[⬆ Back to Top](#table-of-contents)**
142. ### What is the difference between Flow and PropTypes?
Flow is a _static analysis tool_ (static checker) which uses a superset of the language, allowing you to add type annotations to all of your code and catch an entire class of bugs at compile time.
PropTypes is a _basic type checker_ (runtime checker) which has been patched onto React. It can't check anything other than the types of the props being passed to a given component. If you want more flexible typechecking for your entire project Flow/TypeScript are appropriate choices.
**[⬆ Back to Top](#table-of-contents)**
143. ### How to use Font Awesome icons in React?
The below steps followed to include Font Awesome in React:
1. Install `font-awesome`:
```console
$ npm install --save font-awesome
```
2. Import `font-awesome` in your `index.js` file:
```javascript
import "font-awesome/css/font-awesome.min.css";
```
3. Add Font Awesome classes in `className`:
```javascript
function MyComponent {
return <div><i className={'fa fa-spinner'} /></div>
}
```
**[⬆ Back to Top](#table-of-contents)**
144. ### What is React Dev Tools?
_React Developer Tools_ let you inspect the component hierarchy, including component props and state. It exists both as a browser extension (for Chrome and Firefox), and as a standalone app (works with other environments including Safari, IE, and React Native).
The official extensions available for different browsers or environments.
1. **Chrome extension**
2. **Firefox extension**
3. **Standalone app** (Safari, React Native, etc)
**[⬆ Back to Top](#table-of-contents)**
145. ### Why is DevTools not loading in Chrome for local files?
If you opened a local HTML file in your browser (`file://...`) then you must first open _Chrome Extensions_ and check `Allow access to file URLs`.
**[⬆ Back to Top](#table-of-contents)**
146. ### How to use Polymer in React?
You need to follow below steps to use Polymer in React,
1. Create a Polymer element:
```jsx harmony
<link
rel="import"
href="../../bower_components/polymer/polymer.html"
/>;
Polymer({
is: "calendar-element",
ready: function () {
this.textContent = "I am a calendar";
},
});
```
2. Create the Polymer component HTML tag by importing it in a HTML document, e.g. import it in the `index.html` of your React application:
```html
<link
rel="import"
href="./src/polymer-components/calendar-element.html"
/>
```
3. Use that element in the JSX file:
```javascript
export default function MyComponent {
return <calendar-element />;
}
```
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147. ### What are the advantages of React over Vue.js?
React has the following advantages over Vue.js:
1. Gives more flexibility in large apps developing.
2. Easier to test.
3. Suitable for mobile apps creating.
4. More information and solutions available.
**Note:** The above list of advantages are purely opinionated and it vary based on the professional experience. But they are helpful as base parameters.
**[⬆ Back to Top](#table-of-contents)**
148. ### What is the difference between React and Angular?
Let's see the difference between React and Angular in a table format.
| React | Angular |
| ------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------- |
| React is a library and has only the View layer | Angular is a framework and has complete MVC functionality |
| React handles rendering on the server side | AngularJS renders only on the client side but Angular 2 and above renders on the server side |
| React uses JSX that looks like HTML in JS which can be confusing | Angular follows the template approach for HTML, which makes code shorter and easy to understand |
| React Native, which is a React type to build mobile applications are faster and more stable | Ionic, Angular's mobile native app is relatively less stable and slower |
| In React, data flows only in one way and hence debugging is easy | In Angular, data flows both way i.e it has two-way data binding between children and parent and hence debugging is often difficult |
**Note:** The above list of differences are purely opinionated and it vary based on the professional experience. But they are helpful as base parameters.
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149. ### Why React tab is not showing up in DevTools?
When the page loads, _React DevTools_ sets a global named `__REACT_DEVTOOLS_GLOBAL_HOOK__`, then React communicates with that hook during initialization. If the website is not using React or if React fails to communicate with DevTools then it won't show up the tab.
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150. ### What are Styled Components?
`styled-components` is a JavaScript library for styling React applications. It removes the mapping between styles and components, and lets you write actual CSS augmented with JavaScript.
**[⬆ Back to Top](#table-of-contents)**
151. ### Give an example of Styled Components?
Lets create `<Title>` and `<Wrapper>` components with specific styles for each.
```javascript
import React from "react";
import styled from "styled-components";
// Create a <Title> component that renders an <h1> which is centered, red and sized at 1.5em
const Title = styled.h1`
font-size: 1.5em;
text-align: center;
color: palevioletred;
`;
// Create a <Wrapper> component that renders a <section> with some padding and a papayawhip background
const Wrapper = styled.section`
padding: 4em;
background: papayawhip;
`;
```
These two variables, `Title` and `Wrapper`, are now components that you can render just like any other react component.
```jsx harmony
<Wrapper>
<Title>{"Lets start first styled component!"}</Title>
</Wrapper>
```
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152. ### What is Relay?
Relay is a JavaScript framework for providing a data layer and client-server communication to web applications using the React view layer.
**[⬆ Back to Top](#table-of-contents)**
## Miscellaneous
153. ### What are the main features of Reselect library?
Let's see the main features of Reselect library,
1. Selectors can compute derived data, allowing Redux to store the minimal possible state.
2. Selectors are efficient. A selector is not recomputed unless one of its arguments changes.
3. Selectors are composable. They can be used as input to other selectors.
154. #### Give an example of Reselect usage?
Let's take calculations and different amounts of a shipment order with the simplified usage of Reselect:
```javascript
import { createSelector } from "reselect";
const shopItemsSelector = (state) => state.shop.items;
const taxPercentSelector = (state) => state.shop.taxPercent;
const subtotalSelector = createSelector(shopItemsSelector, (items) =>
items.reduce((acc, item) => acc + item.value, 0)
);
const taxSelector = createSelector(
subtotalSelector,
taxPercentSelector,
(subtotal, taxPercent) => subtotal * (taxPercent / 100)
);
export const totalSelector = createSelector(
subtotalSelector,
taxSelector,
(subtotal, tax) => ({ total: subtotal + tax })
);
let exampleState = {
shop: {
taxPercent: 8,
items: [
{ name: "apple", value: 1.2 },
{ name: "orange", value: 0.95 },
],
},
};
console.log(subtotalSelector(exampleState)); // 2.15
console.log(taxSelector(exampleState)); // 0.172
console.log(totalSelector(exampleState)); // { total: 2.322 }
```
**[⬆ Back to Top](#table-of-contents)**
155. ### Can Redux only be used with React?
Redux can be used as a data store for any UI layer. The most common usage is with React and React Native, but there are bindings available for Angular, Angular 2, Vue, Mithril, and more. Redux simply provides a subscription mechanism which can be used by any other code.
**[⬆ Back to Top](#table-of-contents)**
156. ### Do you need to have a particular build tool to use Redux?
Redux is originally written in ES6 and transpiled for production into ES5 with Webpack and Babel. You should be able to use it regardless of your JavaScript build process. Redux also offers a UMD build that can be used directly without any build process at all.
**[⬆ Back to Top](#table-of-contents)**
157. ### How Redux Form `initialValues` get updated from state?
You need to add `enableReinitialize : true` setting.
```javascript
const InitializeFromStateForm = reduxForm({
form: "initializeFromState",
enableReinitialize: true,
})(UserEdit);
```
If your `initialValues` prop gets updated, your form will update too.
**[⬆ Back to Top](#table-of-contents)**
158. ### How React PropTypes allow different types for one prop?
You can use `oneOfType()` method of `PropTypes`.
For example, the height property can be defined with either `string` or `number` type as below:
```javascript
Component.propTypes = {
size: PropTypes.oneOfType([PropTypes.string, PropTypes.number]),
};
```
**[⬆ Back to Top](#table-of-contents)**
159. ### Can I import an SVG file as react component?
You can import SVG directly as component instead of loading it as a file. This feature is available with `
[email protected]` and higher.
```jsx harmony
import { ReactComponent as Logo } from "./logo.svg";
const App = () => (
<div>
{/* Logo is an actual react component */}
<Logo />
</div>
);
```
**Note**: Don't forget about the curly braces in the import.
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160. ### What is render hijacking in react?
The concept of render hijacking is the ability to control what a component will output from another component. It means that you decorate your component by wrapping it into a Higher-Order component. By wrapping, you can inject additional props or make other changes, which can cause changing logic of rendering. It does not actually enable hijacking, but by using HOC you make your component behave differently.
**[⬆ Back to Top](#table-of-contents)**
161. ### How to pass numbers to React component?
We can pass `numbers` as `props` to React component using curly braces `{}` where as `strings` in double quotes `""` or single quotes `''`
```jsx
import React from "react";
const ChildComponent = ({ name, age }) => {
return (
<>
My Name is {name} and Age is {age}
</>
);
};
const ParentComponent = () => {
return (
<>
<ChildComponent name="Chetan" age={30} />
</>
);
};
export default ParentComponent;
```
**[⬆ Back to Top](#table-of-contents)**
162. ### Do I need to keep all my state into Redux? Should I ever use react internal state?
It is up to the developer's decision, i.e., it is developer's job to determine what kinds of state make up your application, and where each piece of state should live. Some users prefer to keep every single piece of data in Redux, to maintain a fully serializable and controlled version of their application at all times. Others prefer to keep non-critical or UI state, such as “is this dropdown currently open”, inside a component's internal state.
Below are the rules of thumb to determine what kind of data should be put into Redux
1. Do other parts of the application care about this data?
2. Do you need to be able to create further derived data based on this original data?
3. Is the same data being used to drive multiple components?
4. Is there value to you in being able to restore this state to a given point in time (ie, time travel debugging)?
5. Do you want to cache the data (i.e, use what's in state if it's already there instead of re-requesting it)?
**[⬆ Back to Top](#table-of-contents)**
163. ### What is the purpose of registerServiceWorker in React?
React creates a service worker for you without any configuration by default. The service worker is a web API that helps you cache your assets and other files so that when the user is offline or on a slow network, he/she can still see results on the screen, as such, it helps you build a better user experience, that's what you should know about service worker for now. It's all about adding offline capabilities to your site.
```jsx
import React from "react";
import ReactDOM from "react-dom";
import App from "./App";
import registerServiceWorker from "./registerServiceWorker";
ReactDOM.render(<App />, document.getElementById("root"));
registerServiceWorker();
```
**[⬆ Back to Top](#table-of-contents)**
164. ### What is React memo function?
Class components can be restricted from re-rendering when their input props are the same using **PureComponent or shouldComponentUpdate**. Now you can do the same with function components by wrapping them in **React.memo**.
```jsx
const MyComponent = React.memo(function MyComponent(props) {
/* only rerenders if props change */
});
```
**[⬆ Back to Top](#table-of-contents)**
165. ### What is React lazy function?
The `React.lazy` function lets you render a dynamic import as a regular component. It will automatically load the bundle containing the `OtherComponent` when the component gets rendered. This must return a Promise which resolves to a module with a default export containing a React component.
```jsx
const OtherComponent = React.lazy(() => import("./OtherComponent"));
function MyComponent() {
return (
<div>
<OtherComponent />
</div>
);
}
```
**Note:**
`React.lazy` and `Suspense` is not yet available for server-side rendering. If you want to do code-splitting in a server rendered app, we still recommend React Loadable.
**[⬆ Back to Top](#table-of-contents)**
166. ### How to prevent unnecessary updates using setState?
You can compare the current value of the state with an existing state value and decide whether to rerender the page or not. If the values are the same then you need to return **null** to stop re-rendering otherwise return the latest state value.
For example, the user profile information is conditionally rendered as follows,
```jsx
getUserProfile = (user) => {
const latestAddress = user.address;
this.setState((state) => {
if (state.address === latestAddress) {
return null;
} else {
return { title: latestAddress };
}
});
};
```
**[⬆ Back to Top](#table-of-contents)**
167. ### How do you render Array, Strings and Numbers in React 16 Version?
**Arrays**: Unlike older releases, you don't need to make sure **render** method return a single element in React16. You are able to return multiple sibling elements without a wrapping element by returning an array.
For example, let us take the below list of developers,
```jsx
const ReactJSDevs = () => {
return [
<li key="1">John</li>,
<li key="2">Jackie</li>,
<li key="3">Jordan</li>,
];
};
```
You can also merge this array of items in another array component.
```jsx
const JSDevs = () => {
return (
<ul>
<li>Brad</li>
<li>Brodge</li>
<ReactJSDevs />
<li>Brandon</li>
</ul>
);
};
```
**Strings and Numbers:** You can also return string and number type from the render method.
```jsx
render() {
return 'Welcome to ReactJS questions';
}
// Number
render() {
return 2018;
}
```
**[⬆ Back to Top](#table-of-contents)**
168. ### What are hooks?
Hooks is a special JavaScript function that allows you use state and other React features without writing a class. This pattern has been introduced as a new feature in React 16.8 and helped to isolate the stateful logic from the components.
Let's see an example of useState hook:
```jsx
import { useState } from "react";
function Example() {
// Declare a new state variable, which we'll call "count"
const [count, setCount] = useState(0);
return (
<>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>Click me</button>
</>
);
}
```
**Note:** Hooks can be used inside an existing function component without rewriting the component.
**[⬆ Back to Top](#table-of-contents)**
169. ### What rules need to be followed for hooks?
You need to follow two rules in order to use hooks,
1. **Call Hooks only at the top level of your react functions:** You should always use hooks at the top level of react function before any early returns. i.e, You shouldn’t call Hooks inside loops, conditions, or nested functions. This will ensure that Hooks are called in the same order each time a component renders and it preserves the state of Hooks between multiple re-renders due to useState and useEffect calls.
2. **Call Hooks from React Functions only:** You shouldn’t call Hooks from regular JavaScript functions or class components. Instead, you should call them from either function components or custom hooks.
**[⬆ Back to Top](#table-of-contents)**
170. ### How to ensure hooks followed the rules in your project?
React team released an ESLint plugin called **eslint-plugin-react-hooks** that enforces Hook's two rules. It is part of Hooks API. You can add this plugin to your project using the below command,
```javascript
npm install eslint-plugin-react-hooks --save-dev
```
And apply the below config in your ESLint config file,
```javascript
// Your ESLint configuration
{
"plugins": [
// ...
"react-hooks"
],
"rules": {
// ...
"react-hooks/rules-of-hooks": "error"
}
}
```
The recommended `eslint-config-react-app` preset already includes the hooks rules of this plugin.
For example, the linter enforce proper naming convention for hooks. If you rename your custom hooks which as prefix "use" to something else then linter won't allow you to call built-in hooks such as useState, useEffect etc inside of your custom hook anymore.
**Note:** This plugin is intended to use in Create React App by default.
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171. ### What are the differences between Flux and Redux?
Below are the major differences between Flux and Redux
| Flux | Redux |
| ---------------------------------------------- | ------------------------------------------ |
| State is mutable | State is immutable |
| The Store contains both state and change logic | The Store and change logic are separate |
| There are multiple stores exist | There is only one store exist |
| All the stores are disconnected and flat | Single store with hierarchical reducers |
| It has a singleton dispatcher | There is no concept of dispatcher |
| React components subscribe to the store | Container components uses connect function |
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172. ### What are the benefits of React Router V4?
Below are the main benefits of React Router V4 module,
1. In React Router v4(version 4), the API is completely about components. A router can be visualized as a single component(`<BrowserRouter>`) which wraps specific child router components(`<Route>`).
2. You don't need to manually set history. The router module will take care history by wrapping routes with `<BrowserRouter>` component.
3. The application size is reduced by adding only the specific router module(Web, core, or native)
**[⬆ Back to Top](#table-of-contents)**
173. ### Can you describe about componentDidCatch lifecycle method signature?
The **componentDidCatch** lifecycle method is invoked after an error has been thrown by a descendant component. The method receives two parameters,
1. error: - The error object which was thrown
2. info: - An object with a componentStack key contains the information about which component threw the error.
The method structure would be as follows
```javascript
componentDidCatch(error, info);
```
**[⬆ Back to Top](#table-of-contents)**
174. ### In which scenarios do error boundaries not catch errors?
Below are the cases in which error boundaries don't work,
1. Inside Event handlers
2. Asynchronous code using **setTimeout or requestAnimationFrame** callbacks
3. During Server side rendering
4. When errors thrown in the error boundary code itself
**[⬆ Back to Top](#table-of-contents)**
175. ### What is the behavior of uncaught errors in react 16?
In React 16, errors that were not caught by any error boundary will result in unmounting of the whole React component tree. The reason behind this decision is that it is worse to leave corrupted UI in place than to completely remove it. For example, it is worse for a payments app to display a wrong amount than to render nothing.
**[⬆ Back to Top](#table-of-contents)**
176. ### What is the proper placement for error boundaries?
The granularity of error boundaries usage is up to the developer based on project needs. You can follow either of these approaches,
1. You can wrap top-level route components to display a generic error message for the entire application.
2. You can also wrap individual components in an error boundary to protect them from crashing the rest of the application.
**[⬆ Back to Top](#table-of-contents)**
177. ### What is the benefit of component stack trace from error boundary?
Apart from error messages and javascript stack, React16 will display the component stack trace with file names and line numbers using error boundary concept.
For example, BuggyCounter component displays the component stack trace as below,
![stacktrace](images/error_boundary.png)
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178. ### What are default props?
The _defaultProps_ can be defined as a property on the component to set the default values for the props. These default props are used when props not supplied(i.e., undefined props), but not for `null` or `0` as props. That means, If you provide null value then it remains null value. It's the same behavior with 0 as well.
For example, let us create color default prop for the button component,
```javascript
function MyButton {
// ...
}
MyButton.defaultProps = {
color: "red",
};
```
If `props.color` is not provided then it will set the default value to 'red'. i.e, Whenever you try to access the color prop it uses the default value
```javascript
function MyButton() {
return <MyButton />; // props.color will contain red value
}
```
**[⬆ Back to Top](#table-of-contents)**
179. ### What is the purpose of displayName class property?
The displayName string is used in debugging messages. Usually, you don’t need to set it explicitly because it’s inferred from the name of the function or class that defines the component. You might want to set it explicitly if you want to display a different name for debugging purposes or when you create a higher-order component.
For example, To ease debugging, choose a display name that communicates that it’s the result of a withSubscription HOC.
```javascript
function withSubscription(WrappedComponent) {
class WithSubscription extends React.Component {
/* ... */
}
WithSubscription.displayName = `WithSubscription(${getDisplayName(
WrappedComponent
)})`;
return WithSubscription;
}
function getDisplayName(WrappedComponent) {
return (
WrappedComponent.displayName || WrappedComponent.name || "Component"
);
}
```
**[⬆ Back to Top](#table-of-contents)**
180. ### What is the browser support for react applications?
React supports all popular browsers, including Internet Explorer 9 and above, although some polyfills are required for older browsers such as IE 9 and IE 10. If you use **es5-shim and es5-sham** polyfill then it even support old browsers that doesn't support ES5 methods.
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181. ### What is code-splitting?
Code-Splitting is a feature supported by bundlers like Webpack and Browserify which can create multiple bundles that can be dynamically loaded at runtime. The react project supports code splitting via dynamic import() feature.
For example, in the below code snippets, it will make moduleA.js and all its unique dependencies as a separate chunk that only loads after the user clicks the 'Load' button.
**moduleA.js**
```javascript
const moduleA = "Hello";
export { moduleA };
```
**App.js**
```javascript
export default function App {
function handleClick() {
import("./moduleA")
.then(({ moduleA }) => {
// Use moduleA
})
.catch((err) => {
// Handle failure
});
};
return (
<div>
<button onClick={this.handleClick}>Load</button>
</div>
);
}
```
<details><summary><b>See Class</b></summary>
<p>
```javascript
import React, { Component } from "react";
class App extends Component {
handleClick = () => {
import("./moduleA")
.then(({ moduleA }) => {
// Use moduleA
})
.catch((err) => {
// Handle failure
});
};
render() {
return (
<div>
<button onClick={this.handleClick}>Load</button>
</div>
);
}
}
export default App;
```
</p>
</details>
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182. ### What are Keyed Fragments?
The Fragments declared with the explicit <React.Fragment> syntax may have keys. The general use case is mapping a collection to an array of fragments as below,
```javascript
function Glossary(props) {
return (
<dl>
{props.items.map((item) => (
// Without the `key`, React will fire a key warning
<React.Fragment key={item.id}>
<dt>{item.term}</dt>
<dd>{item.description}</dd>
</React.Fragment>
))}
</dl>
);
}
```
**Note:** key is the only attribute that can be passed to Fragment. In the future, there might be a support for additional attributes, such as event handlers.
**[⬆ Back to Top](#table-of-contents)**
183. ### Does React support all HTML attributes?
As of React 16, both standard or custom DOM attributes are fully supported. Since React components often take both custom and DOM-related props, React uses the camelCase convention just like the DOM APIs.
Let us take few props with respect to standard HTML attributes,
```javascript
<div tabIndex="-1" /> // Just like node.tabIndex DOM API
<div className="Button" /> // Just like node.className DOM API
<input readOnly={true} /> // Just like node.readOnly DOM API
```
These props work similarly to the corresponding HTML attributes, with the exception of the special cases. It also support all SVG attributes.
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184. ### When component props defaults to true?
If you pass no value for a prop, it defaults to true. This behavior is available so that it matches the behavior of HTML.
For example, below expressions are equivalent,
```javascript
<MyInput autocomplete />
<MyInput autocomplete={true} />
```
**Note:** It is not recommended to use this approach because it can be confused with the ES6 object shorthand (example, `{name}` which is short for `{name: name}`)
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185. ### What is NextJS and major features of it?
Next.js is a popular and lightweight framework for static and server‑rendered applications built with React. It also provides styling and routing solutions. Below are the major features provided by NextJS,
1. Server-rendered by default
2. Automatic code splitting for faster page loads
3. Simple client-side routing (page based)
4. Webpack-based dev environment which supports (HMR)
5. Able to implement with Express or any other Node.js HTTP server
6. Customizable with your own Babel and Webpack configurations
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186. ### How do you pass an event handler to a component?
You can pass event handlers and other functions as props to child components. The functions can be passed to child component as below,
```jsx
function Button({ onClick }) {
return <button onClick={onClick}>Download</button>;
}
export default function downloadExcel() {
function handleClick() {
alert("Downloaded");
}
return <Button onClick={handleClick}></Button>;
}
```
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187. ### How to prevent a function from being called multiple times?
If you use an event handler such as **onClick or onScroll** and want to prevent the callback from being fired too quickly, then you can limit the rate at which callback is executed. This can be achieved in the below possible ways,
1. **Throttling:** Changes based on a time based frequency. For example, it can be used using \_.throttle lodash function
2. **Debouncing:** Publish changes after a period of inactivity. For example, it can be used using \_.debounce lodash function
3. **RequestAnimationFrame throttling:** Changes based on requestAnimationFrame. For example, it can be used using raf-schd lodash function
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188. ### How JSX prevents Injection Attacks?
React DOM escapes any values embedded in JSX before rendering them. Thus it ensures that you can never inject anything that’s not explicitly written in your application. Everything is converted to a string before being rendered.
For example, you can embed user input as below,
```javascript
const name = response.potentiallyMaliciousInput;
const element = <h1>{name}</h1>;
```
This way you can prevent XSS(Cross-site-scripting) attacks in the application.
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189. ### How do you update rendered elements?
You can update UI(represented by rendered element) by passing the newly created element to ReactDOM's render method.
For example, lets take a ticking clock example, where it updates the time by calling render method multiple times,
```javascript
function tick() {
const element = (
<div>
<h1>Hello, world!</h1>
<h2>It is {new Date().toLocaleTimeString()}.</h2>
</div>
);
ReactDOM.render(element, document.getElementById("root"));
}
setInterval(tick, 1000);
```
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190. ### How do you say that props are readonly?
When you declare a component as a function or a class, it must never modify its own props.
Let us take a below capital function,
```javascript
function capital(amount, interest) {
return amount + interest;
}
```
The above function is called “pure” because it does not attempt to change their inputs, and always return the same result for the same inputs. Hence, React has a single rule saying "All React components must act like pure functions with respect to their props."
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191. ### What are the conditions to safely use the index as a key?
There are three conditions to make sure, it is safe use the index as a key.
1. The list and items are static– they are not computed and do not change
2. The items in the list have no ids
3. The list is never reordered or filtered.
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192. ### Should keys be globally unique?
The keys used within arrays should be unique among their siblings but they don’t need to be globally unique. i.e, You can use the same keys with two different arrays.
For example, the below `Book` component uses two arrays with different arrays,
```javascript
function Book(props) {
const index = (
<ul>
{props.pages.map((page) => (
<li key={page.id}>{page.title}</li>
))}
</ul>
);
const content = props.pages.map((page) => (
<div key={page.id}>
<h3>{page.title}</h3>
<p>{page.content}</p>
<p>{page.pageNumber}</p>
</div>
));
return (
<div>
{index}
<hr />
{content}
</div>
);
}
```
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193. ### What is the popular choice for form handling?
`Formik` is a form library for react which provides solutions such as validation, keeping track of the visited fields, and handling form submission.
In detail, You can categorize them as follows,
1. Getting values in and out of form state
2. Validation and error messages
3. Handling form submission
It is used to create a scalable, performant, form helper with a minimal API to solve annoying stuff.
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194. ### What are the advantages of formik over redux form library?
Below are the main reasons to recommend formik over redux form library,
1. The form state is inherently short-term and local, so tracking it in Redux (or any kind of Flux library) is unnecessary.
2. Redux-Form calls your entire top-level Redux reducer multiple times ON EVERY SINGLE KEYSTROKE. This way it increases input latency for large apps.
3. Redux-Form is 22.5 kB minified gzipped whereas Formik is 12.7 kB
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195. ### Why are you not required to use inheritance?
In React, it is recommended to use composition over inheritance to reuse code between components. Both Props and composition give you all the flexibility you need to customize a component’s look and behavior explicitly and safely.
Whereas, If you want to reuse non-UI functionality between components, it is suggested to extract it into a separate JavaScript module. Later components import it and use that function, object, or class, without extending it.
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196. ### Can I use web components in react application?
Yes, you can use web components in a react application. Even though many developers won't use this combination, it may require especially if you are using third-party UI components that are written using Web Components.
For example, let us use `Vaadin` date picker web component as below,
```javascript
import "./App.css";
import "@vaadin/vaadin-date-picker";
export default function App() {
return (
<div className="App">
<vaadin-date-picker label="When were you born?"></vaadin-date-picker>
</div>
);
}
```
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197. ### What is dynamic import?
You can achieve code-splitting in your app using dynamic import.
Let's take an example of addition,
1. **Normal Import**
```javascript
import { add } from "./math";
console.log(add(10, 20));
```
2. **Dynamic Import**
```javascript
import("./math").then((math) => {
console.log(math.add(10, 20));
});
```
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198. ### What are loadable components?
With the release of React 18, React.lazy and Suspense are now available for server-side rendering. However, prior to React 18, it was recommended to use Loadable Components for code-splitting in a server-side rendered app because React.lazy and Suspense were not available for server-side rendering. Loadable Components lets you render a dynamic import as a regular component. For example, you can use Loadable Components to load the OtherComponent in a separate bundle like this:
```javascript
import loadable from "@loadable/component";
const OtherComponent = loadable(() => import("./OtherComponent"));
function MyComponent() {
return (
<div>
<OtherComponent />
</div>
);
}
```
Now OtherComponent will be loaded in a separated bundle
Loadable Components provides additional benefits beyond just code-splitting, such as automatic code reloading, error handling, and preloading. By using Loadable Components, you can ensure that your application loads quickly and efficiently, providing a better user experience for your users.
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199. ### What is suspense component?
If the module containing the dynamic import is not yet loaded by the time parent component renders, you must show some fallback content while you’re waiting for it to load using a loading indicator. This can be done using **Suspense** component.
For example, the below code uses suspense component,
```javascript
const OtherComponent = React.lazy(() => import("./OtherComponent"));
function MyComponent() {
return (
<div>
<Suspense fallback={<div>Loading...</div>}>
<OtherComponent />
</Suspense>
</div>
);
}
```
As mentioned in the above code, Suspense is wrapped above the lazy component.
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200. ### What is route based code splitting?
One of the best place to do code splitting is with routes. The entire page is going to re-render at once so users are unlikely to interact with other elements in the page at the same time. Due to this, the user experience won't be disturbed.
Let us take an example of route based website using libraries like React Router with React.lazy,
```javascript
import { BrowserRouter as Router, Route, Switch } from "react-router-dom";
import React, { Suspense, lazy } from "react";
const Home = lazy(() => import("./routes/Home"));
const About = lazy(() => import("./routes/About"));
const App = () => (
<Router>
<Suspense fallback={<div>Loading...</div>}>
<Switch>
<Route exact path="/" component={Home} />
<Route path="/about" component={About} />
</Switch>
</Suspense>
</Router>
);
```
In the above code, the code splitting will happen at each route level.
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201. ### What is the purpose of default value in context?
The defaultValue argument is only used when a component does not have a matching Provider above it in the tree. This can be helpful for testing components in isolation without wrapping them.
Below code snippet provides default theme value as Luna.
```javascript
const MyContext = React.createContext(defaultValue);
```
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202. ### What is diffing algorithm?
React needs to use algorithms to find out how to efficiently update the UI to match the most recent tree. The diffing algorithms is generating the minimum number of operations to transform one tree into another. However, the algorithms have a complexity in the order of O(n³) where n is the number of elements in the tree.
In this case, displaying 1000 elements would require in the order of one billion comparisons. This is far too expensive. Instead, React implements a heuristic O(n) algorithm based on two assumptions:
1. Two elements of different types will produce different trees.
2. The developer can hint at which child elements may be stable across different renders with a key prop.
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203. ### What are the rules covered by diffing algorithm?
When diffing two trees, React first compares the two root elements. The behavior is different depending on the types of the root elements. It covers the below rules during reconciliation algorithm,
1. **Elements Of Different Types:**
Whenever the root elements have different types, React will tear down the old tree and build the new tree from scratch. For example, elements <a> to <img>, or from <Article> to <Comment> of different types lead a full rebuild.
2. **DOM Elements Of The Same Type:**
When comparing two React DOM elements of the same type, React looks at the attributes of both, keeps the same underlying DOM node, and only updates the changed attributes. Lets take an example with same DOM elements except className attribute,
```javascript
<div className="show" title="ReactJS" />
<div className="hide" title="ReactJS" />
```
3. **Component Elements Of The Same Type:**
When a component updates, the instance stays the same, so that state is maintained across renders. React updates the props of the underlying component instance to match the new element, and calls componentWillReceiveProps() and componentWillUpdate() on the underlying instance. After that, the render() method is called and the diff algorithm recurses on the previous result and the new result.
4. **Recursing On Children:**
when recursing on the children of a DOM node, React just iterates over both lists of children at the same time and generates a mutation whenever there’s a difference. For example, when adding an element at the end of the children, converting between these two trees works well.
```javascript
<ul>
<li>first</li>
<li>second</li>
</ul>
<ul>
<li>first</li>
<li>second</li>
<li>third</li>
</ul>
```
5. **Handling keys:**
React supports a key attribute. When children have keys, React uses the key to match children in the original tree with children in the subsequent tree. For example, adding a key can make the tree conversion efficient,
```javascript
<ul>
<li key="2015">Duke</li>
<li key="2016">Villanova</li>
</ul>
<ul>
<li key="2014">Connecticut</li>
<li key="2015">Duke</li>
<li key="2016">Vill