diff --git a/.github/FUNDING.yml b/.github/FUNDING.yml new file mode 100644 index 00000000..547e44d4 --- /dev/null +++ b/.github/FUNDING.yml @@ -0,0 +1,5 @@ +# These are supported funding model platforms + +github: [sudheerj] +custom: https://buymeacoffee.com/sudheerj + diff --git a/.github/workflows/main.yaml b/.github/workflows/main.yaml deleted file mode 100644 index 8b41896e..00000000 --- a/.github/workflows/main.yaml +++ /dev/null @@ -1,34 +0,0 @@ -name: Documentation - -on: - push: - branches: [ master ] - -env: - FILE_NAME: react-interview-questions - -jobs: - convert_via_pandoc: - runs-on: ubuntu-18.04 - steps: - - uses: actions/checkout@v2 - - - name: Create output directory - run: | - mkdir output # create output dir - - - name: Create PDF - uses: docker://pandoc/latex:2.10 - with: - args: --pdf-engine=xelatex --output=output/${{env.FILE_NAME}}.pdf README.md - - - name: Create epub - uses: docker://pandoc/latex:2.10 - with: - args: --output=output/${{env.FILE_NAME}}.epub README.md - - - name: Upload - uses: actions/upload-artifact@master - with: - name: output - path: output \ No newline at end of file diff --git a/LICENSE b/LICENSE new file mode 100644 index 00000000..beea88e6 --- /dev/null +++ b/LICENSE @@ -0,0 +1,21 @@ +MIT License + +Copyright (c) 2017-Present Sudheer Jonna + +Permission is hereby granted, free of charge, to any person obtaining a copy +of this software and associated documentation files (the "Software"), to deal +in the Software without restriction, including without limitation the rights +to use, copy, modify, merge, publish, distribute, sublicense, and/or sell +copies of the Software, and to permit persons to whom the Software is +furnished to do so, subject to the following conditions: + +The above copyright notice and this permission notice shall be included in all +copies or substantial portions of the Software. + +THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR +IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, +FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE +AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER +LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, +OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE +SOFTWARE. \ No newline at end of file diff --git a/README.md b/README.md index e69de29b..472ebc46 100644 --- a/README.md +++ b/README.md @@ -0,0 +1,9855 @@ +# React Interview Questions & Answers + +> Click :star: if you like the project. Pull Requests are highly appreciated. Follow me [@SudheerJonna](https://twitter.com/SudheerJonna) for technical updates. + +--- + +
+

+ + GreatFrontEnd React Interview Questions + +

+
+ +> 💡 Nail React interviews with questions and solutions from ex-interviewers! [Try GreatFrontEnd →](https://www.greatfrontend.com/questions/react?utm_source=github&utm_medium=referral&utm_campaign=sudheerj-react&gnrs=sudheerj) 💡 + +--- + +
+

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+ +> 🚀 Ace React interview questions with solutions from FAANG+ companies! [Try FrontendLead →](https://www.frontendlead.com/coding-questions?utm_source=github&utm_medium=referral&utm_campaign=sudheerj-react) 🚀 + +--- + +
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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 [Data Structures and Algorithms](https://github.com/sudheerj/datastructures-algorithms) for DSA-related questions or problems. + +### Table of Contents + +
+ +Hide/Show table of contents + + +| No. | Questions | +| --- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | +| | **Core React** | +| 1 | [What is React?](#what-is-react) | +| 2 | [What is the history behind React’s 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 an Element and a Component?](#what-is-the-difference-between-an-element-and-a-component) | +| 6 | [How do you create components in React?](#how-to-create-components-in-react) | +| 7 | [When should you 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 the "key" prop and what is its benefit when used in arrays of elements?](#what-is-key-prop-and-what-is-the-benefit-of-using-it-in-arrays-of-elements) | +| 16 | [What is the Virtual DOM?](#what-is-virtual-dom) | +| 17 | [How does the Virtual DOM work?](#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 the children prop?](#what-is-children-prop) | +| 27 | [How do you 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 does React use className instead of the class attribute?](#why-react-uses-classname-over-class-attribute) | +| 31 | [What are Fragments?](#what-are-fragments) | +| 32 | [Why are Fragments 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 do you apply validation to 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 the react-dom package?](#what-is-the-use-of-react-dom-package) | +| 41 | [What is ReactDOMServer?](#what-is-reactdomserver) | +| 42 | [How do you use innerHTML in React?](#how-to-use-innerhtml-in-react) | +| 43 | [How do you apply styles in React?](#how-to-use-styles-in-react) | +| 44 | [How are events different in React?](#how-events-are-different-in-react) | +| 45 | [What is the impact of using 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 do 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 do you implement Server-Side Rendering (SSR)?](#how-you-implement-server-side-rendering-or-ssr) | +| 50 | [How do you 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 a 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 do you loop inside JSX?](#how-to-loop-inside-jsx) | +| 58 | [How do you access props within attribute quotes?](#how-do-you-access-props-in-attribute-quotes) | +| 59 | [What is a React PropType array with shape?](#what-is-react-proptype-array-with-shape) | +| 60 | [How do you 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 is ReactDOM separated from React?](#why-reactdom-is-separated-from-react) | +| 63 | [How do you use the React label element?](#how-to-use-react-label-element) | +| 64 | [How do you combine multiple inline style objects?](#how-to-combine-multiple-inline-style-objects) | +| 65 | [How do you re-render the view when the browser is resized?](#how-to-re-render-the-view-when-the-browser-is-resized) | +| 66 | [How do you pretty-print JSON with React?](#how-to-pretty-print-json-with-react) | +| 67 | [Why can’t you update props in React?](#why-you-cant-update-props-in-react) | +| 68 | [How do you focus an input element on page load?](#how-to-focus-an-input-element-on-page-load) | +| 69 | [How can you 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 do you 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 do you import and export components using React and ES6?](#how-to-import-and-export-components-using-react-and-es6) | +| 73 | [What are the exceptions to 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 common folder structures for React?](#what-are-the-common-folder-structures-for-react) | +| 76 | [What are popular packages for animation?](#what-are-the-popular-packages-for-animation) | +| 77 | [What are the benefits of style modules?](#what-is-the-benefit-of-styles-modules) | +| 78 | [What are popular React-specific linters?](#what-are-the-popular-react-specific-linters) | +| | **React Router** | +| 79 | [What is React Router?](#what-is-react-router) | +| 80 | [How is React Router different from the history library?](#how-react-router-is-different-from-history-library) | +| 81 | [What are the components of React Router v6?](#what-are-the-router-components-of-react-router-v6) | +| 82 | [What is the purpose of the 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 do you get query parameters in React Router v4?](#how-to-get-query-parameters-in-react-router-v4) | +| 85 | [Why do you get a "Router may have only one child element" warning?](#why-you-get-router-may-have-only-one-child-element-warning) | +| 86 | [How do you pass params to the history.push method in React Router v4?](#how-to-pass-params-to-historypush-method-in-react-router-v4) | +| 87 | [How do you implement a default or NotFound page?](#how-to-implement-default-or-notfound-page) | +| 88 | [How do you get history in React Router v4?](#how-to-get-history-on-react-router-v4) | +| 89 | [How do you perform an 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 do you use FormattedMessage as a placeholder with React Intl?](#how-to-use-formattedmessage-as-placeholder-using-react-intl) | +| 94 | [How do you access the current locale with React Intl?](#how-to-access-current-locale-with-react-intl) | +| 95 | [How do you format a date using React Intl?](#how-to-format-date-using-react-intl) | +| | **React Testing** | +| 96 | [What is the Shallow Renderer in React testing?](#what-is-shallow-renderer-in-react-testing) | +| 97 | [What is the TestRenderer package in React?](#what-is-testrenderer-package-in-react) | +| 98 | [What is the purpose of the 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 | [Can you give a simple example of a 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 you dispatch an action in a reducer?](#can-i-dispatch-an-action-in-reducer) | +| 108 | [How do you access the Redux store outside a component?](#how-to-access-redux-store-outside-a-component) | +| 109 | [What are the drawbacks of the 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 do you 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 do you make an AJAX request in Redux?](#how-to-make-ajax-request-in-redux) | +| 115 | [Should you keep all component states in the Redux store?](#should-i-keep-all-components-state-in-redux-store) | +| 116 | [What is the proper way to access the Redux store?](#what-is-the-proper-way-to-access-redux-store) | +| 117 | [What is the difference between a component and a container in React Redux?](#what-is-the-difference-between-component-and-container-in-react-redux) | +| 118 | [What is the purpose of 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 do you 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 should you 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 do you add multiple middlewares to Redux?](#how-to-add-multiple-middlewares-to-redux) | +| 133 | [How do you set the initial state in Redux?](#how-to-set-initial-state-in-redux) | +| 134 | [How is Relay 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 do you test React Native apps?](#how-to-test-react-native-apps) | +| 138 | [How do you log in React Native?](#how-to-do-logging-in-react-native) | +| 139 | [How do you debug React Native apps?](#how-to-debug-your-react-native) | +| | **React Supported Libraries and Integration** | +| 140 | [What is Reselect and how does it work?](#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 do you use Font Awesome icons in React?](#how-to-use-font-awesome-icons-in-react) | +| 144 | [What is React DevTools?](#what-is-react-dev-tools) | +| 145 | [Why does DevTools not load in Chrome for local files?](#why-is-devtools-not-loading-in-chrome-for-local-files) | +| 146 | [How do you 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 is the React tab not showing up in DevTools?](#why-react-tab-is-not-showing-up-in-devtools) | +| 150 | [What are styled-components?](#what-are-styled-components) | +| 151 | [Can you 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 the Reselect library?](#what-are-the-main-features-of-reselect-library) | +| 154 | [Can you 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 a specific build tool to use Redux?](#do-you-need-to-have-a-particular-build-tool-to-use-redux) | +| 157 | [How do Redux Form initial values get updated from state?](#how-redux-form-initialvalues-get-updated-from-state) | +| 158 | [How do React PropTypes allow different types for one prop?](#how-react-proptypes-allow-different-types-for-one-prop) | +| 159 | [Can you import an SVG file as a 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 do you pass numbers to a React component?](#how-to-pass-numbers-to-react-component) | +| 162 | [Do you need to keep all state in Redux? Should you ever use React’s 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 the React.memo function?](#what-is-react-memo-function) | +| 165 | [What is the React.lazy function?](#what-is-react-lazy-function) | +| 166 | [How do you prevent unnecessary updates using setState?](#how-to-prevent-unnecessary-updates-using-setstate) | +| 167 | [How do you render arrays, strings, and numbers in React v16?](#how-do-you-render-array-strings-and-numbers-in-react-16-version) | +| 168 | [What are Hooks?](#what-are-hooks) | +| 169 | [What rules must be followed for Hooks?](#what-rules-need-to-be-followed-for-hooks) | +| 170 | [How do you ensure Hooks follow 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 the 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 v16?](#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 a component stack trace from an 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 the 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 do component props default to true?](#when-component-props-defaults-to-true) | +| 185 | [What is Next.js and what are its major features?](#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 do you prevent a function from being called multiple times?](#how-to-prevent-a-function-from-being-called-multiple-times) | +| 188 | [How does JSX prevent injection attacks?](#how-jsx-prevents-injection-attacks) | +| 189 | [How do you update rendered elements?](#how-do-you-update-rendered-elements) | +| 190 | [How do you indicate that props are read-only?](#how-do-you-say-that-props-are-read-only) | +| 191 | [What are the conditions for safely using an index as a key?](#what-are-the-conditions-to-safely-use-the-index-as-a-key) | +| 192 | [Do keys need to 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 the Redux Form library?](#what-are-the-advantages-of-formik-over-redux-form-library) | +| 195 | [Why are you not required to use inheritance?](#why-do-you-not-required-to-use-inheritance) | +| 196 | [Can you use web components in a React application?](#can-i-use-web-components-in-react-application) | +| 197 | [What is a dynamic import?](#what-is-dynamic-import) | +| 198 | [What are loadable components?](#what-are-loadable-components) | +| 199 | [What is a 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 the default value in Context?](#what-is-the-purpose-of-default-value-in-context) | +| 202 | [What is the diffing algorithm?](#what-is-diffing-algorithm) | +| 203 | [What rules are covered by the 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 | [Must a prop be named "render" for render props?](#is-it-prop-must-be-named-as-render-for-render-props) | +| 206 | [What are the problems with using render props with Pure Components?](#what-are-the-problems-of-using-render-props-with-pure-components) | +| 207 | [What is the 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 for portals?](#what-is-the-typical-use-case-of-portals) | +| 210 | [How do you set a default value for an 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 the real DOM and the Virtual DOM?](#what-is-the-difference-between-real-dom-and-virtual-dom) | +| 213 | [How do you add Bootstrap to a React application?](#how-to-add-bootstrap-to-a-react-application) | +| 214 | [Can you list the top websites or applications using React as a front-end framework?](#can-you-list-down-top-websites-or-applications-using-react-as-front-end-framework) | +| 215 | [Is it recommended to use the CSS-in-JS technique in React?](#is-it-recommended-to-use-css-in-js-technique-in-react) | +| 216 | [Do you need to rewrite all class components with Hooks?](#do-i-need-to-rewrite-all-my-class-components-with-hooks) | +| 217 | [How do you fetch data with React Hooks?](#how-to-fetch-data-with-react-hooks) | +| 218 | [Do 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 bracket notation) in useState?](#why-do-we-use-array-destructuring-square-brackets-notation-in-usestate) | +| 221 | [What sources were used for introducing Hooks?](#what-are-the-sources-used-for-introducing-hooks) | +| 222 | [How do you access the 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 | [Can you describe 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 you 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 you use JavaScript URLs in React v16.9?](#can-i-use-javascript-urls-in-react169) | +| 233 | [What is the purpose of the ESLint plugin for Hooks?](#what-is-the-purpose-of-eslint-plugin-for-hooks) | +| 234 | [What is the difference between imperative and declarative programming 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 ensure a 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 the new JSX transform?](#what-are-the-benefits-of-new-jsx-transform) | +| 238 | [How is the new JSX transform different from the 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 the useState and useRef Hooks?](#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 the 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 multiple JSX tags must 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 update the 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 the Immer library for state updates?](#how-do-you-use-immer-library-for-state-updates) | +| 260 | [What are the benefits of preventing 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 state?](#what-are-the-preferred-and-non-preferred-array-operations-for-updating-the-state) | +| 262 | [What will happen when 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 follow for writing reducers?](#what-are-the-guidelines-to-be-followed-for-writing-reducers) | +| | **Hooks** | +| 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 work with the useContext hook?](#how-does-context-works-using-usecontext-hook) | +| 268 | [What are the use cases of the useContext hook?](#what-are-the-use-cases-of-usecontext-hook) | +| 269 | [When should you use client and server components?](#when-to-use-client-and-server-components) | +| 270 | [What are the differences between the Page Router and App Router in Next.js?](#what-are-the-differences-between-page-router-and-app-router-in-nextjs) | | | +| | +
+ +### Table of Contents + +
+ +Hide/Show table of contents + + +| 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) | + +
+ +## Core React + +1. ### What is React? + + React (aka React.js or ReactJS) is an **open-source front-end JavaScript library** for building user interfaces based on components. It's used for handling the view layer in web and mobile applications, and allows developers to create reusable UI components and manage the state of those components efficiently. + + React was created by [Jordan Walke](https://github.com/jordwalke), a software engineer at Facebook (now Meta). It was first deployed on Facebook's News Feed in 2011 and on Instagram in 2012. The library was open-sourced in May 2013 and has since become one of the most popular JavaScript libraries for building modern user interfaces. + + **[⬆ 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. + +
+ See deep-dive answer + The evolution of React has a fascinating history that spans over a decade: + + **2010-2011: The Origins** + - The journey began with **XHP**, a PHP extension created at Facebook that allowed HTML components to be used in PHP code + - XHP improved front-end code readability and helped prevent cross-site scripting (XSS) attacks + - However, XHP had limitations with dynamic web applications, requiring frequent server roundtrips and complete UI re-renders for small changes + + **2011-2012: Early Development** + - Jordan Walke created the first prototype called **FaxJS** (later renamed to React), inspired by XHP's component model + - The key innovation was bringing XHP's component model to JavaScript with performance improvements + - React introduced the Virtual DOM concept to solve the performance issues of full page re-renders + - First deployed internally on Facebook's News Feed in 2011 and Instagram in 2012 + + **2013: Public Release** + - React was officially open-sourced at JSConf US in May 2013 + - Initial public reception was mixed, with some developers skeptical about the JSX syntax and the approach of mixing markup with JavaScript + + **2014-2015: Growing Adoption** + - React Native was announced in 2015, extending React's paradigm to mobile app development + - The ecosystem began to grow with tools like Redux for state management + - Companies beyond Facebook began adopting React for production applications + + **2016-2018: Maturation** + - React 16 ("Fiber") was released in 2017 with a complete rewrite of the core architecture + - Introduction of new features like Error Boundaries, Portals, and improved server-side rendering + - React 16.3 introduced the Context API for easier state management + + **2019-Present: Modern React** + - React Hooks were introduced in React 16.8 (February 2019), revolutionizing state management in functional components + - React 17 (October 2020) focused on making React upgrades easier + - React 18 (March 2022) introduced concurrent rendering and automatic batching + - React continues to evolve with Server Components, the new React compiler (React Forget), and other performance improvements +
+ + **Note:** JSX, React's syntax extension, was indeed inspired by XHP's approach of embedding XML-like syntax in code. + + **[⬆ Back to Top](#table-of-contents)** + +3. ### What are the major features of React? + + React offers a powerful set of features that have made it one of the most popular JavaScript libraries for building user interfaces: + + **Core Features:** + + - **Component-Based Architecture**: React applications are built using components - independent, reusable pieces of code that return HTML via a render function. This modular approach enables better code organization, reusability, and maintenance. + + - **Virtual DOM**: React creates an in-memory data structure cache, computes the resulting differences, and efficiently updates only the changed parts in the browser DOM. This approach significantly improves performance compared to direct DOM manipulation. + + - **JSX (JavaScript XML)**: A syntax extension that allows writing HTML-like code in JavaScript. JSX makes the code more readable and expressive while providing the full power of JavaScript. + + - **Unidirectional Data Flow**: React follows a one-way data binding model where data flows from parent to child components. This makes the code more predictable and easier to debug. + + - **Declarative UI**: React allows you to describe what your UI should look like for a given state, and it handles the DOM updates when the underlying data changes. + + **Advanced Features:** + + - **React Hooks**: Introduced in React 16.8, hooks allow using state and other React features in functional components without writing classes. + + - **Context API**: Provides a way to share values between components without explicitly passing props through every level of the component tree. + + - **Error Boundaries**: Components that catch JavaScript errors anywhere in their child component tree and display fallback UI instead of crashing. + + - **Server-Side Rendering (SSR)**: Enables rendering React components on the server before sending HTML to the client, improving performance and SEO. + + - **Concurrent Mode**: A set of new features (in development) that help React apps stay responsive and gracefully adjust to the user's device capabilities and network speed. + + - **React Server Components**: A new feature that allows components to be rendered entirely on the server, reducing bundle size and improving performance. + + - **Suspense**: A feature that lets your components "wait" for something before rendering, supporting code-splitting and data fetching with cleaner code. + + These features collectively make React powerful for building everything from small widgets to complex, large-scale web applications. + + **[⬆ 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 `

` tag is returned as JavaScript function to the render function. + + ```jsx harmony + export default function App() { + return

{"Hello, this is a JSX Code!"}

; + } + ``` + + 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!" + ); + } + ``` + +
See Class +

+ + ```jsx harmony + class App extends React.Component { + render() { + return

{"Hello, this is a JSX Code!"}

; + } + } + ``` + +

+
+ + **Note:** JSX is stricter than HTML + + **[⬆ Back to Top](#table-of-contents)** + +5. ### What is the difference between an Element and a Component? + + **Element:** + - A React **Element** is a plain JavaScript object that describes what you want to see on the UI. It represents a DOM node or a component at a specific point in time. + - Elements are immutable: once created, you cannot change their properties. Instead, you create new elements to reflect updates. + - Elements can be nested within other elements through their `props`. + - Creating an element is a fast, lightweight operation—it does **not** create any actual DOM nodes or render anything to the screen directly. + + **Example (without JSX):** + ```js + const element = React.createElement("button", { id: "login-btn" }, "Login"); + ``` + + **Equivalent JSX syntax:** + ```jsx + + ``` + + **The object returned by `React.createElement`:** + ```js + { + type: 'button', + props: { + id: 'login-btn', + children: 'Login' + } + } + ``` + Elements are then passed to the React DOM renderer (e.g., `ReactDOM.render()`), which translates them to actual DOM nodes. + + --- + + **Component:** + - A **Component** is a function or class that returns an element (or a tree of elements) to describe part of the UI. Components can accept inputs (called **props**) and manage their own state (in case of class or function components with hooks). + - Components allow you to split the UI into independent, reusable pieces, each isolated and composable. + - You can define a component using a function or a class: + + **Example (Function Component with JSX):** + ```jsx + const Button = ({ handleLogin }) => ( + + ); + ``` + + When JSX is compiled, it's transformed into a tree of `React.createElement` calls: + + ```js + const Button = ({ handleLogin }) => + React.createElement( + "button", + { id: "login-btn", onClick: handleLogin }, + "Login" + ); + ``` + + --- + + **In summary:** + - **Elements** are the smallest building blocks in React—objects that describe what you want to see. + - **Components** are functions or classes that return elements and encapsulate logic, structure, and behavior for parts of your UI. + + > Think of **elements** as the instructions for creating UI, and **components** as reusable blueprints that combine logic and structure to generate those instructions. + + **[⬆ 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

{`Hello, ${message}`}

; + } + ``` + + 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

{`Hello, ${this.props.message}`}

; + } + } + ``` + + **[⬆ 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"; + + Something went wrong}> + + ; + ``` + + **[⬆ 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 ( + <> +

Name:{name}

+

Email: {email}

+ + ); + }); + export default function EmployeeRegForm() { + const [name, setName] = useState(""); + const [email, setEmail] = useState(""); + return ( + <> + + +
+ + + ); + } + ``` + + 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 ( +
+

{message}

+
+ ); + } + ``` + + Whenever React calls your component or access `useState` hook, it gives you a snapshot of the state for that particular render. + +
See Class +

+ + ```jsx harmony + import React from "react"; + class User extends React.Component { + constructor(props) { + super(props); + + this.state = { + message: "Welcome to React world", + }; + } + + render() { + return ( +

+

{this.state.message}

+
+ ); + } + } + ``` + +

+
+ + 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 + + ``` + + 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 ( +
+

{props.name}

+

{props.age}

+

{props.gender}

+
+ ); + }; + + const ParentComponent = () => { + return ( +
+ + +
+ ); + }; + ``` + +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 ( +
+

{name}

+

{age}

+

{gender}

+
+ ); +}; +``` + +**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. + +
See Class + 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 ( +
+

{this.props.name}

+

{this.props.age}

+

{this.props.gender}

+
+ ); + } +} + +class ParentComponent extends React.Component { + render() { + return ( +
+ + +
+ ); + } +} +``` + +
+ +**[⬆ 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, but they serve different purposes and have distinct behaviors: + + ### State + - **Definition:** + State is a data structure that is managed within a component. It represents information that can change over the lifetime of the component. + - **Mutability:** + State is mutable, meaning it can be changed using the setter function (`setState` in class components or the updater function from `useState` in functional components). + - **Scope:** + State is local to the component where it is defined. Only that component can modify its own state. + - **Usage:** + State is typically used for data that needs to change in response to user actions, network responses, or other dynamic events. + - **Re-rendering:** + Updating the state triggers a re-render of the component and its descendants. + + ### Props + - **Definition:** + Props (short for “properties”) are inputs to a component, provided by its parent component. + - **Mutability:** + Props are read-only. A component cannot modify its own props; they are immutable from the component’s perspective. + - **Scope:** + Props are used to pass data and event handlers down the component tree, enabling parent components to configure or communicate with their children. + - **Usage:** + Props are commonly used to make components reusable and configurable. They allow the same component to be rendered with different data or behavior. + - **Analogy:** + Think of props as arguments to a function, whereas state is like variables declared inside the function. + + ### Summary Table + + | Feature | State | Props | + |-----------|-------------------------------------|-----------------------------------| + | Managed by| The component itself | Parent component | + | Mutable | Yes | No (read-only) | + | Scope | Local to the component | Passed from parent to child | + | Usage | Manage dynamic data and UI changes | Configure and customize component | + | Update | Using setState/useState | Cannot be updated by the component| + + --- + + **[⬆ 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 + + ``` + + Whereas in React it follows _camelCase_ convention: + + ```jsx harmony +