var Component=(()=>{var p=Object.create;var o=Object.defineProperty;var E=Object.getOwnPropertyDescriptor;var y=Object.getOwnPropertyNames;var u=Object.getPrototypeOf,F=Object.prototype.hasOwnProperty;var m=(l,e)=>()=>(e||l((e={exports:{}}).exports,e),e.exports),f=(l,e)=>{for(var r in e)o(l,r,{get:e[r],enumerable:!0})},c=(l,e,r,s)=>{if(e&&typeof e=="object"||typeof e=="function")for(let a of y(e))!F.call(l,a)&&a!==r&&o(l,a,{get:()=>e[a],enumerable:!(s=E(e,a))||s.enumerable});return l};var g=(l,e,r)=>(r=l!=null?p(u(l)):{},c(e||!l||!l.__esModule?o(r,"default",{value:l,enumerable:!0}):r,l)),b=l=>c(o({},"__esModule",{value:!0}),l);var i=m((C,t)=>{t.exports=_jsx_runtime});var w={};f(w,{default:()=>h,frontmatter:()=>B});var n=g(i()),B={title:"Top reactjs questions asked in interview",description:"Frequently asked reactjs interview question",publishedAt:"2024-06-15",updatedAt:"2024-06-15",author:"GeeksMonk",isPublished:!0,tags:["interview","react"]};function d(l){let e={a:"a",code:"code",figure:"figure",h2:"h2",h3:"h3",hr:"hr",li:"li",p:"p",pre:"pre",span:"span",strong:"strong",ul:"ul",...l.components};return(0,n.jsxs)(n.Fragment,{children:[(0,n.jsxs)(e.h2,{id:"what-is-react-js",children:["What is react Js?",(0,n.jsx)(e.a,{"aria-hidden":"true",tabIndex:"-1",href:"#what-is-react-js",children:(0,n.jsx)(e.span,{className:"icon icon-link"})})]}),` `,(0,n.jsx)(e.p,{children:`React is an open-source front-end JavaScript library that is used for building user interfaces, especially for single-page applications. It is used for handling view layer for web and mobile apps.`}),` `,(0,n.jsx)(e.hr,{}),` `,(0,n.jsxs)(e.h2,{id:"what-are-the-main-features-of-react",children:["What are the main features of react?",(0,n.jsx)(e.a,{"aria-hidden":"true",tabIndex:"-1",href:"#what-are-the-main-features-of-react",children:(0,n.jsx)(e.span,{className:"icon icon-link"})})]}),` `,(0,n.jsx)(e.p,{children:"The major features of React are:"}),` `,(0,n.jsxs)(e.ul,{children:[` `,(0,n.jsx)(e.li,{children:"React uses JSX syntax, a syntax extension of javascript that allows developers to write HTML in their JS code."}),` `,(0,n.jsx)(e.li,{children:"React uses Virtual DOM instead of RealDOM considering that RealDOM manipulations are expensive."}),` `,(0,n.jsx)(e.li,{children:"Supports server-side rendering."}),` `,(0,n.jsx)(e.li,{children:"React follows Unidirectional data flow or data binding."}),` `,(0,n.jsx)(e.li,{children:"Uses reusable/composable UI components to develop the view."}),` `,(0,n.jsx)(e.li,{children:"React can be used to build web as well as mobile application."}),` `]}),` `,(0,n.jsx)(e.hr,{}),` `,(0,n.jsxs)(e.h2,{id:"what-is-jsx",children:["What is JSX?",(0,n.jsx)(e.a,{"aria-hidden":"true",tabIndex:"-1",href:"#what-is-jsx",children:(0,n.jsx)(e.span,{className:"icon icon-link"})})]}),` `,(0,n.jsxs)(e.p,{children:[`JSX is a XML-like syntax extension to ECMAScript. Basically it just provides syntactic sugar for the `,(0,n.jsx)(e.strong,{children:"React.createElement()"}),` function, giving us expressiveness of JavaScript along with HTML like template syntax.`]}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.code,{"data-language":"js","data-theme":"github-dark",children:[(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"export"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" default"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" function"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" App"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"() {"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" return"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" ("})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" <"}),(0,n.jsx)(e.span,{style:{color:"#85E89D"},children:"div"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:">"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" <"}),(0,n.jsx)(e.span,{style:{color:"#85E89D"},children:"h1"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:">Welcome to React world!"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" "})]}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" );"})}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"}"})})]})})}),` `,(0,n.jsx)(e.p,{children:"The above react code can be written in plain javascript like below"}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsx)(e.code,{"data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"React."}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:"createElement"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"("}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:'"div"'}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:", "}),(0,n.jsx)(e.span,{style:{color:"#79B8FF"},children:"null"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:", React."}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:"createElement"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"("}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:'"h1"'}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:", "}),(0,n.jsx)(e.span,{style:{color:"#79B8FF"},children:"null"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:", "}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:'"Welcome to React world!'}),(0,n.jsx)(e.span,{style:{color:"#79B8FF"},children:'\\"'}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:'"'}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"));"})]})})})}),` `,(0,n.jsx)(e.hr,{}),` `,(0,n.jsxs)(e.h2,{id:"what-is-the-difference-between-element-and-component",children:["What is the difference between Element and Component?",(0,n.jsx)(e.a,{"aria-hidden":"true",tabIndex:"-1",href:"#what-is-the-difference-between-element-and-component",children:(0,n.jsx)(e.span,{className:"icon icon-link"})})]}),` `,(0,n.jsx)(e.p,{children:"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 is never mutated."}),` `,(0,n.jsx)(e.p,{children:"The object representation of React Element would be as follows:"}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsx)(e.code,{"data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"const"}),(0,n.jsx)(e.span,{style:{color:"#79B8FF"},children:" element"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" ="}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" React."}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:"createElement"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"("}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:'"div"'}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:", { id: "}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:'"login-btn"'}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" }, "}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:'"Login"'}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:");"})]})})})}),` `,(0,n.jsx)(e.p,{children:"The above React.createElement() function returns an object:"}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.code,{"data-language":"js","data-theme":"github-dark",children:[(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"{"})}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" type"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:": "}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:"'div'"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:","})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" props"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:": {"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" children"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:": "}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:"'Login'"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:","})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" id"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:": "}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:"'login-btn'"})]}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" }"})}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"}"})})]})})}),` `,(0,n.jsx)(e.p,{children:"And finally it renders to the DOM using ReactDOM.render():"}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsx)(e.code,{"data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"<"}),(0,n.jsx)(e.span,{style:{color:"#85E89D"},children:"div"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" id"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"="}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:'"login-btn"'}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:">Login"})]})})})}),` `,(0,n.jsx)(e.p,{children:"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:"}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.code,{"data-language":"js","data-theme":"github-dark",children:[(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"const"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" Button"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" ="}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" ({ "}),(0,n.jsx)(e.span,{style:{color:"#FFAB70"},children:"onLogin"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" }) "}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"=>"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" ("})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" <"}),(0,n.jsx)(e.span,{style:{color:"#85E89D"},children:"div"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" id"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"="}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"{"}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:'"login-btn"'}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"} "}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:"onClick"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"="}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"{onLogin}>"})]}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" Login"})}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" "})]}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:");"})})]})})}),` `,(0,n.jsx)(e.p,{children:"Then JSX gets transpiled to a React.createElement() function tree:"}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.code,{"data-language":"js","data-theme":"github-dark",children:[(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"const"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" Button"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" ="}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" ({ "}),(0,n.jsx)(e.span,{style:{color:"#FFAB70"},children:"onLogin"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" }) "}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"=>"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" React."}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:"createElement"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"("})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:' "div"'}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:","})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" { id: "}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:'"login-btn"'}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:", onClick: onLogin },"})]}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:' "Login"'})}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" );"})})]})})}),` `,(0,n.jsx)(e.hr,{}),` `,(0,n.jsxs)(e.h2,{id:"how-to-create-components-in-react",children:["How to create components in React?",(0,n.jsx)(e.a,{"aria-hidden":"true",tabIndex:"-1",href:"#how-to-create-components-in-react",children:(0,n.jsx)(e.span,{className:"icon icon-link"})})]}),` `,(0,n.jsx)(e.p,{children:"There are two possible ways to create a component."}),` `,(0,n.jsxs)(e.p,{children:[(0,n.jsx)(e.strong,{children:"Function Components"}),": This is the simplest way to create a component. Those are pure JavaScript functions that accept props object as the first parameter and return React elements:"]}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.code,{"data-language":"js","data-theme":"github-dark",children:[(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"function"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" Greeting"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"({ "}),(0,n.jsx)(e.span,{style:{color:"#FFAB70"},children:"message"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" }) {"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" return"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" <"}),(0,n.jsx)(e.span,{style:{color:"#85E89D"},children:"h1"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:">{"}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:"`Hello, ${"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"message"}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:"}`"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"};"})]}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"}"})})]})})}),` `,(0,n.jsxs)(e.p,{children:[(0,n.jsx)(e.strong,{children:"Class Components"}),": You can also use ES6 class to define a component. The above function component can be written as:"]}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.code,{"data-language":"js","data-theme":"github-dark",children:[(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"class"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" Greeting"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" extends"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" React"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"."}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:"Component"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" {"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" render"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"() {"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" return"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" <"}),(0,n.jsx)(e.span,{style:{color:"#85E89D"},children:"h1"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:">{"}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:"`Hello, ${"}),(0,n.jsx)(e.span,{style:{color:"#79B8FF"},children:"this"}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:"."}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"props"}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:"."}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"message"}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:"}`"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"};"})]}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" }"})}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"}"})})]})})}),` `,(0,n.jsxs)(e.p,{children:["After the introduction of ",(0,n.jsx)(e.strong,{children:"React Hooks"})," in react, class based components are hardly used."]}),` `,(0,n.jsx)(e.hr,{}),` `,(0,n.jsxs)(e.h2,{id:"what-are-pure-components-in-reactjs",children:["What are Pure Components in Reactjs?",(0,n.jsx)(e.a,{"aria-hidden":"true",tabIndex:"-1",href:"#what-are-pure-components-in-reactjs",children:(0,n.jsx)(e.span,{className:"icon icon-link"})})]}),` `,(0,n.jsx)(e.p,{children:"In functional programming, A function is considerd to be pure when"}),` `,(0,n.jsxs)(e.ul,{children:[` `,(0,n.jsx)(e.li,{children:"Its return value is only determined by its input values"}),` `,(0,n.jsx)(e.li,{children:"Its return value is always the same for the same input values"}),` `]}),` `,(0,n.jsx)(e.p,{children:`In react a component is said to be pure when it renders the same output for the same state and props. For class based component, React provides the PureComponent base class. Class components that extend the React.PureComponent class are treated as pure components.`}),` `,(0,n.jsxs)(e.p,{children:[`Pure components have some performance improvements and render optimizations since React implements the `,(0,n.jsx)(e.strong,{children:"shouldComponentUpdate()"})," method for them with a shallow comparison for props and state."]}),` `,(0,n.jsxs)(e.h2,{id:"are-react-functional-components-pure",children:["Are React functional components pure?",(0,n.jsx)(e.a,{"aria-hidden":"true",tabIndex:"-1",href:"#are-react-functional-components-pure",children:(0,n.jsx)(e.span,{className:"icon icon-link"})})]}),` `,(0,n.jsx)(e.p,{children:`Functional components cannot leverage the performance improvements and render optimizations that come with React.PureComponent since by definition, they are not classes, so they are not pure component.`}),` `,(0,n.jsx)(e.p,{children:`If you want React to treat a functional component as a pure component, you\u2019ll have to convert the functional component to a class component that extends React.PureComponent.`}),` `,(0,n.jsx)(e.p,{children:`With React.memo(), you can create memoized functional components that bail out of rendering on unnecessary updates using shallow comparison of props.`}),` `,(0,n.jsx)(e.p,{children:"Using the React.memo() API, the previous functional component can be wrapped as follows:"}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.code,{"data-language":"js","data-theme":"github-dark",children:[(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"import"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" { memo } "}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"from"}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:' 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`,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" return"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" ("})]}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" <>"})}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" <"}),(0,n.jsx)(e.span,{style:{color:"#85E89D"},children:"h2"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:">My Todos"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" {todos."}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:"map"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"(("}),(0,n.jsx)(e.span,{style:{color:"#FFAB70"},children:"todo"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:", "}),(0,n.jsx)(e.span,{style:{color:"#FFAB70"},children:"index"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:") 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`,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"export"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" default"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" memo"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"(Todos);"})]})]})})}),` `,(0,n.jsx)(e.p,{children:`For one, React.memo() is a higher-order component. It takes a React component as its first argument and returns a special type of React component that allows the renderer to render the component while memoizing the output. Therefore, if the component\u2019s props are shallowly equal, the React.memo() component will bail out the updates.`}),` `,(0,n.jsx)(e.p,{children:`React.memo() works with all React components. The first argument passed to React.memo() can be any type of React component. However, for class components, you should use React.PureComponent instead of using React.memo(). React.memo() also works with components rendered from the server using ReactDOMServer.`}),` `,(0,n.jsx)(e.hr,{}),` `,(0,n.jsxs)(e.h2,{id:"what-is-state-in-react",children:["What is state in React?",(0,n.jsx)(e.a,{"aria-hidden":"true",tabIndex:"-1",href:"#what-is-state-in-react",children:(0,n.jsx)(e.span,{className:"icon icon-link"})})]}),` `,(0,n.jsx)(e.p,{children:`State of a component is an object that holds some information that may change over the lifetime of the component. We should always try to make our state as simple as possible and minimize the number of stateful components.`}),` `,(0,n.jsx)(e.p,{children:"Let's create a user component with message state,"}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.code,{"data-language":"js","data-theme":"github-dark",children:[(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"import"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" React, { useState } "}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"from"}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:' "react"'}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:";"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"function"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" User"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"() {"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" const"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" ["}),(0,n.jsx)(e.span,{style:{color:"#79B8FF"},children:"message"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:", "}),(0,n.jsx)(e.span,{style:{color:"#79B8FF"},children:"setMessage"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"] "}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"="}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" useState"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"("}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:'"Welcome to React world"'}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:");"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" return"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" ("})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" <"}),(0,n.jsx)(e.span,{style:{color:"#85E89D"},children:"div"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:">"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" <"}),(0,n.jsx)(e.span,{style:{color:"#85E89D"},children:"h1"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:">{message}"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" "})]}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" );"})}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"}"})})]})})}),` `,(0,n.jsx)(e.hr,{}),` `,(0,n.jsxs)(e.h2,{id:"what-are-props-in-react",children:["What are props in React?",(0,n.jsx)(e.a,{"aria-hidden":"true",tabIndex:"-1",href:"#what-are-props-in-react",children:(0,n.jsx)(e.span,{className:"icon icon-link"})})]}),` `,(0,n.jsx)(e.p,{children:"Props are inputs to components. They are single values or objects containing a set of values that are passed to components on creation using a naming convention similar to HTML-tag attributes. They are data passed down from a parent component to a child component."}),` `,(0,n.jsx)(e.p,{children:"The primary purpose of props in React is to provide following component functionality:"}),` `,(0,n.jsxs)(e.ul,{children:[` `,(0,n.jsx)(e.li,{children:"Pass custom data to your component."}),` `,(0,n.jsx)(e.li,{children:"Trigger state changes."}),` `,(0,n.jsx)(e.li,{children:"Use via this.props.reactProp inside component's render() method."}),` `]}),` `,(0,n.jsx)(e.p,{children:"For example, let us create an element with reactProp property:"}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsx)(e.code,{"data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"<"}),(0,n.jsx)(e.span,{style:{color:"#79B8FF"},children:"Element"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" reactProp"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"="}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"{"}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:'"1"'}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"} />"})]})})})}),` `,(0,n.jsx)(e.p,{children:"This reactProp (or whatever you came up with) name then becomes a property attached to React's native props object which originally already exists on all components created using React library."}),` 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Most often we use ID from our data as key:"}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsx)(e.code,{"data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"const"}),(0,n.jsx)(e.span,{style:{color:"#79B8FF"},children:" todoItems"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" ="}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" todos."}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:"map"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"(("}),(0,n.jsx)(e.span,{style:{color:"#FFAB70"},children:"todo"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:") "}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"=>"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" <"}),(0,n.jsx)(e.span,{style:{color:"#85E89D"},children:"li"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" key"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"="}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"{todo.id}>{todo.text});"})]})})})}),` `,(0,n.jsx)(e.p,{children:"When you don't have stable IDs for rendered items, you may use the item index as a key as a last resort:"}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.code,{"data-language":"js","data-theme":"github-dark",children:[(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"const"}),(0,n.jsx)(e.span,{style:{color:"#79B8FF"},children:" todoItems"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" ="}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" todos."}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:"map"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"(("}),(0,n.jsx)(e.span,{style:{color:"#FFAB70"},children:"todo"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:", "}),(0,n.jsx)(e.span,{style:{color:"#FFAB70"},children:"index"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:") "}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"=>"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" ("})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" <"}),(0,n.jsx)(e.span,{style:{color:"#85E89D"},children:"li"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" key"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"="}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"{index}>{todo.text}"})]}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"));"})})]})})}),` `,(0,n.jsx)(e.p,{children:"Note:"}),` `,(0,n.jsxs)(e.ul,{children:[` `,(0,n.jsx)(e.li,{children:"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."}),` `,(0,n.jsx)(e.li,{children:"If you extract list item as separate component then apply keys on list component instead of li tag."}),` `,(0,n.jsx)(e.li,{children:"There will be a warning message in the console if the key prop is not present on list items."}),` `]}),` `,(0,n.jsx)(e.hr,{}),` `,(0,n.jsxs)(e.h2,{id:"what-is-react-hooks",children:["What is React Hooks?",(0,n.jsx)(e.a,{"aria-hidden":"true",tabIndex:"-1",href:"#what-is-react-hooks",children:(0,n.jsx)(e.span,{className:"icon icon-link"})})]}),` `,(0,n.jsx)(e.p,{children:"React Hooks are the built-in functions that permit developers for using the state and lifecycle methods within React components"}),` `,(0,n.jsx)(e.p,{children:"Each lifecycle of a component is having 3 phases which include mount, unmount, and update. Along with that, components have properties and states. Hooks will allow using these methods by developers for improving the reuse of code with higher flexibility navigating the component tree."}),` `,(0,n.jsx)(e.p,{children:"Using Hook, all features of React can be used without writing class components. For example, before React version 16.8, it required a class component for managing the state of a component. But now using the useState hook, we can keep the state in a functional component."}),` `,(0,n.jsxs)(e.ul,{children:[` `,(0,n.jsx)(e.li,{children:"useState: It declares a state variable that you can update directly."}),` `,(0,n.jsx)(e.li,{children:"useReducer: It declares a state variable with the update logic inside a reducer function."}),` `,(0,n.jsx)(e.li,{children:"useContext: It reads and subscribes to a context."}),` `,(0,n.jsx)(e.li,{children:"useRef declares a ref. You can hold any value in it, but most often it\u2019s used to hold a DOM node."}),` `,(0,n.jsx)(e.li,{children:"useImperativeHandle lets you customize the ref exposed by your component. This is rarely used."}),` `,(0,n.jsx)(e.li,{children:"useEffect connects a component to an external system."}),` `,(0,n.jsx)(e.li,{children:"useMemo lets you cache the result of an expensive calculation."}),` `,(0,n.jsx)(e.li,{children:"useCallback lets you cache a function definition before passing it down to an optimized component."}),` `]}),` `,(0,n.jsx)(e.hr,{}),` `,(0,n.jsxs)(e.h2,{id:"what-is-the-use-of-react-state-hooks",children:["What is the use of React state Hooks?",(0,n.jsx)(e.a,{"aria-hidden":"true",tabIndex:"-1",href:"#what-is-the-use-of-react-state-hooks",children:(0,n.jsx)(e.span,{className:"icon icon-link"})})]}),` `,(0,n.jsx)(e.p,{children:`React State lets a component remember information so that you can use it latter. In below example, we are defining a react state using useState and holding a initial value 0.`}),` `,(0,n.jsx)(e.p,{children:`useState returns an array containing two value. The first one is current state and second one is a function which lets you change the state.`}),` `,(0,n.jsx)(e.p,{children:"The set function set the new state and whenever a state is updated , component gets re-renderd."}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.code,{"data-language":"js","data-theme":"github-dark",children:[(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"import"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" {useState} "}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"from"}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:' "react"'})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"export"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" default"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" function"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" Counter"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"() {"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" const"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" ["}),(0,n.jsx)(e.span,{style:{color:"#79B8FF"},children:"count"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:", "}),(0,n.jsx)(e.span,{style:{color:"#79B8FF"},children:"setCount"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"] "}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"="}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" useState"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"("}),(0,n.jsx)(e.span,{style:{color:"#79B8FF"},children:"0"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"); "}),(0,n.jsx)(e.span,{style:{color:"#6A737D"},children:"// <--- react state"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:" return"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" ("})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" <"}),(0,n.jsx)(e.span,{style:{color:"#85E89D"},children:"div"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:">"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" <"}),(0,n.jsx)(e.span,{style:{color:"#85E89D"},children:"h1"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:">Hello react"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" <"}),(0,n.jsx)(e.span,{style:{color:"#85E89D"},children:"h1"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:">{count}"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" <"}),(0,n.jsx)(e.span,{style:{color:"#85E89D"},children:"button"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" onClick"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"="}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"{()"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"=>"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:"setCount"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"("}),(0,n.jsx)(e.span,{style:{color:"#FFAB70"},children:"prev"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"=>"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"prev"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"+"}),(0,n.jsx)(e.span,{style:{color:"#79B8FF"},children:"1"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:")}>Increase Count"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" "})]}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" )"})}),` `,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"}"})})]})})}),` `,(0,n.jsx)(e.p,{children:"set function can directly set a new state to new value"}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsx)(e.code,{"data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:"setCount"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"(count"}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"+"}),(0,n.jsx)(e.span,{style:{color:"#79B8FF"},children:"1"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:")"})]})})})}),` `,(0,n.jsx)(e.hr,{}),` `,(0,n.jsxs)(e.h2,{id:"what-is-the-use-of-useeffect-hooks-in-reactjs",children:["What is the use of useEffect Hooks in reactjs?",(0,n.jsx)(e.a,{"aria-hidden":"true",tabIndex:"-1",href:"#what-is-the-use-of-useeffect-hooks-in-reactjs",children:(0,n.jsx)(e.span,{className:"icon icon-link"})})]}),` `,(0,n.jsx)(e.p,{children:"useEffect lets you invokes side effects from within functional components."}),` `,(0,n.jsxs)(e.h3,{id:"what-is-side-effects",children:["what is side effects?",(0,n.jsx)(e.a,{"aria-hidden":"true",tabIndex:"-1",href:"#what-is-side-effects",children:(0,n.jsx)(e.span,{className:"icon icon-link"})})]}),` `,(0,n.jsxs)(e.p,{children:["A ",(0,n.jsx)(e.strong,{children:"side effect"}),` is anything that affects something outside the scope of the function being executed. These can be a network request, which has your code communicating with a third party api, timer function like setTimeout.`]}),` `,(0,n.jsx)(e.p,{children:`useEffect is the hooks that lets you decouple the logic of rendering from side effects. Below code show data fetching side effects from an api. useEffect() starts a fetch request by calling fetchPosts() async function after the initial rendering. When the request completes, setPosts(posts) updates the posts state with the just fetched posts list.`}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.code,{"data-language":"js","data-theme":"github-dark",children:[(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"import"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" { useEffect, useState } "}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"from"}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:" 'react'"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:";"})]}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"function"}),(0,n.jsx)(e.span,{style:{color:"#B392F0"},children:" FetchPost"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"() 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`,(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:"}"})})]})})}),` `,(0,n.jsx)(e.p,{children:"useEffect needs two arguments"}),` `,(0,n.jsxs)(e.ul,{children:[` `,(0,n.jsx)(e.li,{children:"A setup function which perform the side efects logic. It should return a cleanup function."}),` `,(0,n.jsx)(e.li,{children:`A list of dependencies including every value from your component used inside of those functions. React calls your setup and cleanup functions whenever it\u2019s necessary, which may happen multiple times:`}),` `]}),` `,(0,n.jsx)(e.p,{children:"Setup code runs when component is added to the page (mounts). 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In the future, React may add more features that take advantage of throwing away the cache\u2014for example, if React adds built-in support for virtualized lists in the future, it would make sense to throw away the cache for items that scroll out of the virtualized table viewport. This should match your expectations if you rely on useMemo solely as a performance optimization. Otherwise, a state variable or a ref may be more appropriate."}),` `]}),` `,(0,n.jsxs)(e.h3,{id:"usage",children:["Usage",(0,n.jsx)(e.a,{"aria-hidden":"true",tabIndex:"-1",href:"#usage",children:(0,n.jsx)(e.span,{className:"icon icon-link"})})]}),` `,(0,n.jsx)(e.p,{children:"By default, React will re-run the entire body of your component every time that it re-renders. 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The list of dependencies must have a constant number of items and be written inline like [dep1, dep2, dep3]. React will compare each dependency with its previous value using the Object.is comparison algorithm."}),` `]}),` `]}),` `,(0,n.jsx)(e.p,{children:"useCallback returns the fn function you have passed.During subsequent renders, it will either return an already stored function from the last render (if the dependencies haven\u2019t changed), or return the fn function you have passed during this render."}),` `,(0,n.jsxs)(e.h3,{id:"caveats-1",children:["Caveats",(0,n.jsx)(e.a,{"aria-hidden":"true",tabIndex:"-1",href:"#caveats-1",children:(0,n.jsx)(e.span,{className:"icon icon-link"})})]}),` `,(0,n.jsxs)(e.ul,{children:[` `,(0,n.jsxs)(e.li,{children:[` `,(0,n.jsx)(e.p,{children:"useCallback is a Hook, so you can only call it at the top level of your component or your own Hooks. You can\u2019t call it inside loops or conditions. 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Thanks to useCallback together with memo, it\u2019s fast despite the artificial slowdown! ShippingForm skipped re-rendering because the handleSubmit function has not changed. 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In most cases, developing similar components might not be an issue but, while developing larger applications we need to keep our code DRY, therefore, we want an abstraction that allows us to define this logic in a single place and share it across components. HOC allows us to create that abstraction."}),` `,(0,n.jsx)(e.p,{children:"Example of a HOC:"}),` `,(0,n.jsx)(e.figure,{"data-rehype-pretty-code-figure":"",children:(0,n.jsx)(e.pre,{style:{backgroundColor:"#24292e",color:"#e1e4e8"},tabIndex:"0","data-language":"js","data-theme":"github-dark",children:(0,n.jsxs)(e.code,{"data-language":"js","data-theme":"github-dark",children:[(0,n.jsx)(e.span,{"data-line":"",children:(0,n.jsx)(e.span,{style:{color:"#6A737D"},children:"// HOC.js"})}),` `,(0,n.jsx)(e.span,{"data-line":""}),` `,(0,n.jsxs)(e.span,{"data-line":"",children:[(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"import"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:" React, {Component} "}),(0,n.jsx)(e.span,{style:{color:"#F97583"},children:"from"}),(0,n.jsx)(e.span,{style:{color:"#9ECBFF"},children:" 'react'"}),(0,n.jsx)(e.span,{style:{color:"#E1E4E8"},children:";"})]}),` `,(0,n.jsx)(e.span,{"data-line":""}),` 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#react

What’s New in React19: Key Features and Enhancements

React

Top reactjs questions asked in interview

interview