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First we have to understand two main conceptsSynchronous programmingAsynchronous programmingSYNCHRONOUS PROGRAMMINGIt waits for each statement to complete its execution before going to next statement.This approach can slow down application process if statements are not dependent on each other but still they are waiting for execution as they are in queue.ASYNCHRONOUS PROGRAMMINGIt does not wait for current statement to complete its execution before moving to next statement. e.g. calling a web service and executing file copy in JavaScript.The call to web service can take some time to return a result meanwhile we can complete some other actions.Once server provides the result, ... Read More
Styling in React.js can be done in below two wayscss style sheetsinline styleLet’s see CSS style sheets firstWe have App.js file as shown below −import React, {Component} from 'react'; import './App.css'; class App extends Component { render(){ return ( Styling React Components ); } } export default App;In App.js file we have imported an App.css file which contains css class myColoredTextPlease noteWe used name of css class in double quotes with attribute classNameJSX uses ... Read More
Generally the return statement in React’s render method returns a single div element enclosing all the child jsx like below −render(){ return ( Header Test ); }Here we cannot simply return multiple elements, we need to have a parent container similar to div as shown above.If no parent container element, then we can even return an array like below −While returning an array we will require an unique key to be given to each element of an arrayrender(){ return ( ... Read More
Before React Router v4, the routing in react was static but after v4 update its dynamic. In single page applications, we have only one page index.html and different components which gets displayed based on routes.In non spa web applications, server returns a page on request. To start with routing , first install the routing package −npm install –save react-router-domOnce we create the create the project with create-react-app, we will see there is only one html file and that is index.html and a single component named as AppNow, we will create two more components AboutUs.jsx and ContactUs.jsxAboutUs.jsximport React from 'react' class ... Read More
There are two main methods in error handling. These method are used in the error boundary mechanism in React. We wrap the component ( from where there is a possibility of error occurrences ) in a class which handles the below methods.Static method getDerivedStateFromError(error)componentDidCatch(error, info)Static getDerivedStateFromError(error): As name of the method suggest, we can update state object here based on error received from descended component.componentDidCatch(error, info): we can log the errors using api call. This is helpful in displaying useful error message on screen instead of technical errors.A class can be termed as an error boundary if it implements at ... Read More
The update lifecycle can be triggered by two events −Update of props by parent componentChange in local stateUpdate of props by parent componentcomponentWillReceiveProps(nextProps) −This is the first method which gets called on prop change. It contains one argument nextProps means the newly changed props.Here, we can synchronize local state with props if requiredIf state is not needed to synchronize with props then probably no need to implement this method.Causing side effects should be avoided here. Because if side effects used here, it may results into re-render or performance issues.Now, componentWillReceiveProps is deprecated and replaced by static method getDerivedStateFromProps(props, state). As ... Read More
ComponentWillUnmount is the only method that executes in unmount phase.Component enters into this phase when there is no matching in element tree for this component.Just before the component gets removed from actual DOM, this method gets called.Along with removal of this component from DOM tree, all children of this component also gets removed automatically.Once component is removed from the DOM, it gets available for the garbage collection in React.Cleanup activities can be done in this method. E.g. clear localstorage variables used in app, clear session, clean up charts, cleanup timers, cancel pending api requests etc.componentWillUnmount(){ this.resetSession(); //example method to ... Read More
Each stateful means class based component goes through four types of lifecycle phases.Mounting or CreationUpdatingUnmounting or DestroyError handling phaseReact.js Component Lifecycle - Mounting phaseThe methods that executes during creation lifecycle are −constructorcomponentWillMount (Only available till version 17 )rendercomponentDidMountconstructor ()It’s an ES6 feature and not a React provided method.React uses the constructor to pass the props to parent Component extended from React library.constructor(props){ super( props ); }Passing of props to parent Component helps us to use this.props in the component.We can initialize the state for the component in constructor. Important point is we should not use setState method in constructor. ... Read More
We have a lifecycle method called shouldComponentUpdate which by default returns true (Boolean) value.The purpose of the shouldComponentUpdate is we can custom implement the default behavior and decide when react should update or re-render the component.Generally we use state or props value to decide the update cycle. React has now provided us a PureComponent which does the comparison of state and props to decide the update cycle. We don’t need to override shouldComponentUpdate if we extend class with PureComponent.React does the shallow comparisons of current state and props with new props and state to decide whether to continue with next ... Read More
We have an app.jsx componentimport React, { Component } from 'react'; import { Link, Route, Switch } from 'react-router-dom'; import Category from './Category'; class App extends Component { render() { return ( Main Page Users ... Read More