React Component Lifecycle

React component lifecycle is a serial of methods that are automatically called by react at certain points during. This method are an optional methods. We can optionally define inside our class-based components.

Basically all the React components lifecycle methods has split in four phases: initialization, mounting, updating and unmounting. Let’s take a closer look at each one of them.

The following image is the visual representation of the phases and the methods of ReactJs lifecycle.


Initialization

This is the first stage in the ReactJS lifecycle. In this phase the developer has to define the props (properties) and initial state of the component this is generally done in the constructor of the component. The following example describes the initialization process

constructor(props) { 
   super(props); 
   this.state = { hello : "World!" }; 
}

Mounting

Mounting means putting elements into the DOM.

Mounting is the phase of the component lifecycle when the initialization of the component is completed and the component is mounted on the DOM and rendered for the first time in the webpage. Mounting phase consists of two such predefined functions as described below.

  • componentWillMount() Function: As the name clearly suggests, this function is invoked right before the component is mounted on the DOM i.e. this function gets invoked once before the render() function is executed for the first time.
  • componentDidMount() Function: This method runs after the component output has been rendered to the DOM i.e. this function gets invoked once after the render() function is executed for the first time.

Updation

Updation is the phase where the states and props of a component are updated followed by some user events such as clicking, pressing a key on keyboard etc. The following are the descriptions of functions that are invoked at different points of Updation phase.

componentWillRecieveProps() Function: This is a Props exclusive Function and is independent of States. This function is invoked before a mounted component gets its props reassigned. The function is passed the new set of Props which may or may not be identical to the original Props. Thus checking is a mandatory step in this regards. The following code snippet shows a sample use-case.

componentWillRecieveProps(newProps) 
{ 
    if (this.props !== newProps) { 
        console.log(" New Props have been assigned "); 
        // Use this.setState() to rerender the page. 
    } 
}
  • setState() Function: This is not particularly a Lifecycle function and can be invoked explicitly at any instant. This function is used to update the State of a component. You may refer to this article for detailed information.
  • shouldComponentUpdate() Function: By default, every state or props update re-render the page but this may not always be the desired outcome, sometimes it is desired that on updating the page will not be repainted. The shouldComponentUpdate() Function fulfills the requirement by letting React know whether the component’s output will be affected by the update or not.

    shouldComponentUpdate() is invoked before rendering an already mounted component when new props or state are being received. If returned false then the subsequent steps of rendering will not be carried out. This function can’t be used in case of forceUpdate(). The Function takes the new Props and new State as the arguments and returns whether to re-render or not.
  • componentWillUpdate() Function: As the name clearly suggests, this function is invoked before the component is rerendered i.e. this function gets invoked once before the render() function is executed after the updation of State or Props.
  • componentDidUpdate() Function: Similarly this function is invoked after the component is rerendered i.e. this function gets invoked once after the render() function is executed after the updation of State or Props.

Unmounting

This is the final phase of the lifecycle of the component that is the phase of unmounting the component from the DOM. The following function is the sole member of this phase.

  • componentWillUnmount() Function: This function is invoked before the component is finally unmounted from the DOM i.e. this function gets invoked once before the component is removed from the page and this denotes the end of the lifecycle.

We have so far discussed every predefined function there was in the lifecycle of the component and we have also specified the order of execution of the function. Let us now see one final example to finish of the article while revising what’s discussed above.

Creating new component

Create new component and put the below code on it and import in the index.js file

import React from 'react'; 
import ReactDOM from 'react-dom'; 
  
class ComponentLifecycle extends React.Component { 
    
    constructor(props) { 
        super(props); 
        this.state = { hello : "World!" }; 
    } 
  
    componentWillMount() { 
        console.log("componentWillMount()"); 
    } 
  
    componentDidMount() { 
        console.log("componentDidMount()"); 
    } 
  
    changeState() { 
        this.setState({ hello : "W3path!" }); 
    } 
    
    shouldComponentUpdate(nextProps, nextState) { 
        console.log("shouldComponentUpdate()"); 
        return true; 
    } 
  
    componentWillUpdate() { 
        console.log("componentWillUpdate()"); 
    } 
  
    componentDidUpdate() { 
        console.log("componentDidUpdate()"); 
    } 

    componentWillUnmount() {
        console.log('componentWillUnmount()');
    }

    render() 
    { 
        return ( 
            <div className="mt-5"> 
                <h2>Hello { this.state.hello }</h2> 
                <button className="btn btn-primary mt-2" onClick={this.changeState.bind(this)}>Press Here!</button>
            </div>
        ); 
    }
}

export default ComponentLifecycle;

Running the App

Run application using npm start.

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