How to change body class before component is mounted in react?

We can change body class before the component is mounted in ReactJS using the following approach:

Prerequisite: React Component LifeCycle

The lifecycle of react components is as follows:

1. Initialization
2. Mounting
3. Updating
4. Unmounting

Example: Changing the body class before mounting in the Initialization stage. The sequence of Execution:

Javascript

import React,{ Component }  from 'react';

class App extends Component {

  // Gets called 1st place
  constructor(props) {
    super(props);
  }

  // Gets called at 2nd place
  render() {
    return (
        <h1>GeeksforGeeks</h1>
    );
  }

  // Gets called at 3rd place
  componentDidMount(){
     // Code
  }
}

export default App;

Explanation: In the Initialization stage, the first thing that gets called is the Constructor of the component. We will use the Constructor to make our changes. We can get the reference of the body element in the constructor using the following code:

let bodyElement = document.getElementsByTagName('body')[0];

Creating React Application:

Step 1: Create a React application using the following command:

npx create-react-app foldername

Step 2: After creating your project folder i.e. foldername, move to it using the following command:

cd foldername

Project Structure: It will look like the following.

Project Structure

Filename-App.css:

css

* {
  margin: 30px;
}

h1 {
  color: white;
  text-shadow: 1px 1px black;
}

.active {
  background-color: green;
}

Filename-App.js:

Javascript

import React, { Component } from 'react';
import "./App.css"

class App extends Component {

  constructor(props) {

    // This will call the constructor of the parent
    super(props);

    // Taking reference of body element
    let bodyElement = document.getElementsByTagName('body')[0];

    // Changing the class of body Before mounting
    bodyElement.className = "active";
  }

  componentDidMount() {
  }

  render() {
    return (
      <h1>Geeks For Geeks</h1>
    );
  }
}

export default App;

Step to Run Application: Run the application using the following command from the root directory of the project:

npm start

Output: Now open your browser and go to http://localhost:3000/, you will see the following output:

• Before Component Is Mounted:

• After Component Is Mounted: