React set class when component is loaded

MyComponent.js

import React from 'react';

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      didMount: false,
    };
  }

  componentDidMount() {
    // componentDidMount() is only executed once so
    // we're using setInterval() instead of setTimeout()
    // to schedule it periodically
    this.delay = setInterval(() =>
      this.setState(prevState => ({ didMount: !prevState.test })), 500);
  }

  // Called when a component is being removed from the DOM
  // Perfect timing to clear our delay
  componentWillUnmount() {
    clearInterval(this.delay);
  }

  render() {
    const { didMount } = this.state;

    return (
      <div className={`${didMount && 'loaded'}`}>
        <p>Hello, World!</p>
      </div>
    );
  }
}

export default MyComponent;

• setInterval() documentation
• componentDidMount() documentation
• componentWillUnmount() documentation