React - useEffect

useEffect is a React Hook that lets you synchronize a component with an external system.

useEffect(setup, dependencies?) 

In my experience, it mainly used for initialization or need to use some variables to render the current component

Reference

import { useEffect } from 'react';
import { createConnection } from './chat.js';

function ChatRoom({ roomId }) {
  const [serverUrl, setServerUrl] = useState('https://localhost:1234');

  useEffect(() => {
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
    return () => {
      connection.disconnect();
    };
  }, [serverUrl, roomId]);
  // ...
}