what does useeffect do ? in typescript

useEffect is a hook in React that allows you to perform side effects in function components. This hook can be used to fetch data, set up subscriptions, and manually trigger re-renders, among other things. When you call useEffect, you pass in a function that will be executed after the component has rendered. You can also provide an array of dependencies, so that the effect will only run if any of those dependencies change. Here's an example of useEffect in TypeScript:

index.ts
import React, { useEffect } from 'react';

const ExampleComponent: React.FC = () => {
  // This effect will run after the first render
  useEffect(() => {
    console.log('Component has rendered');
    
    // Cleanup function (optional), will run before the component unmounts
    return () => {
      console.log('Component is unmounting');
    };
  }, []); // Empty dependency array ensures the effect runs only once

  return <div>Hello, World!</div>;
};

export default ExampleComponent;
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In this example, the useEffect hook is being used to log a message after rendering the component. The empty dependency array [] indicates that the effect should only run once (equivalent to componentDidMount in class components).