side effects react hook code example

Example 1: react useEffect

import React, { useEffect, useState } from 'react';
import ReactDOM from 'react-dom';

function LifecycleDemo() {
  // It takes a function
  useEffect(() => {
    // This gets called after every render, by default
    // (the first one, and every one after that)
    console.log('render!');

    // If you want to implement componentWillUnmount,
    // return a function from here, and React will call
    // it prior to unmounting.
    return () => console.log('unmounting...');
  }, [ // dependencies to watch = leave blank to run once or you will get a stack overflow  ]);

  return "I'm a lifecycle demo";
}

function App() {
  // Set up a piece of state, just so that we have
  // a way to trigger a re-render.
  const [random, setRandom] = useState(Math.random());

  // Set up another piece of state to keep track of
  // whether the LifecycleDemo is shown or hidden
  const [mounted, setMounted] = useState(true);

  // This function will change the random number,
  // and trigger a re-render (in the console,
  // you'll see a "render!" from LifecycleDemo)
  const reRender = () => setRandom(Math.random());

  // This function will unmount and re-mount the
  // LifecycleDemo, so you can see its cleanup function
  // being called.
  const toggle = () => setMounted(!mounted);

  return (
    <>
      <button onClick={reRender}>Re-render</button>
      <button onClick={toggle}>Show/Hide LifecycleDemo</button>
      {mounted && <LifecycleDemo/>}
    </>
  );
}

ReactDOM.render(<App/>, document.querySelector('#root'));

Example 2: componentwillunmount hooks

useEffect(() => {
  window.addEventListener('mousemove', () => {});

  // returned function will be called on component unmount 
  return () => {
    window.removeEventListener('mousemove', () => {})
  }
}, [])

Example 3: React looping hooks to display in other hook

import React, { useState } from "react";

function App() {

  const [list, setList] = useState(" ");

  const [items, updateOnClick] = useState([ ]);

  function updateList(event) {
    const valueEntered = event.target.value;
    setList(valueEntered);
  }

  function updateClick(){
     updateOnClick(list);
     }
 return (
           <input onChange={updateList} type="text" value={list} />
            <button onClick={updateClick}>
              <span>Add</span>
            </button>
            <div>
              <ul>
                {items.map(item => <li>{item}</li>) }
              </ul>
            </div>);}
export default App;