React Hooks : Demystifying the useEffect hook in under 15 minutes [examples + video]

import { useState, useEffect} from "react"; 

const Effect = props => { 
  const [counter, setCounter] = useState(0);
  useEffect(() => { 
    console.log('In Effect.js, [AFTER RENDER]');
  }); 
  return ( 
    <div>
      <button onClick={() => setCounter(prevCounter => prevCounter + 1)}>Increment Counter</button>
      <h1>Current Value : {counter}</h1>
      <button onClick={() => setCounter(prevCounter => prevCounter - 1)}>Decrement Counter</button>
    </div>
  )
}

export default Effect;

import { useState, useEffect } from "react"; 

const APICallEffect = props => { 
  const [todo, setTodo] = useState(null); 
  const [id, setId] = useState(1); 
  const fetchTodo = (id) => { 
    return fetch('https://jsonplaceholder.typicode.com/todos/' + id)
    .then(response => response.json()); 
  }
  useEffect(() => { 
    console.log('Effect ran...');
    fetchTodo(id)
     .then(setTodo); 
  },[id]);

  const incrementId = () => setId(prevId => prevId + 1); 

  if(todo === null) return <h1>Loading...</h1>
  return ( 
     <div>
        <h1>id : {id}, todo : {todo.title}</h1>
        <button onClick={incrementId}>incrementId</button>
     </div>
  )
}
export default APICallEffect;

import { useState, useEffect, Fragment  } from "react"; 

const EffectWithCleanup = props => { 
  const[xCoordinate, setXCoordinate] = useState(0); 
  const[yCoordinate, setYCoordinate] = useState(0); 
  const handleCoordinateUpdate = event => { 
    console.log(event.clientX, event.clientY); 
    setXCoordinate(event.clientX); 
    setYCoordinate(event.clientY); 
  }

  useEffect(() => { 
    console.log('useEffect ran...'); 
    window.addEventListener('mousemove', handleCoordinateUpdate); 
    return () => {
      console.log('cleanup....');
      window.removeEventListener('mousemove',handleCoordinateUpdate);
    }
  },[]);
  return ( 
    <Fragment> 
      <div>
      <h1>X coordinate: {xCoordinate}</h1>
      <h1>Y coordinate: {yCoordinate} </h1>
      </div>
    </Fragment>
  )
}
export default EffectWithCleanup;