ReactJS – Cleaning up with useEffect hook

In this article, we are going to see how to clean up the subscriptions set up in the useEffect hook in the functional component.

Once the effects are created, then they are needed to be cleaned up before the component gets removed from the DOM. For this, cleaning up effect is used to remove the previous useEffect hook’s effect before using this hook of the same component again.

Syntax

useEffect(()=>{
   return ()=>{}
}
,[]);

Example

In this example, we will build a React application which displays the coordinates of the mouse pointer when it is moved over the screen. For this to implement, we will write the code with both cleanup effects and without it.

Without Clean Up Effect

Example

App.jsx

import React, { useEffect, useState } from 'react';

function App() {
   return (
      <div className="App">
      <Comp />
      </div>
   );
}

function Comp() {

   useEffect(() => {
      document.addEventListener('mousemove', mouseHandler);
   }, []);

   const mouseHandler = (e) => {
      console.log(e.clientX, e.clientY);
   };

   return (
      <div>
         <h1>Tutorialspoint</h1>
      </div>
   );
}
export default App;

In the above example, we are not removing the previous useEffect hook’s data, which is affecting the new data returned by this hook.

Output

This will produce the following result.

With Clean Up Effect

Example

App.jsx

import React, { useEffect, useState } from 'react';

function App() {
   return (
      <div className="App">
         <Comp />
      </div>
   );
}

function Comp() {

   useEffect(() => {
      document.addEventListener('mousemove', mouseHandler);
      return () => {
         document.removeEventListener('mousemove', mouseHandler);
      };
   }, []);

   const mouseHandler = (e) => {
      console.log(e.clientX, e.clientY);
   };
   return (
      <div>
         <h1>Tutorialspoint</h1>
      </div>
   );
}
export default App;

In the above example, useEffect hook is called with the cleanup effect and thus, the effect of this hook will get removed every time the component gets destroyed.

Output

This will produce the following result.