CODEWITHSUNDEEP
reactjsuse-effect
Sam Archer
Sam Archer

Reputation: 301

Basic React question. why is useEffect needed in this example from the React documentation?

This example is given in the react documentation to describe the use of useEffect.

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

function Example() {
  const [count, setCount] = useState(0);

  // Similar to componentDidMount and componentDidUpdate:
  useEffect(() => {
    // Update the document title using the browser API
    document.title = `You clicked ${count} times`;
  });

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

Could this not be re-written without using useEffect as below? Why is useEffect preferred?

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

function Example() {
  const [count, setCount] = useState(0);

  const handleChange = () => {
      setCount(count + 1);
      document.title = `You clicked ${count} times`;
  }

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={handleChange}>
        Click me
      </button>
    </div>
  );
}

Upvotes: 1

Views: 39

Answers (1)

CertainPerformance
CertainPerformance

Reputation: 370669

There are two issues here:

  • Calling a state setter won't reassign the stateful const variable in the current render - you need to wait until the next render for count to be updated, so

    const handleChange = () => {
    setCount(count + 1);
    document.title = `You clicked ${count} times`;
}

    won't work. Your second snippet will result in You clicked 0 times after a single click.

  • Even if that were possible, when there are multiple places where a state setter may be called, putting something that should happen afterwards after every single one isn't very maintainable - it'd be better to be able to put that code to run afterwards in only one place.

Upvotes: 1

Related Questions