Why does useEffect run two times?

How to fix the useEffect runs twice problem

React 18 released in March 2022 changed the default behavior of useEffect().

I didn’t even realize it at first, reading the React 18 launch post, buried along a lot of new features.

If your application is behaving strangely after updating to React 18, the default behavior of useEffect changed to run it 2 times.

Just in development mode, but this is the mode everyone builds their application on.

And just in strict mode, but that’s now the default for applications built using create-react-app or Next.js.

There are reasons for this.

It’s not a problem of your code - it’s how things work now in React.

The only way to disable this behavior is to disable strict mode.

Strict mode is important so this is a temporary workaround until you can fix any issue this change introduced.

You can do it in Next.js by adding the option

reactStrictMode: false

in your next.config.js file.

In create-react-app you can go in your index.js file and remove the StrictMode higher order component, from:

import React, { StrictMode } from 'react';
import { createRoot } from 'react-dom/client';

import App from './App';

const rootElement = document.getElementById('root');
const root = createRoot(rootElement);

root.render(
  <StrictMode>
    <App />
  </StrictMode>
);

to

import React from 'react';
import { createRoot } from 'react-dom/client';

import App from './App';

const rootElement = document.getElementById('root');
const root = createRoot(rootElement);

root.render(
  <App />
);

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