I recently upgraded my React app to version 18, and I noticed that the useEffect hook is running twice when the component mounts. This behavior wasn’t happening before, and it only seems to occur in development mode.

Here's a simplified version of the component:

1import { useEffect } from 'react';
2
3function MyComponent() {
4  useEffect(() => {
5    console.log("Effect ran");
6  }, []);
7
8  return <div>Hello World</div>;
9}
10

1import { useEffect } from 'react';
2
3function MyComponent() {
4  useEffect(() => {
5    console.log("Effect ran");
6  }, []);
7
8  return <div>Hello World</div>;
9}
10

I see the console log twice on initial load. Why is this happening, and should I be concerned about side effects running multiple times? Is this related to React's Strict Mode?

Why Does `useEffect` Run Twice in Development Mode in React 18?

In React 18, you may notice that the useEffect hook runs twice when a component mounts in development mode. This behavior is intentional and is primarily related to React's Strict Mode.

Explanation

Strict Mode:
- React's Strict Mode is a tool for highlighting potential problems in an application. It activates additional checks and warnings for its descendants.
- In development mode, React intentionally invokes certain lifecycle methods (including useEffect) twice to help identify side effects that may not be handled correctly. This is to ensure that your effects are idempotent and can handle being called multiple times without causing issues.
Purpose:
- The double invocation helps developers catch bugs related to side effects that might not be apparent when effects run only once. It encourages writing effects that are resilient and can handle being executed multiple times.
Production Behavior:
- It's important to note that this behavior only occurs in development mode. In production builds, useEffect will run as expected, only once on mount.

Should You Be Concerned?

No Immediate Concern: If your effect is designed to be idempotent (i.e., it can run multiple times without adverse effects), then you should not be concerned.
Testing for Side Effects: Use this opportunity to ensure that your side effects are correctly implemented and can handle being called multiple times.

Example Component

Here’s your simplified component for reference:

1import { useEffect } from 'react';
2
3function MyComponent() {
4  useEffect(() => {
5    console.log("Effect ran");
6  }, []);
7
8  return <div>Hello World</div>;
9}

1import { useEffect } from 'react';
2
3function MyComponent() {
4  useEffect(() => {
5    console.log("Effect ran");
6  }, []);
7
8  return <div>Hello World</div>;
9}

In this example, you will see "Effect ran" logged twice in the console during development mode due to the reasons mentioned above.

Conclusion

In summary, the double invocation of useEffect in development mode is a feature of React's Strict Mode designed to help developers identify potential issues with side effects. It does not affect the production build, so you can focus on ensuring your effects are robust and handle multiple invocations gracefully.