Embrace the "effects" running twice in ReactJS

May 29th, 2022

Gaurav Thakur

As per the React documentation, whenever a component mounts for the first time, React will unmount and mount it again. It is important to note that this behavior only occurs in development with the strict mode on. In production, all components will be mounted only once.

1export default function App() {
2    useLayoutEffect(() => {
3        console.log('useLayoutEffect: effect called');
4    }, []);
5
6    useEffect(() => {
7        console.log('useEffect: effect called');
8    }, []);
9
10  return (
11      <div>
12        <p>hello world</p>
13      </div>
14  );
15}
16

Due to this behavior, all the effects (useEffect and useLayoutEffect) will run twice. Running the above component in React 18 will display the below results

console output with effect called twice

In the beginning, it may seem like an odd behavior from React, but later in the post we will explore the reason behind it and why we should embrace it rather than consider it a bug.

So far, we have created a mental model that an empty dependency array will only run the effect once. It becomes a problem when we write effects in such a way that calling them twice with the same values breaks the application or causes a bug. With the help of Strict mode, the React team is trying to identify these issues early in development.

In your mind you may think, "this is specific to stick mode, and if we opt out of stick mode then we don't need to worry about how effect reacts to multiple calls because empty dependencies will invoke effect only once". In fact, this scenario is very likely to occur even if the strict mode is removed.

Consider a scenario in which the user clicks on a link and then returns to the original screen immediately. In this case, the component will first mount, and then as the user navigates to another screen, it will unmount, and when the user returns to the original screen, it will mount again.

1function ChatScreen() {
2    useEffect(() => {
3       const {connect, connection} = dummyServer();
4       connect();
5       connection.on(() => {
6           console.log('connected');
7       })
8    }, []);
9
10    return (
11        <div>
12            <p>Chat Screen</p>
13            <a href='/'>Go to home page</a>
14        </div>
15    )
16}
17

Here in the above example, we are connecting the user to the chat server as soon as ChatScreen component mounts. Without the Strict mode turned on, the effect will only run once and the user will only be connected to the server once. But, things can get messy if a user clicks on the "Get back to home page" link then immediately moves back to the original screen, mounting the ChatScreen component all over again. As a result, the two connections will be created for the same user, causing the bug later on or now depending on the logic.

It is easy to capture these types of issues using React's Strict mode early in the development, so we do not have to wait for support requests to come in. So, what should be done in such a case? The fix is simple: add a cleanup function.

1function Chat() {
2    useEffect(() => {
3       const {connect, disconnect, connection} = dummyServer();
4       connect();
5       connection.on(() => {
6           console.log('connected');
7       });
8       return disconnect; // cleanup
9    }, []);
10
11    return (
12        <div>
13            <p>Chat Screen</p>
14            <a href='/'>Go to home page</a>
15        </div>
16    )
17}
18

A cleanup function like this will ensure only a single connection can be made, regardless of how many times the component is mounted.

Conclusion#

The React uses Strict mode to identify issues as early as possible during the development process, which involves running the effect twice. Instead of finding a workaround to solve these problems we should sit back and see why our logic fails with multiple effect calls. It will help us to make our components future-proof and ensure that they are edge case free.

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Written By Gaurav Thakur

Gaurav is a software engineer and JavaScript enthusiast. He is a full stack developer, more inclined to frontend development. He enjoys talking about React, JavaScript, and web development in general. He usually writes about what he learns or experiences in his daily life. His roots are from Kullu in Himachal Pradesh, but he now resides in Hyderabad because of his job. Learn more about Gaurav.

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