Lifecycle of Reactive Effects

The lifecycle of an Effect

Every React component goes through the same lifecycle:

• A component mounts when it’s added to the screen.
• A component updates when it receives new props or state. This usually happens in response to an interaction.
• A component unmounts when it’s removed from the screen.

It’s a good way to think about components, but not about Effects. Instead, try to think about each Effect independently from your component’s lifecycle. An Effect describes how to synchronize an external system to the current props and state. As your code changes, this synchronization will need to happen more or less often.

To illustrate this point, consider this Effect connecting your component to a chat server:

const serverUrl = 'https://localhost:1234';

function ChatRoom({ roomId }) {
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
    return () => {
      connection.disconnect();
    };
  }, [roomId]);
  // ...
}

Your Effect’s body specifies how to start synchronizing:

    // ...
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
    return () => {
      connection.disconnect();
    };
    // ...

The cleanup function returned by your Effect specifies how to stop synchronizing:

    // ...
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
    return () => {
      connection.disconnect();
    };
    // ...

Intuitively, you might think that React would start synchronizing when your component mounts and stop synchronizing when your component unmounts. However, this is not the end of the story! Sometimes, it may also be necessary to start and stop synchronizing multiple times while the component remains mounted.

Let’s look at why this is necessary, when it happens, and how you can control this behavior.

Why synchronization may need to happen more than once

Imagine this ChatRoom component receives a roomId prop that the user picks in a dropdown. Let’s say that initially the user picks the "general" room as the roomId. Your app displays the "general" chat room:

const serverUrl = 'https://localhost:1234';

function ChatRoom({ roomId /* "general" */ }) {
  // ...
  return <h1>Welcome to the {roomId} room!</h1>;
}

After the UI is displayed, React will run your Effect to start synchronizing. It connects to the "general" room:

function ChatRoom({ roomId /* "general" */ }) {
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId); // Connects to the "general" room
    connection.connect();
    return () => {
      connection.disconnect(); // Disconnects from the "general" room
    };
  }, [roomId]);
  // ...

So far, so good.

Later, the user picks a different room in the dropdown (for example, "travel"). First, React will update the UI:

function ChatRoom({ roomId /* "travel" */ }) {
  // ...
  return <h1>Welcome to the {roomId} room!</h1>;
}

Pause to think about what should happen next. The user sees that "travel" is the selected chat room in the UI. However, the Effect that ran the last time is still connected to the "general" room. The roomId prop has changed, so whatever your Effect did back then (connecting to the "general" room) no longer matches the UI.

At this point, you want React to do two things:

1. Stop synchronizing with the old roomId (disconnect from the "general" room)
2. Start synchronizing with the new roomId (connect to the "travel" room)

Luckily, you’ve already taught React how to do both of these things! Your Effect’s body specifies how to start synchronizing, and your cleanup function specifies how to stop synchronizing. All that React needs to do now is to call them in the correct order and with the correct props and state. Let’s see how exactly that happens.

How React re-synchronizes your Effect

Recall that your ChatRoom component has received a new value for its roomId prop. It used to be "general", and now it is "travel". React needs to re-synchronize your Effect to re-connect you to a different room.

To stop synchronizing, React will call the cleanup function that your Effect returned after connecting to the "general" room. Since roomId was "general", the cleanup function disconnects from the "general" room:

function ChatRoom({ roomId /* "general" */ }) {
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId); // Connects to the "general" room
    connection.connect();
    return () => {
      connection.disconnect(); // Disconnects from the "general" room
    };
    // ...

Then React will run the Effect that you’ve provided during this render. This time, roomId is "travel" so it will start synchronizing to the "travel" chat room (until its cleanup function is eventually called too):

function ChatRoom({ roomId /* "travel" */ }) {
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId); // Connects to the "travel" room
    connection.connect();
    // ...

Thanks to this, you’re now connected to the same room that the user chose in the UI. Disaster averted!

Every time after your component re-renders with a different roomId, your Effect will re-synchronize. For example, let’s say the user changes roomId from "travel" to "music". React will again stop synchronizing your Effect by calling its cleanup function (disconnecting you from the "travel" room). Then it will start synchronizing again by running its body with the new roomId prop (connecting you to the "music" room).

Finally, when the user goes to a different screen, ChatRoom unmounts. Now there is no need to stay connected at all. React will stop synchronizing your Effect one last time and disconnect you from the "music" chat room.

Thinking from the Effect’s perspective

Let’s recap everything that’s happened from the ChatRoom component’s perspective:

1. ChatRoom mounted with roomId set to "general"
2. ChatRoom updated with roomId set to "travel"
3. ChatRoom updated with roomId set to "music"
4. ChatRoom unmounted

During each of these points in the component’s lifecycle, your Effect did different things:

1. Your Effect connected to the "general" room
2. Your Effect disconnected from the "general" room and connected to the "travel" room
3. Your Effect disconnected from the "travel" room and connected to the "music" room
4. Your Effect disconnected from the "music" room

Now let’s think about what happened from the perspective of the Effect itself:

  useEffect(() => {
    // Your Effect connected to the room specified with roomId...
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
    return () => {
      // ...until it disconnected
      connection.disconnect();
    };
  }, [roomId]);

This code’s structure might inspire you to see what happened as a sequence of non-overlapping time periods:

1. Your Effect connected to the "general" room (until it disconnected)
2. Your Effect connected to the "travel" room (until it disconnected)
3. Your Effect connected to the "music" room (until it disconnected)

Previously, you were thinking from the component’s perspective. When you looked from the component’s perspective, it was tempting to think of Effects as “callbacks” or “lifecycle events” that fire at a specific time like “after a render” or “before unmount”. This way of thinking gets complicated very fast, so it’s best to avoid it.

Instead, always focus on a single start/stop cycle at a time. It shouldn’t matter whether a component is mounting, updating, or unmounting. All you need to do is to describe how to start synchronization and how to stop it. If you do it well, your Effect will be resilient to being started and stopped as many times as it’s needed.

This might remind you how you don’t think whether a component is mounting or updating when you write the rendering logic that creates JSX. You describe what should be on the screen, and React figures out the rest.

How React verifies that your Effect can re-synchronize

Here is a live example that you can play with. Press “Open chat” to mount the ChatRoom component:

Notice that when the component mounts for the first time, you see three logs:

1. ✅ Connecting to "general" room at https://localhost:1234... (development-only)
2. ❌ Disconnected from "general" room at https://localhost:1234. (development-only)
3. ✅ Connecting to "general" room at https://localhost:1234...

The first two logs are development-only. In development, React always remounts each component once. In other words, React verifies that your Effect can re-synchronize by forcing it to do that immediately in development. This might remind you how you might open the door and close it an extra time to check that the door lock works. React starts and stops your Effect one extra time in development to check you’ve implemented its cleanup well.

The main reason your Effect will re-synchronize in practice is if some data it uses has changed. In the sandbox above, change the selected chat room. Notice how, when the roomId changes, your Effect re-synchronizes.

However, there are also more unusual cases in which re-synchronization is necessary. For example, try editing the serverUrl in the sandbox above while the chat is open. Notice how the Effect re-synchronizes in response to your edits to the code. In the future, React may add more features that take advantage of re-synchronization.

How React knows that it needs to re-synchronize the Effect

You might be wondering how React knew that your Effect needed to re-synchronize after roomId changes. It’s because you told React that this Effect’s code depends on roomId by including it in the list of dependencies:

function ChatRoom({ roomId }) { // The roomId prop may change over time
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId); // This Effect reads roomId 
    connection.connect();
    return () => {
      connection.disconnect();
    };
  }, [roomId]); // So you tell React that this Effect "depends on" roomId
  // ...

Here’s how this works:

1. You knew roomId is a prop, which means it can change over time.
2. You knew that your Effect reads roomId (so its logic depends on a value that may change later).
3. This is why you specified it as your Effect’s dependency (so that it re-synchronizes when roomId changes).

Every time after your component re-renders, React will look at the array of dependencies that you have passed. If any of the values in the array is different from the value at the same spot that you passed during the previous render, React will re-synchronize your Effect. For example, if you passed ["general"] during the initial render, and later you passed ["travel"] during the next render, React will compare "general" and "travel". These are different values (compared with Object.is), so React will re-synchronize your Effect. On the other hand, if your component re-renders but roomId has not changed, your Effect will remain connected to the same room.

Each Effect represents a separate synchronization process

Resist adding unrelated logic to your Effect only because this logic needs to run at the same time as an Effect you already wrote. For example, let’s say you want to send an analytics event when the user visits the room. You already have an Effect that depends on roomId, so you might feel tempted to add the analytics call right there:

function ChatRoom({ roomId }) {
  useEffect(() => {
    logVisit(roomId);
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
    return () => {
      connection.disconnect();
    };
  }, [roomId]);
  // ...
}

But imagine you later add another dependency to this Effect that needs to re-establish the connection. If this Effect re-synchronizes, it will also call logVisit(roomId) for the same room, which you did not intend. Logging the visit is a separate process from connecting. This is why they should be written as two separate Effects:

function ChatRoom({ roomId }) {
  useEffect(() => {
    logVisit(roomId);
  }, [roomId]);

  useEffect(() => {
    const connection = createConnection(serverUrl, roomId);
    // ...
  }, [roomId]);
  // ...
}

Each Effect in your code should represent a separate and independent synchronization process.

In the above example, deleting one Effect wouldn’t break the other Effect’s logic. This is a good indication that they synchronize different things, and so it made sense to split them up. On the other hand, if you split up a cohesive piece of logic into separate Effects, the code may look “cleaner” but will be more difficult to maintain. This is why you should think whether the processes are same or separate, not whether the code looks cleaner.

Effects “react” to reactive values

Your Effect reads two variables (serverUrl and roomId), but you only specified roomId as a dependency:

const serverUrl = 'https://localhost:1234';

function ChatRoom({ roomId }) {
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
    return () => {
      connection.disconnect();
    };
  }, [roomId]);
  // ...
}

Why doesn’t serverUrl need to be a dependency?

This is because the serverUrl never changes due to a re-render. It’s always the same no matter how many times and with which props and state the component re-renders. Since serverUrl never changes, it wouldn’t make sense to specify it as a dependency. After all, dependencies only do something when they change over time!

On the other hand, roomId may be different on a re-render. Props, state, and other values declared inside the component are reactive because they’re calculated during rendering and participate in the React data flow.

If serverUrl was a state variable, it would be reactive. Reactive values must be included in dependencies:

function ChatRoom({ roomId }) { // Props change over time
  const [serverUrl, setServerUrl] = useState('https://localhost:1234'); // State may change over time

  useEffect(() => {
    const connection = createConnection(serverUrl, roomId); // Your Effect reads props and state
    connection.connect();
    return () => {
      connection.disconnect();
    };
  }, [roomId, serverUrl]); // So you tell React that this Effect "depends on" on props and state
  // ...
}

By including serverUrl as a dependency, you ensure that the Effect re-synchronizes after it changes.

Try changing the selected chat room or edit the server URL in this sandbox:

Whenever you change a reactive value like roomId or serverUrl, the Effect re-connects to the chat server.

What an Effect with empty dependencies means

What happens if you move both serverUrl and roomId outside the component?

const serverUrl = 'https://localhost:1234';
const roomId = 'general';

function ChatRoom() {
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
    return () => {
      connection.disconnect();
    };
  }, []); // ✅ All dependencies declared
  // ...
}

Now your Effect’s code does not use any reactive values, so its dependencies can be empty ([]).

If you think from the component’s perspective, the empty [] dependency array means this Effect connects to the chat room only when the component mounts, and disconnects only when the component unmounts. (Keep in mind that React would still re-synchronize it an extra time in development to stress-test your Effect’s logic.)

However, if you think from the Effect’s perspective, you don’t need to think about mounting and unmounting at all. What’s important is you’ve specified what your Effect does to start and stop synchronizing. Today, it has no reactive dependencies. But if you ever want the user to change roomId or serverUrl over time (and so they’d have to become reactive), your Effect’s code won’t change. You will only need to add them to the dependencies.

All variables declared in the component body are reactive

Props and state aren’t the only reactive values. Values that you calculate from them are also reactive. If the props or state change, your component will re-render, and the values calculated from them will also change. This is why all variables from the component body used by the Effect should also be in the Effect dependency list.

Let’s say that the user can pick a chat server in the dropdown, but they can also configure a default server in settings. Suppose you’ve already put the settings state in a context so you read the settings from that context. Now you calculate the serverUrl based on the selected server from props and the default server from context:

function ChatRoom({ roomId, selectedServerUrl }) { // roomId is reactive
  const settings = useContext(SettingsContext); // settings is reactive
  const serverUrl = selectedServerUrl ?? settings.defaultServerUrl; // serverUrl is reactive
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId); // Your Effect reads roomId and serverUrl
    connection.connect();
    return () => {
      connection.disconnect();
    };
  }, [roomId, serverUrl]); // So it needs to re-synchronize when either of them changes!
  // ...
}

In this example, serverUrl is not a prop or a state variable. It’s a regular variable that you calculate during rendering. But it’s calculated during rendering, so it can change due to a re-render. This is why it’s reactive.

All values inside the component (including props, state, and variables in your component’s body) are reactive. Any reactive value can change on a re-render, so you need to include reactive values as Effect’s dependencies.

In other words, Effects “react” to all values from the component body.

React verifies that you specified every reactive value as a dependency

If your linter is configured for React, it will check that every reactive value used by your Effect’s code is declared as its dependency. For example, this is a lint error because both roomId and serverUrl are reactive:

This may look like a React error, but really React is pointing out a bug in your code. Both roomId and serverUrl may change over time, but you’re forgetting to re-synchronize your Effect when they change. As a result, you will remain connected to the initial roomId and serverUrl even after the user picks different values in the UI.

To fix the bug, follow the linter’s suggestion to specify roomId and serverUrl as dependencies of your Effect:

function ChatRoom({ roomId, serverUrl }) {
  const [serverUrl, setServerUrl] = useState('https://localhost:1234'); // serverUrl is reactive
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
    return () => {
      connection.disconnect();
    };
  }, [serverUrl, roomId]); // ✅ All dependencies declared
  // ...
}

Try this fix in the sandbox above. Verify that the linter error is gone, and that the chat re-connects when needed.

What to do when you don’t want to re-synchronize

In the previous example, you’ve fixed the lint error by listing roomId and serverUrl as dependencies.

However, you could instead “prove” to the linter that these values aren’t reactive values, i.e. that they can’t change as a result of a re-render. For example, if serverUrl and roomId don’t depend on rendering and always have the same values, you can move them outside the component. Now they don’t need to be dependencies:

const serverUrl = 'https://localhost:1234'; // serverUrl is not reactive
const roomId = 'general'; // roomId is not reactive

function ChatRoom() {
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
    return () => {
      connection.disconnect();
    };
  }, []); // ✅ All dependencies declared
  // ...
}

You can also move them inside the Effect. They aren’t calculated during rendering, so they’re not reactive:

function ChatRoom() {
  useEffect(() => {
    const serverUrl = 'https://localhost:1234'; // serverUrl is not reactive
    const roomId = 'general'; // roomId is not reactive
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
    return () => {
      connection.disconnect();
    };
  }, []); // ✅ All dependencies declared
  // ...
}

Effects are reactive blocks of code. They re-synchronize when the values you read inside of them change. Unlike event handlers, which only run once per interaction, Effects run whenever synchronization is necessary.

You can’t “choose” your dependencies. Your dependencies must include every reactive value you read in the Effect. The linter enforces this. Sometimes this may lead to problems like infinite loops and to your Effect re-synchronizing too often. Don’t fix these problems by suppressing the linter! Here’s what to try instead:

• Check that your Effect represents an independent synchronization process. If your Effect doesn’t synchronize anything, it might be unnecessary. If it synchronizes several independent things, split it up.

• If you want to read the latest value of props or state without “reacting” to it and re-synchronizing the Effect, you can split your Effect into a reactive part (which you’ll keep in the Effect) and a non-reactive part (which you’ll extract into something called an Event function). Read more about separating Events from Effects.

• Avoid relying on objects and functions as dependencies. If you create objects and functions during rendering and then read them from an Effect, they will be different on every render. This will cause your Effect to re-synchronize every time. Read more about removing unnecessary dependencies from your Effects.

useEffect(() => {
  // ...
  // 🔴 Avoid suppressing the linter like this:
  // eslint-ignore-next-line react-hooks/exhaustive-dependencies
}, []);