Separating Events from Effects

Choosing between event handlers and Effects

First, let’s recap the difference between event handlers and Effects.

Imagine you’re implementing a chat room component. Your requirements look like this:

1. Your component should automatically connect to the selected chat room.
2. When you click the “Send” button, it should send a message to the chat.

Let’s say you’ve already implemented the code for them, but you’re not sure where to put it. Should you use event handlers or Effects? Every time you need to answer this question, consider why the code needs to run.

Event handlers run in response to specific interactions

From the user’s perspective, sending a message should happen because the particular “Send” button was clicked. The user will get rather upset if you send their message at any other time or for any other reason. This is why sending a message should be an event handler. Event handlers let you handle specific interactions like clicks:

function ChatRoom({ roomId }) {
  const [message, setMessage] = useState('');
  // ...
  function handleSendClick() {
    sendMessage(message);
  }
  // ...
  return (
    <>
      <input value={message} onChange={e => setMessage(e.target.value)} />
      <button onClick={handleSendClick}>Send</button>;
    </>
  );
}

With an event handler, you can be sure that sendMessage(message) will only run if the user presses the button.

Effects run whenever synchronization is needed

Recall that you also need to keep the component connected to the chat room. Where does that code go?

The reason to run this code is not some particular interaction. It doesn’t matter why or how the user navigated to the chat room screen. Now that they’re looking at it and could interact with it, the component needs to stay connected to the selected chat server. Even if the chat room component was the initial screen of your app, and the user has not performed any interactions at all, you would still need to connect. This is why it’s an Effect:

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

With this code, you can be sure that there is always an active connection to the currently selected chat server, regardless of the specific interactions performed by the user. Whether the user has only opened your app, selected a different room, or navigated to another screen and back, your Effect will ensure that the component will remain synchronized with the currently selected room, and will re-connect whenever it’s necessary.

Reactive values and reactive logic

Intuitively, you could say that event handlers are always triggered “manually”, for example by clicking a button. Effects, on the other hand, are “automatic”: they run and re-run as often as it’s needed to stay synchronized.

There is a more precise way to think about this.

Props, state, and variables declared inside your component’s body are called reactive values. In this example, serverUrl is not a reactive value, but roomId and message are. They participate in the rendering data flow:

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

function ChatRoom({ roomId }) {
  const [message, setMessage] = useState('');

  // ...
}

Reactive values like these can change due to a re-render. For example, the user may edit the message or choose a different roomId in a dropdown. Event handlers and Effects are different in how they respond to changes:

• Logic inside event handlers is not reactive. It will not run again unless the user performs the same interaction (for example, a click) again. Event handlers can read reactive values, but they don’t “react” to their changes.
• Logic inside Effects is reactive. If your Effect reads a reactive value, you have to specify it as a dependency. Then, if a re-render causes that value to change, React will re-run your Effect’s logic again with the new value.

Let’s revisit the previous example to illustrate this difference.

Logic inside event handlers is not reactive

Take a look at this line of code. Should this logic be reactive or not?

    // ...
    sendMessage(message);
    // ...

From the user’s perspective, a change to the message does not mean that they want to send a message. It only means that the user is typing. In other words, the logic that sends a message should not be reactive. It should not run again only because the reactive value has changed. That’s why you placed this logic in the event handler:

  function handleSendClick() {
    sendMessage(message);
  }

Event handlers aren’t reactive, so sendMessage(message) will only run when the user clicks the Send button.

Logic inside Effects is reactive

Now let’s return to these lines:

    // ...
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
    // ...

From the user’s perspective, a change to the roomId does mean that they want to connect to a different room. In other words, the logic for connecting to the room should be reactive. You want these lines of code to “keep up” with the reactive value, and to run again if that value is different. That’s why you put this logic inside an Effect:

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

Effects are reactive, so createConnection(serverUrl, roomId) and connection.connect() will run for every distinct value of roomId. Your Effect keeps the chat connection synchronized to the currently selected room.

Extracting non-reactive logic out of Effects

Things get more tricky when you want to mix reactive logic with non-reactive logic.

For example, imagine that you want to show a notification when the user connects to the chat. You read the current theme (dark or light) from the props so that you can show the notification in the correct color:

function ChatRoom({ roomId, theme }) {
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId);
    connection.on('connected', () => {
      showNotification('Connected!', theme);
    });
    connection.connect();
    // ...

However, theme is a reactive value (it can change as a result of re-rendering), and every reactive value read by an Effect must be declared as its dependency. So now you have to specify theme as a dependency of your Effect:

function ChatRoom({ roomId, theme }) {
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId);
    connection.on('connected', () => {
      showNotification('Connected!', theme);
    });
    connection.connect();
    return () => {
      connection.disconnect()
    };
  }, [roomId, theme]); // ✅ All dependencies declared
  // ...

Play with this example and see if you can spot the problem with this user experience:

When the roomId changes, the chat re-connects as you would expect. But since theme is also a dependency, the chat also re-connects every time you switch between the dark and the light theme. That’s not great!

In other words, you don’t want this line to be reactive, even though it is inside an Effect (which is reactive):

      // ...
      showNotification('Connected!', theme);
      // ...

You need a way to separate this non-reactive logic from the reactive Effect around it.

Declaring an Event function

Use a special Hook called useEvent to extract this non-reactive logic out of your Effect:

import { useEffect, useEvent } from 'react';

function ChatRoom({ roomId, theme }) {
  const onConnected = useEvent(() => {
    showNotification('Connected!', theme);
  });
  // ...

Here, onConnected is called an Event function. It’s a part of your Effect logic, but it behaves a lot more like an event handler. The logic inside it is not reactive, and it always “sees” the latest values of your props and state.

Now you can call the onConnected Event function from inside your Effect:

function ChatRoom({ roomId, theme }) {
  const onConnected = useEvent(() => {
    showNotification('Connected!', theme);
  });

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

This solves the problem. Similar to the set functions returned from useState, all Event functions are stable: they never change on a re-render. This is why you can skip them in the dependency list. They are not reactive.

Verify that the new behavior works as you would expect:

You can think of Event functions as being very similar to event handlers. The main difference is that event handlers run in response to a user interactions, whereas Event functions are triggered by you from Effects. Event functions let you “break the chain” between the reactivity of Effects and some code that should not be reactive.

Reading latest props and state with Event functions

Event functions let you fix many patterns where you might be tempted to suppress the dependency linter.

For example, say you have an Effect to log the page visits:

function Page() {
  useEffect(() => {
    logVisit();
  }, []);
  // ...
}

Later, you add multiple routes to your site. Now your Page component receives a url prop with the current path. You want to pass the url as a part of your logVisit call, but the dependency linter complains:

function Page({ url }) {
  useEffect(() => {
    logVisit(url);
  }, []); // 🔴 React Hook useEffect has a missing dependency: 'url'
  // ...
}

Think about what you want the code to do. You want to log a separate visit for different URLs since each URL represents a different page. In other words, this logVisit call should be reactive with respect to the url. This is why, in this case, it makes sense to follow the dependency linter, and add url as a dependency:

function Page({ url }) {
  useEffect(() => {
    logVisit(url);
  }, [url]); // ✅ All dependencies declared
  // ...
}

Now let’s say you want to include the number of items in the shopping cart together with every page visit:

function Page({ url }) {
  const { items } = useContext(ShoppingCartContext);
  const numberOfItems = items.length;

  useEffect(() => {
    logVisit(url, numberOfItems);
  }, [url]); // 🔴 React Hook useEffect has a missing dependency: 'numberOfItems'
  // ...
}

You used numberOfItems inside the Effect, so the linter asks you to add it as a dependency. However, you don’t want the logVisit call to be reactive with respect to numberOfItems. If the user puts something into the shopping cart, and the numberOfItems changes, this does not mean that the user visited the page again. In other words, visiting the page feels similar to an event. You want to be very precise about when you say it’s happened.

Split the code in two parts:

function Page({ url }) {
  const { items } = useContext(ShoppingCartContext);
  const numberOfItems = items.length;

  const onVisit = useEvent(visitedUrl => {
    logVisit(visitedUrl, numberOfItems);
  });

  useEffect(() => {
    onVisit(url);
  }, [url]); // ✅ All dependencies declared
  // ...
}

Here, onVisit is an Event function. The code inside it isn’t reactive. This is why you can use numberOfItems (or any other reactive value!) without worrying that it will cause the surrounding code to re-execute on changes.

On the other hand, the Effect itself remains reactive. Code inside the Effect uses the url prop, so the Effect will re-run after every re-render with a different url. This, in turn, will call the onVisit event function.

As a result, you will call logVisit for every change to the url, and always read the latest numberOfItems. However, if numberOfItems changes on its own, this will not cause any of the code to re-run.

  const onVisit = useEvent(() => {
    logVisit(url, numberOfItems);
  });

  useEffect(() => {
    onVisit();
  }, [url]);

  const onVisit = useEvent(visitedUrl => {
    logVisit(visitedUrl, numberOfItems);
  });

  useEffect(() => {
    onVisit(url);
  }, [url]);

  const onVisit = useEvent(visitedUrl => {
    logVisit(visitedUrl, numberOfItems);
  });

  useEffect(() => {
    setTimeout(() => {
      onVisit(url);
    }, 5000); // Delay logging visits
  }, [url]);

function Page({ url }) {
  const { items } = useContext(ShoppingCartContext);
  const numberOfItems = items.length;

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

import { useState, useEffect } from 'react';

export default function App() {
  const [position, setPosition] = useState({ x: 0, y: 0 });
  const [canMove, setCanMove] = useState(true);

  function handleMove(e) {
    if (canMove) {
      setPosition({ x: e.clientX, y: e.clientY });
    }
  }

  useEffect(() => {
    window.addEventListener('pointermove', handleMove);
    return () => window.removeEventListener('pointermove', handleMove);
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, []);

  return (
    <>
      <label>
        <input type="checkbox"
          checked={canMove}
          onChange={e => setCanMove(e.target.checked)} 
        />
        The dot is allowed to move
      </label>
      <hr />
      <div style={{
        position: 'absolute',
        backgroundColor: 'pink',
        borderRadius: '50%',
        opacity: 0.6,
        transform: `translate(${position.x}px, ${position.y}px)`,
        pointerEvents: 'none',
        left: -20,
        top: -20,
        width: 40,
        height: 40,
      }} />
    </>
  );
}

import { useState, useEffect } from 'react';
import { useEvent } from './useEvent.js';

export default function App() {
  const [position, setPosition] = useState({ x: 0, y: 0 });
  const [canMove, setCanMove] = useState(true);

  const onMove = useEvent(e => {
    if (canMove) {
      setPosition({ x: e.clientX, y: e.clientY });
    }
  });

  useEffect(() => {
    window.addEventListener('pointermove', onMove);
    return () => window.removeEventListener('pointermove', onMove);
  }, [onMove]); // TODO: Linter will allow [] in the future

  return (
    <>
      <label>
        <input type="checkbox"
          checked={canMove}
          onChange={e => setCanMove(e.target.checked)} 
        />
        The dot is allowed to move
      </label>
      <hr />
      <div style={{
        position: 'absolute',
        backgroundColor: 'pink',
        borderRadius: '50%',
        opacity: 0.6,
        transform: `translate(${position.x}px, ${position.y}px)`,
        pointerEvents: 'none',
        left: -20,
        top: -20,
        width: 40,
        height: 40,
      }} />
    </>
  );
}

Limitations of Event functions

At the moment, Event functions are very limited in how you can use them:

• Only call them from inside Effects.
• Never pass them to other components or Hooks.

For example, don’t declare and pass an Event function like this:

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

  const onTick = useEvent(() => {
    setCount(count + 1);
  });

  useTimer(onTick, 1000); // 🔴 Avoid: Passing event functions

  return <h1>{count}</h1>
}

function useTimer(callback, delay) {
  useEffect(() => {
    const id = setInterval(() => {
      callback();
    }, delay);
    return () => {
      clearInterval(id);
    };
  }, [delay, callback]); // Need to specify "callback" in dependencies
}

Instead, always declare Event functions directly next to the Effects that use them:

function Timer() {
  const [count, setCount] = useState(0);
  useTimer(() => {
    setCount(count + 1);
  }, 1000);
  return <h1>{count}</h1>
}

function useTimer(callback, delay) {
  const onTick = useEvent(() => {
    callback();
  });

  useEffect(() => {
    const id = setInterval(() => {
      onTick(); // ✅ Good: Only called locally inside an Effect
    }, delay);
    return () => {
      clearInterval(id);
    };
  }, [delay]); // No need to specify "onTick" (an Event function) as a dependency
}

It’s possible that in the future, some of these restrictions will be lifted. But for now, you can think of Event functions as non-reactive “pieces” of your Effect code, so they should be close to the Effect using them.