Removing Effect Dependencies

Dependencies should match the code

When you write an Effect, you first specify how to start and stop whatever you want your Effect to be doing:

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

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

Then, if you leave the Effect dependencies empty ([]), the linter will suggest the correct dependencies:

Fill them in according to what the linter says:

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

Effects “react” to reactive values. Since roomId is a reactive value (it can change due to a re-render), the linter verifies that you’ve specified it as a dependency. If roomId receives a different value, React will re-synchronize your Effect. This ensures that the chat stays connected to the selected room and “reacts” to the dropdown:

To remove a dependency, prove that it’s not a dependency

Notice that you can’t “choose” the dependencies of your Effect. Every reactive value used by your Effect’s code must be declared in your dependency list. Your Effect’s dependency list is determined by the surrounding code:

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

function ChatRoom({ roomId }) { // This is a reactive value
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId); // This Effect reads that reactive value
    connection.connect();
    return () => connection.disconnect();
  }, [roomId]); // ✅ So you must specify that reactive value as a dependency of your Effect
  // ...
}

Reactive values include props and all variables and functions declared directly inside of your component. Since roomId is a reactive value, you can’t remove it from the dependency list. The linter wouldn’t allow it:

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

function ChatRoom({ roomId }) {
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
    return () => connection.disconnect();
  }, []); // 🔴 React Hook useEffect has a missing dependency: 'roomId'
  // ...
}

And the linter would be right! Since roomId may change over time, this would introduce a bug in your code.

To remove a dependency, you need to “prove” to the linter that it doesn’t need to be a dependency. For example, you can move roomId out of your component to prove that it’s not reactive and won’t change on re-renders:

const serverUrl = 'https://localhost:1234';
const roomId = 'music'; // Not a reactive value anymore

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

Now that roomId is not a reactive value (and can’t change on a re-render), it doesn’t need to be a dependency:

This is why you could now specify an empty ([]) dependency list. Your Effect really doesn’t depend on any reactive value anymore, so it really doesn’t need to re-run when any of the component’s props or state change.

To change the dependencies, change the code

You might have noticed a pattern in your workflow:

1. First, you change the code of your Effect or how your reactive values are declared.
2. Then, you follow the linter and adjust the dependencies to match the code you have changed.
3. If you’re not happy with the list of dependencies, you go back to the first step (and change the code again).

The last part is important. If you want to change the dependencies, change the surrounding code first. You can think of the dependency list as a list of all the reactive values used by your Effect’s code. You don’t intentionally choose what to put on that list. The list describes your code. To change the dependency list, change the code.

This might feel like solving an equation. You might start with a goal (for example, to remove a dependency), and you need to “find” the exact code matching that goal. Not everyone finds solving equations fun, and the same thing could be said about writing Effects! Luckily, there is a list of common recipes that you can try below.

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

import { useState, useEffect } from 'react';

export default function Timer() {
  const [count, setCount] = useState(0);
  const [increment, setIncrement] = useState(1);

  function onTick() {
	setCount(count + increment);
  }

  useEffect(() => {
    const id = setInterval(onTick, 1000);
    return () => clearInterval(id);
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, []);

  return (
    <>
      <h1>
        Counter: {count}
        <button onClick={() => setCount(0)}>Reset</button>
      </h1>
      <hr />
      <p>
        Every second, increment by:
        <button disabled={increment === 0} onClick={() => {
          setIncrement(i => i - 1);
        }}>–</button>
        <b>{increment}</b>
        <button onClick={() => {
          setIncrement(i => i + 1);
        }}>+</button>
      </p>
    </>
  );
}

Removing unnecessary dependencies

Every time you adjust the Effect’s dependencies to reflect the code, look at the dependency list. Does it make sense for the Effect to re-run when any of these dependencies change? Sometimes, the answer is “no”:

• Sometimes, you want to re-execute different parts of your Effect under different conditions.
• Sometimes, you want to only read the latest value of some dependency instead of “reacting” to its changes.
• Sometimes, a dependency may change too often unintentionally because it’s an object or a function.

To find the right solution, you’ll need to answer a few questions about your Effect. Let’s walk through them.

Should this code move to an event handler?

The first thing you should think about is whether this code should be an Effect at all.

Imagine a form. On submit, you set the submitted state variable to true. You need to send a POST request and show a notification. You’ve decided to put this logic inside an Effect that “reacts” to submitted being true:

function Form() {
  const [submitted, setSubmitted] = useState(false);

  useEffect(() => {
    if (submitted) {
      // 🔴 Avoid: Event-specific logic inside an Effect
      post('/api/register');
      showNotification('Successfully registered!');
    }
  }, [submitted]);

  function handleSubmit() {
    setSubmitted(true);
  }

  // ...
}

Later, you want to style the notification message according to the current theme, so you read the current theme. Since theme is declared in the component body, it is a reactive value, and you must declare it as a dependency:

function Form() {
  const [submitted, setSubmitted] = useState(false);
  const theme = useContext(ThemeContext);

  useEffect(() => {
    if (submitted) {
      // 🔴 Avoid: Event-specific logic inside an Effect
      post('/api/register');
      showNotification('Successfully registered!', theme);
    }
  }, [submitted, theme]); // ✅ All dependencies declared

  function handleSubmit() {
    setSubmitted(true);
  }  

  // ...
}

But by doing this, you’ve introduced a bug. Imagine you submit the form first and then switch between Dark and Light themes. The theme will change, the Effect will re-run, and so it will display the same notification again!

The problem here is that this shouldn’t be an Effect in the first place. You want to send this POST request and show the notification in response to submitting the form, which is a particular interaction. When you want to run some code in response to particular interaction, put that logic directly into the corresponding event handler:

function Form() {
  const theme = useContext(ThemeContext);

  function handleSubmit() {
    // ✅ Good: Event-specific logic is called from event handlers
    post('/api/register');
    showNotification('Successfully registered!', theme);
  }  

  // ...
}

Now that the code is in an event handler, it’s not reactive—so it will only run when the user submits the form. Read more about choosing between event handlers and Effects and how to delete unnecessary Effects.

Is your Effect doing several unrelated things?

The next question you should ask yourself is whether your Effect is doing several unrelated things.

Imagine you’re creating a shipping form where the user needs to choose their city and area. You fetch the list of cities from the server according to the selected country so that you can show them as dropdown options:

function ShippingForm({ country }) {
  const [cities, setCities] = useState(null);
  const [city, setCity] = useState(null);

  useEffect(() => {
    let ignore = false;
    fetch(`/api/cities?country=${country}`)
      .then(response => response.json())
      .then(json => {
        if (!ignore) {
          setCities(json);
        }
      });
    return () => {
      ignore = true;
    };
  }, [country]); // ✅ All dependencies declared

  // ...

This is a good example of fetching data in an Effect. You are synchronizing the cities state with the network according to the country prop. You can’t do this in an event handler because you need to fetch as soon as ShippingForm is displayed and whenever the country changes (no matter which interaction causes it).

Now let’s say you’re adding a second select box for city areas, which should fetch the areas for the currently selected city. You might start by adding a second fetch call for the list of areas inside the same Effect:

function ShippingForm({ country }) {
  const [cities, setCities] = useState(null);
  const [city, setCity] = useState(null);
  const [areas, setAreas] = useState(null);

  useEffect(() => {
    let ignore = false;
    fetch(`/api/cities?country=${country}`)
      .then(response => response.json())
      .then(json => {
        if (!ignore) {
          setCities(json);
        }
      });
    // 🔴 Avoid: A single Effect synchronizes two independent processes
    if (city) {
      fetch(`/api/areas?city=${city}`)
        .then(response => response.json())
        .then(json => {
          if (!ignore) {
            setAreas(json);
          }
        });
    }
    return () => {
      ignore = true;
    };
  }, [country, city]); // ✅ All dependencies declared

  // ...

However, since the Effect now uses the city state variable, you’ve had to add city to the list of dependencies. That, in turn, has introduced a problem. Now, whenever the user selects a different city, the Effect will re-run and call fetchCities(country). As a result, you will be unnecessarily refetching the list of cities many times.

The problem with this code is that you’re synchronizing two different unrelated things:

1. You want to synchronize the cities state to the network based on the country prop.
2. You want to synchronize the areas state to the network based on the city state.

Split the logic into two Effects, each of which reacts to the prop that it needs to synchronize with:

function ShippingForm({ country }) {
  const [cities, setCities] = useState(null);
  useEffect(() => {
    let ignore = false;
    fetch(`/api/cities?country=${country}`)
      .then(response => response.json())
      .then(json => {
        if (!ignore) {
          setCities(json);
        }
      });
    return () => {
      ignore = true;
    };
  }, [country]); // ✅ All dependencies declared

  const [city, setCity] = useState(null);
  const [areas, setAreas] = useState(null);
  useEffect(() => {
    if (city) {
      let ignore = false;
      fetch(`/api/areas?city=${city}`)
        .then(response => response.json())
        .then(json => {
          if (!ignore) {
            setAreas(json);
          }
        });
      return () => {
        ignore = true;
      };
    }
  }, [city]); // ✅ All dependencies declared

  // ...

Now the first Effect only re-runs if the country changes, while the second Effect re-runs when the city changes. You’ve separated them by purpose: two different things are synchronized by two separate Effects. Two separate Effects have two separate dependency lists, so they will no longer trigger each other unintentionally.

The final code is longer than the original, but splitting these Effects is still correct. Each Effect should represent an independent synchronization process. In this example, deleting one Effect doesn’t break the other Effect’s logic. This is a good indication that they synchronize different things, and it made sense to split them up. If the duplication feels concerning, you can further improve this code by extracting repetitive logic into a custom Hook.

Are you reading some state to calculate the next state?

This Effect updates the messages state variable with a newly created array every time a new message arrives:

function ChatRoom({ roomId }) {
  const [messages, setMessages] = useState([]);
  useEffect(() => {
    const connection = createConnection();
    connection.connect();
    connection.on('message', (receivedMessage) => {
      setMessages([...messages, receivedMessage]);
    });
    // ...

It uses the messages variable to create a new array starting with all the existing messages and adds the new message at the end. However, since messages is a reactive value read by an Effect, it must be a dependency:

function ChatRoom({ roomId }) {
  const [messages, setMessages] = useState([]);
  useEffect(() => {
    const connection = createConnection();
    connection.connect();
    connection.on('message', (receivedMessage) => {
      setMessages([...messages, receivedMessage]);
    });
    return () => connection.disconnect();
  }, [roomId, messages]); // ✅ All dependencies declared
  // ...

And making messages a dependency introduces a problem.

Every time you receive a message, setMessages() causes the component to re-render with a new messages array that includes the received message. However, since this Effect now depends on messages, this will also re-synchronize the Effect. So every new message will make the chat re-connect. The user would not like that!

To fix the issue, don’t read messages inside the Effect. Instead, pass an updater function to setMessages:

function ChatRoom({ roomId }) {
  const [messages, setMessages] = useState([]);
  useEffect(() => {
    const connection = createConnection();
    connection.connect();
    connection.on('message', (receivedMessage) => {
      setMessages(msgs => [...msgs, receivedMessage]);
    });
    return () => connection.disconnect();
  }, [roomId]); // ✅ All dependencies declared
  // ...

Notice how your Effect does not read the messages variable at all now. You only need to pass an updater function like msgs => [...msgs, receivedMessage]. React puts your updater function in a queue and will provide the msgs argument to it during the next render. This is why the Effect itself doesn’t need to depend on messages anymore. As a result of this fix, receiving a chat message will no longer make the chat re-connect.

Do you want to read a value without “reacting” to its changes?

Suppose that you want to play a sound when the user receives a new message unless isMuted is true:

function ChatRoom({ roomId }) {
  const [messages, setMessages] = useState([]);
  const [isMuted, setIsMuted] = useState(false);

  useEffect(() => {
    const connection = createConnection();
    connection.connect();
    connection.on('message', (receivedMessage) => {
      setMessages(msgs => [...msgs, receivedMessage]);
      if (!isMuted) {
        playSound();
      }
    });
    // ...

Since your Effect now uses isMuted in its code, you have to add it to the dependencies:

function ChatRoom({ roomId }) {
  const [messages, setMessages] = useState([]);
  const [isMuted, setIsMuted] = useState(false);

  useEffect(() => {
    const connection = createConnection();
    connection.connect();
    connection.on('message', (receivedMessage) => {
      setMessages(msgs => [...msgs, receivedMessage]);
      if (!isMuted) {
        playSound();
      }
    });
    return () => connection.disconnect();
  }, [roomId, isMuted]); // ✅ All dependencies declared
  // ...

The problem is that every time isMuted changes (for example, when the user presses the “Muted” toggle), the Effect will re-synchronize, and reconnect to the chat server. This is not the desired user experience! (In this example, even disabling the linter would not work—if you do that, isMuted would get “stuck” with its old value.)

To solve this problem, you need to extract the logic that shouldn’t be reactive out of the Effect. You don’t want this Effect to “react” to the changes in isMuted. Move this non-reactive piece of logic into an Event function:

import { useState, useEffect, useEvent } from 'react';

function ChatRoom({ roomId }) {
  const [messages, setMessages] = useState([]);
  const [isMuted, setIsMuted] = useState(false);

  const onMessage = useEvent(receivedMessage => {
    setMessages(msgs => [...msgs, receivedMessage]);
    if (!isMuted) {
      playSound();
    }
  });

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

Event functions let you split an Effect into reactive parts (which should “react” to reactive values like roomId and their changes) and non-reactive parts (which only read their latest values, like onMessage reads isMuted). Now that you read isMuted inside an Event function, it doesn’t need to be a dependency of your Effect. As a result, the chat won’t re-connect when you toggle the “Muted” setting on and off, solving the original issue!

Wrapping an event handler from the props

You might run into a similar problem when your component receives an event handler as a prop:

function ChatRoom({ roomId, onReceiveMessage }) {
  const [messages, setMessages] = useState([]);

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

Suppose that the parent component passes a different onReceiveMessage function on every render:

<ChatRoom
  roomId={roomId}
  onReceiveMessage={receivedMessage => {
    // ...
  }}
/>

Since onReceiveMessage is a dependency of your Effect, it would cause the Effect to re-synchronize after every parent re-render. This would make it re-connect to the chat. To solve this, wrap the call in an Event function:

function ChatRoom({ roomId, onReceiveMessage }) {
  const [messages, setMessages] = useState([]);

  const onMessage = useEvent(receivedMessage => {
    onReceiveMessage(receivedMessage);
  });

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

Event functions aren’t reactive, so you don’t need to specify them as dependencies. As a result, the chat will no longer re-connect even if the parent component passes a function that’s different on every re-render.

Separating reactive and non-reactive code

In this example, you want to log a visit every time roomId changes. You want to include the current notificationCount with every log, but you don’t want a change to notificationCount to trigger a log event.

The solution is again to split out the non-reactive code into an Event function:

function Chat({ roomId, notificationCount }) {
  const onVisit = useEvent(visitedRoomId => {
    logVisit(visitedRoomId, notificationCount);
  });

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

You want your logic to be reactive with regards to roomId, so you read roomId inside of your Effect. However, you don’t want a change to notificationCount to log an extra visit, so you read notificationCount inside of the Event function. Learn more about reading the latest props and state from Effects using Event functions.

Does some reactive value change unintentionally?

Sometimes, you do want your Effect to “react” to a certain value, but that value changes more often than you’d like—and might not reflect any actual change from the user’s perspective. For example, let’s say that you create an options object in the body of your component, and then read that object from inside of your Effect:

function ChatRoom({ roomId }) {
  // ...
  const options = {
    serverUrl: serverUrl,
    roomId: roomId
  };

  useEffect(() => {
    const connection = createConnection(options);
    connection.connect();
    // ...

This object is declared in the component body, so it’s a reactive value. When you read a reactive value like this inside an Effect, you declare it as a dependency. This ensures your Effect “reacts” to its changes:

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

It is important to declare it as a dependency! This ensures, for example, that if the roomId changes, then your Effect will re-connect to the chat with the new options. However, there is also a problem with the code above. To see the problem, try typing into the input in the sandbox below, and watch what happens in the console:

In the sandbox above, the input only updates the message state variable. From the user’s perspective, this should not affect the chat connection. However, every time you update the message, your component re-renders. When your component re-renders, the code inside of it runs again from scratch.

This means that a new options object is created from scratch on every re-render of the ChatRoom component. React sees that the options object is a different object from the options object created during the last render. This is why it re-synchronizes your Effect (which depends on options), and the chat re-connects as you type.

This problem affects objects and functions in particular. In JavaScript, each newly created object and function is considered distinct from all the others. It doesn’t matter that the contents inside of them may be the same!

// During the first render
const options1 = { serverUrl: 'https://localhost:1234', roomId: 'music' };

// During the next render
const options2 = { serverUrl: 'https://localhost:1234', roomId: 'music' };

// These are two different objects!
console.log(Object.is(options1, options2)); // false

Object and function dependencies create a risk that your Effect will re-synchronize more often than you need.

This is why, whenever possible, you should try to avoid objects and functions as your Effect’s dependencies. Instead, try moving them outside the component, inside the Effect, or extracting primitive values out of them.

Move static objects and functions outside your component

If the object does not depend on any props and state, you can move that object outside your component:

const options = {
  serverUrl: 'https://localhost:1234',
  roomId: 'music'
};

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

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

This way, you prove to the linter that it’s not reactive. It can’t change as a result of a re-render, so it doesn’t need to be a dependency of your Effect. Now re-rendering ChatRoom won’t cause your Effect to re-synchronize.

This works for functions too:

function createOptions() {
  return {
    serverUrl: 'https://localhost:1234',
    roomId: 'music'
  };
}

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

  useEffect(() => {
    const options = createOptions();
    const connection = createConnection();
    connection.connect();
    return () => connection.disconnect();
  }, []); // ✅ All dependencies declared
  // ...

Since createOptions is declared outside your component, it’s not a reactive value. This is why it doesn’t need to be specified in your Effect’s dependencies, and why it won’t ever cause your Effect to re-synchronize.

Move dynamic objects and functions inside your Effect

If your object depends on some reactive value that may change as a result of a re-render, like a roomId prop, you can’t pull it outside your component. You can, however, move its creation inside of your Effect’s code:

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

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

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

Now that options is declared inside of your Effect, it is no longer a dependency of your Effect. Instead, the only reactive value used by your Effect is roomId. Since roomId is not an object or function, you can be sure that it won’t be unintentionally different. In JavaScript, numbers and strings are compared by their content:

// During the first render
const roomId1 = 'music';

// During the next render
const roomId2 = 'music';

// These two strings are the same!
console.log(Object.is(roomId1, roomId2)); // true

Thanks to this fix, the chat no longer re-connects if you edit the input:

However, it does re-connect when you change the roomId dropdown, as you would expect.

This works for functions, too:

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

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

  useEffect(() => {
    function createOptions() {
      return {
        serverUrl: serverUrl,
        roomId: roomId
      };
    }

    const options = createOptions();
    const connection = createConnection(options);
    connection.connect();
    return () => connection.disconnect();
  }, [roomId]); // ✅ All dependencies declared
  // ...

You can write your own functions to group pieces of logic inside your Effect. As long as you also declare them inside your Effect, they’re not reactive values, and so they don’t need to be dependencies of your Effect.

Read primitive values from objects

Sometimes, you may receive an object from props:

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

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

The risk here is that the parent component will create the object during rendering:

<ChatRoom
  roomId={roomId}
  options={{
    serverUrl: serverUrl,
    roomId: roomId
  }}
/>

This would cause your Effect to re-connect every time the parent component re-renders. To fix this, read all the necessary information from the object outside the Effect, and avoid having objects and functions dependencies:

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

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

The logic gets a little repetitive (you read some values from an object outside an Effect, and then create an object with the same values inside the Effect). But it makes it very explicit what information your Effect actually depends on. If an object is re-created unintentionally by the parent component, the chat would not re-connect. However, if options.roomId or options.serverUrl actually change, the chat would re-connect as you’d expect.

Calculate primitive values from functions

The same approach can work for functions. For example, suppose the parent component passes a function:

<ChatRoom
  roomId={roomId}
  getOptions={() => {
    return {
      serverUrl: serverUrl,
      roomId: roomId
    };
  }}
/>

To avoid making it a dependency (and thus causing it to re-connect on re-renders), call it outside the Effect. This gives you the roomId and serverUrl values that aren’t objects, and that you can read from inside your Effect:

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

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

This only works for pure functions because they are safe to call during rendering. If your function is an event handler, but you don’t want its changes to re-synchronize your Effect, wrap it into an Event function instead.