Reusing Logic with Custom Hooks

Custom Hooks: Sharing logic between components

Imagine you’re developing an app that heavily relies on the network (as most apps do). You want to warn the user if their network connection has accidentally gone off while they were using your app. How would you go about it?

It seems like you’ll need two things in your component:

1. A piece of state that tracks whether the network is online.
2. An Effect that subscribes to the global online and offline events, and updates that state.

This will keep your component synchronized with the network status. You might start with something like this:

Try turning your network on and off, and notice how this StatusBar updates in response to your actions.

Now imagine you also want to use the same logic in a different component. You want to implement a Save button that will become disabled and show “Reconnecting…” instead of “Save” while the network is off.

To start, you can copy and paste the isOnline state and the Effect into SaveButton:

Verify that, if you turn off the network, the button will change its appearance.

These two components work fine, but the duplication in logic between them is unfortunate. It seems like even though they have different visual appearance, you want to reuse the logic between them.

Extracting your own custom Hook from a component

Imagine for a moment that, similar to useState and useEffect, there was a built-in useOnlineStatus Hook. Then both of these components could be simplified and you could remove the duplication between them:

function StatusBar() {
  const isOnline = useOnlineStatus();
  return <h1>{isOnline ? '✅ Online' : '❌ Disconnected'}</h1>;
}

function SaveButton() {
  const isOnline = useOnlineStatus();

  function handleSaveClick() {
    console.log('✅ Progress saved');
  }

  return (
    <button disabled={!isOnline} onClick={handleSaveClick}>
      {isOnline ? 'Save progress' : 'Reconnecting...'}
    </button>
  );
}

Although there is no such built-in Hook, you can write it yourself. Declare a function called useOnlineStatus and move all the duplicated code into it from the components you wrote earlier:

function useOnlineStatus() {
  const [isOnline, setIsOnline] = useState(true);
  useEffect(() => {
    function handleOnline() {
      setIsOnline(true);
    }
    function handleOffline() {
      setIsOnline(false);
    }
    window.addEventListener('online', handleOnline);
    window.addEventListener('offline', handleOffline);
    return () => {
      window.removeEventListener('online', handleOnline);
      window.removeEventListener('offline', handleOffline);
    };
  }, []);
  return isOnline;
}

At the end of the function, return isOnline. This lets your components read that value:

Verify that switching the network on and off updates both components.

Now your components don’t have as much repetitive logic. More importantly, the code inside them describes what they want to do (use the online status!) rather than how to do it (by subscribing to the browser events).

When you extract logic into custom Hooks, you can hide the gnarly details of how you deal with some external system or a browser API. The code of your components expresses your intent, not the implementation.

Hook names always start with use

React applications are built from components. Components are built from Hooks, whether built-in or custom. You’ll likely often use custom Hooks created by others, but occasionally you might write one yourself!

You must follow these naming conventions:

1. React component names must start with a capital letter, like StatusBar and SaveButton. React components also need to return something that React knows how to display, like a piece of JSX.
2. Hook names must start with use followed by a capital letter, like useState (built-in) or useOnlineStatus (custom, like earlier on the page). Hooks may return arbitrary values.

This convention guarantees that you can always look at a component and know where its state, Effects, and other React features might “hide”. For example, if you see a getColor() function call inside your component, you can be sure that it can’t possibly contain React state inside because its name doesn’t start with use. However, a function call like useOnlineStatus() will most likely contain calls to other Hooks inside!

// 🔴 Avoid: A Hook that doesn't use Hooks
function useSorted(items) {
  return items.slice().sort();
}

// ✅ Good: A regular function that doesn't use Hooks
function getSorted(items) {
  return items.slice().sort();
}

function List({ items, shouldSort }) {
  let displayedItems = items;
  if (shouldSort) {
    // ✅ It's ok to call getSorted() conditionally because it's not a Hook
    displayedItems = getSorted(items);
  }
  // ...
}

// ✅ Good: A Hook that uses other Hooks
function useAuth() {
  return useContext(Auth);
}

// ✅ Good: A Hook that will likely some other Hooks later
function useAuth() {
  // TODO: Replace with this line when authentication is implemented:
  // return useContext(Auth);
  return TEST_USER;
}

Custom Hooks let you share stateful logic, not state itself

In the earlier example, when you turned the network on and off, both components updated together. However, it’s wrong to think that a single isOnline state variable is shared between them. Look at this code:

function StatusBar() {
  const isOnline = useOnlineStatus();
  // ...
}

function SaveButton() {
  const isOnline = useOnlineStatus();
  // ...
}

It works the same way as before you extracted the duplication:

function StatusBar() {
  const [isOnline, setIsOnline] = useState(true);
  useEffect(() => {
    // ...
  }, []);
  // ...
}

function SaveButton() {
  const [isOnline, setIsOnline] = useState(true);
  useEffect(() => {
    // ...
  }, []);
  // ...
}

These are two completely independent state variables and Effects! They only happened to have the same value at the same time because you synchronized them with the same external value (whether the network is on).

To better illustrate this, we’ll need a different example. Consider this Form component:

There’s some repetitive logic for each form field:

1. There’s a piece of state (firstName and lastName).
2. There’s a change handler (handleFirstNameChange and handleLastNameChange).
3. There’s a piece of JSX that specifies the value and onChange attributes for that input.

You can extract the repetitive logic into this useFormInput custom Hook:

Notice that it only declares one state variable called value.

However, the Form component calls useFormInput two times:

function Form() {
  const firstNameProps = useFormInput('Mary');
  const lastNameProps = useFormInput('Poppins');
  // ...

This is why it works like declaring two separate state variables!

Custom Hooks let you share stateful logic but not state itself. Each call to a Hook is completely independent from every other call to the same Hook. This is why the two sandboxes above are completely equivalent. If you’d like, scroll back up and compare them. The behavior before and after extracting a custom Hook is identical.

When you need to share the state itself between multiple components, lift it up and pass it down instead.

Passing reactive values between Hooks

The code inside your custom Hooks will re-run during every re-render of your component. This is why, like components, custom Hooks need to be pure. Think of custom Hooks’ code as part of your component’s body!

Because custom Hooks re-render together with your component, they always receive the latest props and state. To see what this means, consider this chat room example. Change the server URL or the selected chat room:

When you change serverUrl or roomId, the Effect “reacts” to your changes and re-synchronizes. You can tell by the console messages that the chat re-connects every time that you change your Effect’s dependencies.

Now move the Effect’s code into a custom Hook:

export function useChatRoom({ serverUrl, roomId }) {
  useEffect(() => {
    const options = {
      serverUrl: serverUrl,
      roomId: roomId
    };
    const connection = createConnection(options);
    connection.connect();
    connection.on('message', (msg) => {
      showNotification('New message: ' + msg);
    });
    return () => connection.disconnect();
  }, [roomId, serverUrl]);
}

This lets your ChatRoom component call your custom Hook without worrying about how it works inside:

export default function ChatRoom({ roomId }) {
  const [serverUrl, setServerUrl] = useState('https://localhost:1234');

  useChatRoom({
    roomId: roomId,
    serverUrl: serverUrl
  });

  return (
    <>
      <label>
        Server URL:
        <input value={serverUrl} onChange={e => setServerUrl(e.target.value)} />
      </label>
      <h1>Welcome to the {roomId} room!</h1>
    </>
  );
}

This looks much simpler! (But it does the same thing.)

Notice that the logic still responds to prop and state changes. Try editing the server URL or the selected room:

Notice how you’re taking the return value of one Hook:

export default function ChatRoom({ roomId }) {
  const [serverUrl, setServerUrl] = useState('https://localhost:1234');

  useChatRoom({
    roomId: roomId,
    serverUrl: serverUrl
  });
  // ...

and pass it as an input to another Hook:

export default function ChatRoom({ roomId }) {
  const [serverUrl, setServerUrl] = useState('https://localhost:1234');

  useChatRoom({
    roomId: roomId,
    serverUrl: serverUrl
  });
  // ...

Every time your ChatRoom component re-renders, it passes the latest roomId and serverUrl to your Hook. This is why your Effect re-connects to the chat whenever their values are different after a re-render. (If you ever worked with music processing software, chaining Hooks like this might remind you of chaining multiple audio effects, like adding reverb or chorus. It’s as if the output of useState “feeds into” the input of the useChatRoom.)

Passing event handlers to custom Hooks

As you start using useChatRoom in more components, you might want to let different components customize its behavior. For example, currently, the logic for what to do when a message arrives is hardcoded inside the Hook:

export function useChatRoom({ serverUrl, roomId }) {
  useEffect(() => {
    const options = {
      serverUrl: serverUrl,
      roomId: roomId
    };
    const connection = createConnection(options);
    connection.connect();
    connection.on('message', (msg) => {
      showNotification('New message: ' + msg);
    });
    return () => connection.disconnect();
  }, [roomId, serverUrl]);
}

Let’s say you want to move this logic back to your component:

export default function ChatRoom({ roomId }) {
  const [serverUrl, setServerUrl] = useState('https://localhost:1234');

  useChatRoom({
    roomId: roomId,
    serverUrl: serverUrl,
    onReceiveMessage(msg) {
      showNotification('New message: ' + msg);
    }
  });
  // ...

To make this work, change your custom Hook to take onReceiveMessage as one of its named options:

export function useChatRoom({ serverUrl, roomId, onReceiveMessage }) {
  useEffect(() => {
    const options = {
      serverUrl: serverUrl,
      roomId: roomId
    };
    const connection = createConnection(options);
    connection.connect();
    connection.on('message', (msg) => {
      onReceiveMessage(msg);
    });
    return () => connection.disconnect();
  }, [roomId, serverUrl, onReceiveMessage]); // ✅ All dependencies declared
}

This will work, but there’s one more improvement you can do when your custom Hook accepts event handlers.

Adding a dependency on onReceiveMessage is not ideal because it will cause the chat to re-connect every time the component re-renders. Wrap this event handler into an Event function to remove it from the dependencies:

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

export function useChatRoom({ serverUrl, roomId, onReceiveMessage }) {
  const onMessage = useEvent(onReceiveMessage);

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

Now the chat won’t re-connect every time that the ChatRoom component re-renders. Here is a fully working demo of passing an event handler to a custom Hook that you can play with:

Notice how you no longer need to know how useChatRoom works in order to use it. You could add it to any other component, pass any other options, and it would work the same way. That’s the power of custom Hooks.

When to use custom Hooks

You don’t need to extract a custom Hook for every little duplicated bit of code. Some duplication is fine. For example, extracting a useFormInput Hook to wrap a single useState call like earlier is probably unnecessary.

However, whenever you write an Effect, consider whether it would be clearer to also wrap it in a custom Hook. You shouldn’t need Effects very often, so if you’re writing one, it means that you need to “step outside React” to synchronize with some external system or to do something that React doesn’t have a built-in API for. Wrapping your Effect into a custom Hook lets you precisely communicate your intent and how the data flows through it.

For example, consider a ShippingForm component that displays two dropdowns: one shows the list of cities, and another shows the list of areas in the selected city. You might start with some code that looks like this:

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

  const [city, setCity] = useState(null);
  const [areas, setAreas] = useState(null);
  // This Effect fetches areas for the selected city
  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]);

  // ...

Although this code is quite repetitive, it’s correct to keep these Effects separate from each other. They synchronize two different things, so you shouldn’t merge them into one Effect. Instead, you can simplify the ShippingForm component above by extracting the common logic between them into your own useData Hook:

function useData(url) {
  const [data, setData] = useState(null);
  useEffect(() => {
    if (url) {
      let ignore = false;
      fetch(url)
        .then(response => response.json())
        .then(json => {
          if (!ignore) {
            setData(json);
          }
        });
      return () => {
        ignore = true;
      };
    }
  }, [url]);
  return data;
}

Now you can replace both Effects in the ShippingForm components with calls to useData:

function ShippingForm({ country }) {
  const cities = useData(`/api/cities?country=${country}`);
  const [city, setCity] = useState(null);
  const areas = useData(city ? `/api/areas?city=${city}` : null);
  // ...

Extracting a custom Hook makes the data flow explicit. You feed the url in and you get the data out. By “hiding” your Effect inside useData, you also prevent someone working on the ShippingForm component from adding unnecessary dependencies to it. Ideally, with time, most of your app’s Effects will be in custom Hooks.

function ChatRoom({ roomId }) {
  const [serverUrl, setServerUrl] = useState('https://localhost:1234');

  // 🔴 Avoid: using custom "lifecycle" Hooks
  useMount(() => {
    const connection = createConnection({ roomId, serverUrl });
    connection.connect();

    post('/analytics/event', { eventName: 'visit_chat' });
  });
  // ...
}

// 🔴 Avoid: creating custom "lifecycle" Hooks
function useMount(fn) {
  useEffect(() => {
    fn();
  }, []); // 🔴 React Hook useEffect has a missing dependency: 'fn'
}

function ChatRoom({ roomId }) {
  const [serverUrl, setServerUrl] = useState('https://localhost:1234');

  // ✅ Good: two raw Effects separated by purpose

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

  useEffect(() => {
    post('/analytics/event', { eventName: 'visit_chat', roomId });
  }, [roomId]);

  // ...
}

function ChatRoom({ roomId }) {
  const [serverUrl, setServerUrl] = useState('https://localhost:1234');

  // ✅ Great: custom Hooks named after their purpose
  useChatRoom({ serverUrl, roomId });
  useImpressionLog('visit_chat', { roomId });
  // ...
}

Custom Hooks help you migrate to better patterns

Effects are an “escape hatch”: you use them when you need to “step outside React” and when there is no better built-in solution for your use case. With time, the React team’s goal is to reduce the number of the Effects in your app to the minimum by providing more specific solutions to more specific problems. Wrapping Effects in custom Hooks makes it easier to upgrade your code when these solutions become available. Let’s return to this example:

In the above example, useOnlineStatus is implemented with a pair of useState and useEffect. However, this isn’t the best possible solution. There is a number of edge cases it doesn’t consider. For example, it assumes that when the component mounts, isOnline is already true, but this may be wrong if the network already went offline. You can use the browser navigator.onLine API to check for that, but using it directly would break if you run your React app on the server to generate the initial HTML. In short, this code could be improved.

Luckily, React 18 includes a dedicated API called useSyncExternalStore which takes care of all of these problems for you. Here is how your useOnlineStatus Hook, rewritten to take advantage of this new API:

Notice how you didn’t need to change any of the components to make this migration:

function StatusBar() {
  const isOnline = useOnlineStatus();
  // ...
}

function SaveButton() {
  const isOnline = useOnlineStatus();
  // ...
}

This is another reason for why wrapping Effects in custom Hooks is often beneficial:

1. You make the data flow to and from your Effects very explicit.
2. You let your components focus on the intent rather than on the exact implementation of your Effects.
3. When React adds new features, you can remove those Effects without changing any of your components.

Similar to a design system, you might find it helpful to start extracting common idioms from your app’s components into custom Hooks. This will keep your components’ code focused on the intent, and let you avoid writing raw Effects very often. There are also many excellent custom Hooks maintained by the React community.

import { use } from 'react'; // Not available yet!

function ShippingForm({ country }) {
  const cities = use(fetch(`/api/cities?country=${country}`));
  const [city, setCity] = useState(null);
  const areas = city ? use(fetch(`/api/areas?city=${city}`)) : null;
  // ...

There is more than one way to do it

Let’s say you want to implement a fade-in animation from scratch using the browser requestAnimationFrame API. You might start with an Effect that sets up an animation loop. During each frame of the animation, you could change the opacity of the DOM node you hold in a ref until it reaches 1. Your code might start like this:

To make the component more readable, you might extract the logic into a useFadeIn custom Hook:

You could keep the useFadeIn code as is, but you could also refactor it more. For example, you could extract the logic for setting up the animation loop out of useFadeIn into a new custom Hook called useAnimationLoop:

However, you didn’t have to do that. As with regular functions, ultimately you decide where to draw the boundaries between different parts of your code. For example, you could also take a very different approach. Instead of keeping the logic in the Effect, you could move most of the imperative logic inside a JavaScript class:

Effects let you connect React to external systems. The more coordination between Effects is needed (for example, to chain multiple animations), the more it makes sense to extract that logic out of Effects and Hooks completely like in the sandbox above. Then, the code you extracted becomes the “external system”. This lets your Effects stay simple because they only need to send messages to the system you’ve moved outside React.

The examples above assume that the fade-in logic needs to be written in JavaScript. However, this particular fade-in animation is both simpler and much more efficient to implement with a plain CSS Animation:

Sometimes, you don’t even need a Hook!