useSyncExternalStore

const snapshot = useSyncExternalStore(subscribe, getSnapshot, getServerSnapshot?)

• Usage
  • Subscribing to an external store
  • Subscribing to a browser API
  • Extracting the logic to a custom Hook
  • Adding support for server rendering
• Reference
  • useSyncExternalStore(subscribe, getSnapshot, getServerSnapshot?)
• Troubleshooting
  • I’m getting an error: “The result of getSnapshot should be cached”
  • My subscribe function gets called after every re-render

Usage

Subscribing to an external store

Most of your React components will only read data from their props, state, and context. However, sometimes a component needs to read some data from some store outside of React that changes over time. This includes:

• Third-party state management libraries that hold state outside of React.
• Browser APIs that expose a mutable value and events to subscribe to its changes.

Call useSyncExternalStore at the top level of your component to read a value from an external data store.

import { useSyncExternalStore } from 'react';
import { todosStore } from './todoStore.js';

function TodosApp() {
  const todos = useSyncExternalStore(todosStore.subscribe, todosStore.getSnapshot);
  // ...
}

It returns the snapshot of the data in the store. You need to pass two functions as arguments:

1. The subscribe function should subscribe to the store and return a function that unsubscribes.
2. The getSnapshot function should read a snapshot of the data from the store.

React will use these functions to keep your component subscribed to the store and re-render it on changes.

For example, in the sandbox below, todosStore is implemented as an external store that stores data outside of React. The TodosApp component connects to that external store with the useSyncExternalStore Hook.

Subscribing to a browser API

Another reason to add useSyncExternalStore is when you want to subscribe to some value exposed by the browser that changes over time. For example, suppose that you want your component to display whether the network connection is active. The browser exposes this information via a property called navigator.onLine. This value can change over time without React’s knowledge, so you need to read it with useSyncExternalStore.

import { useSyncExternalStore } from 'react';

function ChatIndicator() {
  const isOnline = useSyncExternalStore(subscribe, getSnapshot);
  // ...
}

To implement the getSnapshot function, read the current value from the browser API:

function getSnapshot() {
  return navigator.onLine;
}

Next, you need to implement the subscribe function. For example, when navigator.onLine changes, the browser fires the online and offline events on the window object. You need to subscribe the callback argument to the corresponding events, and then return a function that cleans up the subscriptions:

function subscribe(callback) {
  window.addEventListener('online', callback);
  window.addEventListener('offline', callback);
  return () => {
    window.removeEventListener('online', callback);
    window.removeEventListener('offline', callback);
  };
}

Now React knows how to read the value from the external navigator.onLine API and how to subscribe to its changes. Try to disconnect your device from the network and notice that the component re-renders in response:

Extracting the logic to a custom Hook

Usually you won’t write useSyncExternalStore directly in your components. Instead, you’ll typically call it from your own custom Hook. This lets you use the same external store from different components.

For example, this custom useOnlineStatus Hook tracks whether the network is online:

import { useSyncExternalStore } from 'react';

export function useOnlineStatus() {
  const isOnline = useSyncExternalStore(subscribe, getSnapshot);
  return isOnline;
}

function getSnapshot() {
  // ...
}

function subscribe(callback) {
  // ...
}

Now different components can call useOnlineStatus without repeating the underlying implementation:

Adding support for server rendering

If your React app uses server rendering, your React components will also run outside the browser environment to generate the initial HTML. This creates a few challenges when connecting to an external store:

• If you’re connecting to a browser-only API, it won’t work because it does not exist on the server.
• If you’re connecting to a third-party data store, you’ll need its data to match between the server and client.

To solve these issues, pass a getServerSnapshot function as the third argument to useSyncExternalStore:

import { useSyncExternalStore } from 'react';

export function useOnlineStatus() {
  const isOnline = useSyncExternalStore(subscribe, getSnapshot, getServerSnapshot);
  return isOnline;
}

function getSnapshot() {
  return navigator.onLine;
}

function getServerSnapshot() {
  return true; // Always show "Online" for server-generated HTML
}

function subscribe(callback) {
  // ...
}

The getServerSnapshot function is similar to getSnapshot, but it runs only in two situations:

• It runs on the server when generating the HTML.
• It runs on the client during hydration, i.e. when React takes the server HTML and makes it interactive.

This lets you provide the initial snapshot value which will be used before the app becomes interactive. If there is no meaningful initial value for the server rendering, you can force the component to render only on the client.

Reference

useSyncExternalStore(subscribe, getSnapshot, getServerSnapshot?)

Call useSyncExternalStore at the top level of your component to read a value from an external data store.

import { useSyncExternalStore } from 'react';
import { todosStore } from './todoStore.js';

function TodosApp() {
  const todos = useSyncExternalStore(todosStore.subscribe, todosStore.getSnapshot);
  // ...
}

It returns the snapshot of the data in the store. You need to pass two functions as arguments:

1. The subscribe function should subscribe to the store and return a function that unsubscribes.
2. The getSnapshot function should read a snapshot of the data from the store.

See more examples above.

Parameters

• subscribe: A function that takes a single callback argument and subscribes it to the store. When the store changes, it should invoke the provided callback. This will cause the component to re-render. The subscribe function should return a function that cleans up the subscription.

• getSnapshot: A function that returns a snapshot of the data in the store that’s needed by the component. While the store has not changed, repeated calls to getSnapshot must return the same value. If the store changes and the returned value is different (as compared by Object.is), React will re-render the component.

• optional getServerSnapshot: A function that returns the initial snapshot of the data in the store. It will be used only during server rendering and during hydration of server-rendered content on the client. The server snapshot must be the same between the client and the server, and is usually serialized and passed from the server to the client. If this function is not provided, rendering the component on the server will throw an error.

Returns

The current snapshot of the store which you can use in your rendering logic.

Caveats

• The store snapshot returned by getSnapshot must be immutable. If the underlying store has mutable data, return a new immutable snapshot if the data has changed. Otherwise, return a cached last snapshot.

• If a different subscribe function is passed during a re-render, React will re-subscribe to the store using the newly passed subscribe function. You can prevent this by declaring subscribe outside the component.

Troubleshooting

I’m getting an error: “The result of getSnapshot should be cached”

If you get this error, it means your getSnapshot function returns a new object every time it’s called, for example:

function getSnapshot() {
  // 🔴 Do not return always different objects from getSnapshot
  return {
    todos: myStore.todos
  };
}

React will re-render the component if getSnapshot return value is different from the last time. This is why, if you always return a different value, you will enter an infinite loop and get this error.

Your getSnapshot object should only return a different object if something has actually changed. If your store contains immutable data, you can return that data directly:

function getSnapshot() {
  // ✅ You can return immutable data
  return myStore.todos;
}

If your store data is mutable, your getSnapshot function should return an immutable snapshot of it. This means it does need to create new objects, but it shouldn’t do this for every single call. Instead, it should store the last calculated snapshot, and return the same snapshot as the last time if the data in the store has not changed. How you determine whether mutable data has changed depends on how your mutable store is implemented.

My subscribe function gets called after every re-render

This subscribe function is defined inside a component so it is different on every re-render:

function ChatIndicator() {
  const isOnline = useSyncExternalStore(subscribe, getSnapshot);
  
  // 🚩 Always a different function, so React will resubscribe on every re-render
  function subscribe() {
    // ...
  }

  // ...
}

React will resubscribe to your store if you pass a different subscribe function between re-renders. If this causes performance issues and you’d like to avoid resubscribing to the store, move the subscribe function outside:

function ChatIndicator() {
  const isOnline = useSyncExternalStore(subscribe, getSnapshot);
  // ...
}

// ✅ Always the same function, so React won't need to resubscribe
function subscribe() {
  // ...
}

Alternatively, wrap subscribe into useCallback to only resubscribe when some argument changes:

function ChatIndicator({ userId }) {
  const isOnline = useSyncExternalStore(subscribe, getSnapshot);
  
  // ✅ Same function as long as userId doesn't change
  const subscribe = useCallback(() => {
    // ...
  }, [userId]);

  // ...
}