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Escape Hatches

Reusing Logic with Custom Hooks

React comes with several built-in Hooks like useState, useContext, and useEffect. Sometimes, you’ll wish that there was a Hook for some more specific purpose: for example, to fetch data, to keep track of whether the user is online, or to connect to a chat room. You might not find these Hooks in React, but you can create your own Hooks for your application’s needs.

You will learn

  • What custom Hooks are, and how to write your own
  • How to reuse logic between components
  • How to name and structure your custom Hooks
  • When and why to extract 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:

import { useState, useEffect } from 'react';

export default function StatusBar() {
  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 <h1>{isOnline ? '✅ Online' : '❌ Disconnected'}</h1>;
}

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:

import { useState, useEffect } from 'react';

export default function SaveButton() {
  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);
    };
  }, []);

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

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

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);
    };
  },