How to Implement useDebugValue Hook in React

The useDebugValue Hook is a built-in React Hook designed to improve the debugging experience of custom Hooks within React Developer Tools. This article provides a quick, practical guide on how to implement useDebugValue in your React applications, demonstrating its syntax, usage, and how to optimize its performance with formatting functions.

What is useDebugValue?

In React, useDebugValue is used to display a custom label next to your custom Hooks in React DevTools. This makes it significantly easier to inspect the state and behavior of your custom Hooks during development without cluttering your actual UI or console logs.

It is important to note that you should not add debug values to every custom Hook. It is most valuable for custom Hooks that are part of shared libraries or those with complex, hard-to-inspect internal states.

Basic Implementation

To implement useDebugValue, you call it inside your custom Hook and pass the value you want to display in React DevTools.

Here is a basic implementation using a custom Hook that tracks online status:

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

function useOnlineStatus() {
  const [isOnline, setIsOnline] = useState(true);

  useEffect(() => {
    const handleOnline = () => setIsOnline(true);
    const handleOffline = () => setIsOnline(false);

    window.addEventListener('online', handleOnline);
    window.addEventListener('offline', handleOffline);

    return () => {
      window.removeEventListener('online', handleOnline);
      window.removeEventListener('offline', handleOffline);
    };
  }, []);

  // Implement useDebugValue to show the status in React DevTools
  useDebugValue(isOnline ? 'Online' : 'Offline');

  return isOnline;
}

When you inspect a component utilizing useOnlineStatus in React DevTools, you will see OnlineStatus: "Online" or OnlineStatus: "Offline" next to the Hook’s entry.

Deferring Formatting for Performance

In some cases, formatting a debug value can be computationally expensive (for example, parsing a large date object or a complex data structure). To prevent this formatting from slowing down your application’s rendering, useDebugValue accepts an optional second argument: a formatting function.

The formatting function only runs when React DevTools is open and the Hook is actually inspected.

Here is how to implement deferred formatting:

import { useState, useDebugValue } from 'react';

function useDateAdded(date) {
  const [creationDate] = useState(date);

  // The formatting function (toDateString) will only run when DevTools is open
  useDebugValue(creationDate, date => date.toDateString());

  return creationDate;
}

In this example, the toDateString() method will not execute during standard application renders, protecting your app’s performance while still providing detailed debug information when you inspect the Hook.