How to Update useDebugValue Hook in React

This article explains how to use and update the useDebugValue hook in React to improve your debugging experience in React DevTools. You will learn the purpose of this hook, how to implement it within custom hooks, how its values update dynamically when hook state changes, and how to optimize its performance using an optional formatting function.

Understanding useDebugValue

The useDebugValue hook is a built-in React hook used to assign a custom, readable label to custom hooks inside React DevTools. It does not affect the runtime logic of your application.

Unlike hooks like useState or useReducer, useDebugValue does not have a companion “updater” function (such as setValue). Instead, it updates automatically in React DevTools whenever the state or props passed into it change.

How to Implement and Update the Value

To update the displayed debug value, pass a reactive state variable or prop directly to useDebugValue inside your custom hook. When the underlying state updates, React automatically recalculates and updates the debug label in the DevTools UI.

Here is a practical example of a custom hook that tracks online status and updates its debug value dynamically:

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

export 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);
    };
  }, []);

  // The debug value updates automatically whenever isOnline state changes
  useDebugValue(isOnline ? 'User is Online' : 'User is Offline');

  return isOnline;
}

Whenever the browser’s network status changes, setIsOnline triggers a re-render. Consequently, useDebugValue receives the new value and updates the label displayed in React DevTools in real time.

Optimizing Updates with a Formatting Function

If formatting the debug value requires heavy computation (such as complex date parsing or string manipulation), passing the formatted value directly can slow down your application’s rendering performance.

To resolve this, you can pass a formatting function as the second argument to useDebugValue. React will only execute this function when React DevTools is open and the hook is actively being inspected.

import { useState, useDebugValue } from 'react';

export function useDateTracker() {
  const [date, setDate] = useState(new Date());

  // The expensive formatting function only runs when DevTools is open
  useDebugValue(date, formattedDate => formattedDate.toDateString());

  return [date, setDate];
}

In this setup, useDebugValue still updates its value whenever the date state updates, but the potentially slow toDateString() calculation is deferred until it is actually needed for inspection.