In this article, we will understand how useDebugValue is used in Zustand’s source code.

useDebugValue is used in a function named useStoreWithEquality. useDebugValue is a React Hook that lets you add a label to a custom Hook in React DevTools.

Call useDebugValue at the top level of your custom Hook to display a readable debug value:

// Pulled from https://react.dev/reference/react/useDebugValue
import { useDebugValue } from 'react';

function useOnlineStatus() {
  // ...
  useDebugValue(isOnline ? 'Online' : 'Offline');
  // ...
}

In Zustand, useDebugValue is used on slice an object that looks like below:

const slice = useSyncExternalStoreWithSelector(
  api.subscribe,
  api.getState,
  api.getInitialState,
  selector,
  equalityFn,
)

useDebugValue is also used in src/react.ts

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References:

1. https://github.com/pmndrs/zustand/blob/main/src/traditional.ts#L51

2. https://react.dev/reference/react/useDebugValue

3. https://github.com/pmndrs/zustand/blob/0a4f9d0f71477c5ef399191acc19e25674d0d3c4/src/react.ts#L42