veritatisdoloremque/docs-exp/auth-types.auth.onauthstatechanged.md

Monorepo for the Firebase JavaScript SDK

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[Home](./index.md) &gt; [@firebase/auth-types](./auth-types.md) &gt; [Auth](./auth-types.auth.md) &gt; [onAuthStateChanged](./auth-types.auth.onauthstatechanged.md)

## Auth.onAuthStateChanged() method

Adds an observer for changes to the user's sign-in state.

<b>Signature:</b>

```typescript
onAuthStateChanged(
    nextOrObserver: NextOrObserver<User | null>,
    error?: ErrorFn,
    completed?: CompleteFn
  ): Unsubscribe;
```

## Parameters

|  Parameter | Type | Description |
|  --- | --- | --- |
|  nextOrObserver | [NextOrObserver](./auth-types.nextorobserver.md)<!-- -->&lt;[User](./auth-types.user.md) \| null&gt; | callback triggered on change. |
|  error | ErrorFn | callback triggered on error. |
|  completed | CompleteFn | callback triggered when observer is removed. |

<b>Returns:</b>

Unsubscribe

## Remarks

To keep the old behavior, see [Auth.onIdTokenChanged()](./auth-types.auth.onidtokenchanged.md)<!-- -->.