[Home](./index.md) > [@firebase/auth](./auth.md) > [Auth](./auth.auth.md) > [onAuthStateChanged](./auth.auth.onauthstatechanged.md) ## Auth.onAuthStateChanged() method Adds an observer for changes to the user's sign-in state. Signature: ```typescript onAuthStateChanged(nextOrObserver: NextOrObserver, error?: ErrorFn, completed?: CompleteFn): Unsubscribe; ``` ## Parameters | Parameter | Type | Description | | --- | --- | --- | | nextOrObserver | [NextOrObserver](./auth.nextorobserver.md)<[User](./auth.user.md) \| null> | callback triggered on change. | | error | ErrorFn | callback triggered on error. | | completed | CompleteFn | callback triggered when observer is removed. | Returns: Unsubscribe ## Remarks To keep the old behavior, see [Auth.onIdTokenChanged()](./auth.auth.onidtokenchanged.md).