[Home](./index.md) > [@firebase/auth](./auth.md) > [onAuthStateChanged](./auth.onauthstatechanged.md) ## onAuthStateChanged() function Adds an observer for changes to the user's sign-in state. Signature: ```typescript export declare function onAuthStateChanged(auth: Auth, nextOrObserver: NextOrObserver, error?: ErrorFn, completed?: CompleteFn): Unsubscribe; ``` ## Parameters | Parameter | Type | Description | | --- | --- | --- | | auth | [Auth](./auth.auth.md) | The Auth instance. | | nextOrObserver | [NextOrObserver](./auth.nextorobserver.md)<[User](./auth.user.md)> | 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 [onIdTokenChanged()](./auth.onidtokenchanged.md).