voluptasmollitia/docs-exp/auth.signinwithcustomtoken.md

Monorepo for the Firebase JavaScript SDK

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[Home](./index.md) &gt; [@firebase/auth](./auth.md) &gt; [signInWithCustomToken](./auth.signinwithcustomtoken.md)

## signInWithCustomToken() function

Asynchronously signs in using a custom token.

<b>Signature:</b>

```typescript
export declare function signInWithCustomToken(auth: Auth, customToken: string): Promise<UserCredential>;
```

## Parameters

|  Parameter | Type | Description |
|  --- | --- | --- |
|  auth | [Auth](./auth.auth.md) | The Auth instance. |
|  customToken | string | The custom token to sign in with. |

<b>Returns:</b>

Promise&lt;[UserCredential](./auth.usercredential.md)<!-- -->&gt;

## Remarks

Custom tokens are used to integrate Firebase Auth with existing auth systems, and must be generated by an auth backend using the [createCustomToken](https://firebase.google.com/docs/reference/admin/node/admin.auth.Auth#createcustomtoken) method in the [Admin SDK](https://firebase.google.com/docs/auth/admin) .

Fails with an error if the token is invalid, expired, or not accepted by the Firebase Auth service.