> For the complete documentation index, see [llms.txt](/docs/llms.txt)

# Facebook Reconcile Lambda | FusionAuth Docs

An overview of the Facebook Reconcile lambda.

# Facebook Reconcile Lambda

When a Facebook identity provider is used to complete a federated login request, FusionAuth will use the [configured linking strategy](/docs/lifecycle/authenticate-users/identity-providers/#linking-strategies) to reconcile the user. FusionAuth will attempt to match the user information returned from the Facebook identity provider to an existing user account or create a new one.

You may optionally utilize a lambda to customize the user and user registration during the authentication event.

When you create a new lambda using the FusionAuth administrative user interface, you will be provided an empty function to implement.

Starting in version 1.65.0, you can use [attribute mappings](/docs/lifecycle/authenticate-users/identity-providers/#attribute-mappings) to map IdP claims directly to FusionAuth user and registration fields without writing a lambda. Reconcile lambdas and attribute mappings are mutually exclusive. Attempting to configure both on a given IdP results in a validation error.

## Lambda Structure[#](#lambda-structure)

If you are using the API to create the lambda you will need to ensure your function has the following signature:

```
function reconcile(user, registration, facebookUser, context) {
  // Lambda code goes here
}
```

This lambda must contain a function named `reconcile` that accepts the following parameters:

*   `user` - the FusionAuth User object. You can modify this object. However, the **email** and **username** attributes may not be modified after the user has been linked.
*   `registration` - the FusionAuth UserRegistration object. You can modify this object.

*   `facebookUser` - the User returned from the Facebook Me API. This object is read-only.
*   `context` - Available since 1.64.0 - an object containing the context of the request, including access to [secrets](/docs/extend/code/lambdas#secrets). This object is read-only.

The two FusionAuth objects are well documented in the [User API](/docs/apis/users) and [Registration API](/docs/apis/registrations) documentation. The `facebookUser` may contain various user claims depending upon the user's Facebook configuration.

## Assigning The Lambda[#](#assigning-the-lambda)

Once a lambda is created, you may assign it to the Facebook identity provider in the IdP configuration.

Navigate to Settings -> Identity Providers and select your existing Facebook configuration or click Add provider and select Facebook if it has not yet been configured.

## Default Lambda[#](#default-lambda)

A default Facebook reconcile lambda is available in FusionAuth that may be used or modified. The default Facebook lambda function is documented below.

```
// This is the default Facebook reconcile, modify this to your liking.
function reconcile(user, registration, facebookUser) {
  // Un-comment this line to see the facebookUser object printed to the event log
  // console.info(JSON.stringify(facebookUser, null, 2));

  user.firstName = facebookUser.first_name;
  user.middleName = facebookUser.middle_name;
  user.lastName = facebookUser.last_name;
  user.fullName = facebookUser.name;

  if (facebookUser.picture && !facebookUser.picture.data.is_silhouette) {
    user.imageUrl = facebookUser.picture.data.url;
  }

  if (facebookUser.birthday) {
    // Convert MM/dd/yyyy -> YYYY-MM-DD
    var parts = facebookUser.birthday.split('/');
    user.birthDate = parts[2] + '-' +  parts[0] + '-' +  parts[1];
  }
}
```