Spec: Alias

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The Alias method is an advanced method used to merge 2 unassociated user identities, effectively connecting 2 sets of user data in one profile.

Alias and Unify

Alias calls can’t be used to merge profiles in Unify. For more information on how Unify merges user profiles, view the Identity Resolution documentation.

Alias is an advanced method

The Alias method allows you to explicitly change the ID of a tracked user. This should only be done when it’s required for downstream destination compatibility. See the Best Practices for Identifying Users docs for more information.


The Alias call has the following fields:

Field   Type Description
userId   String The userId is a string that will be the user’s new identity, or an existing identity that you wish to merge with the previousId. See the User ID docs for more detail.
previousId optional String The previousId is the existing ID you’ve referred to the user by. It might be an Anonymous ID assigned to that user or a User ID you previously identified them with using Segment’s Identify call.
options optional Object A dictionary of options. For example, enable or disable specific destinations for the call.
callback optional Function A function that is executed after a timeout of 300 ms, giving the browser time to make outbound requests first.

The Alias method follows the format below:

analytics.alias(userId, [previousId], [options], [callback]);

Here’s the payload of a basic Alias call that will associate this user’s existing id (email address) with a new one (a database ID), with most common fields removed:

  "type": "alias",
  "previousId": "jen@email.com",
  "userId": "507f191e81"

Here’s the corresponding JavaScript event that would generate the above payload. If you’re using Segment’s JavaScript library, Segment automatically passes in the user’s anonymousId as previousId for you:


If you’re instrumenting a website, the Anonymous ID is generated in the browser so you must call Alias from the client-side. If you’re using a server-side session ID as the Anonymous ID, then you must call Alias from the server-side.

Based on the library you use, the syntax in the examples might be different. You can find library-specific documentation on the Sources Overview page.


Here’s a complete example of an Alias call:

  "anonymousId": "507f191e810c19729de860ea",
  "channel": "browser",
  "context": {
    "ip": "",
    "userAgent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/40.0.2214.115 Safari/537.36"
  "integrations": {
    "All": true,
    "Mixpanel": false,
    "Salesforce": false
  "messageId": "022bb90c-bbac-11e4-8dfc-aa07a5b093db",
  "previousId": "39239-239239-239239-23923",
  "receivedAt": "2015-02-23T22:28:55.387Z",
  "sentAt": "2015-02-23T22:28:55.111Z",
  "timestamp": "2015-02-23T22:28:55.111Z",
  "type": "alias",
  "userId": "507f191e81",
  "version": "1.1"

This page was last modified: 24 Jan 2024

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