Managing identity in Analytics.js

This page explains how Analytics.js identifies users, and passes userID and anonymousID data, and how to override and change this information.

Segment ID Persistence

To ensure high fidelity, first-party customer data, Segment writes the user’s IDs to the user’s local storage, and uses that as the Segment ID on the cookie whenever possible. Local Storage is meant for storing this type of first-party customer information.

If a user returns to your site after the cookie expires, Analytics.js looks for an old ID in the user’s localStorage, and if one is found, sets it as the user’s ID again in the new cookie. If a user clears their cookies and localstorage, all of the IDs are removed, and the user gets a completely new anonymousID when they next visit the page.

Anonymous IDs

Analytics.js generates a universally unique ID (UUID) for the viewer during the library’s initialization phase, and sets this as anonymousId for each new visitor to your site. This happens before Analytics.js loads any device-mode destinations, and so before these destination-libraries can generate their own user IDs.



You can override the default-generated anonymousID in code using the methods described below:

Retrieve the Anonymous ID

You can get the user’s current anonymousId using the following call:


If the user’s anonymousId is null (meaning not set) when you call this function, Analytics.js automatically generated and sets a new anonymousId for the user.

If you are using the npm library, the previous call returns a promise for user(). As a workaround, you’ll need to grab the user’s current anonymousId in the following way:


Refreshing the Anonymous ID

A user’s anonymousId changes when any of the following conditions are met.

  • The user clears their cookies and localstorage.
  • Your site or app calls analytics.reset() during in the user’s browser session.
  • Your site or app calls analytics.identify() with a userId that is different from the current userId.
  • Your site or app is setting ajs_user_id to an empty string or calling analytics.user().id('') before calling analytics.identify(). This sequence of events will result in a new anonymousId being set when analytics.identify() is called.

Override the Anonymous ID from the Segment snippet

You can also set the anonymousId immediately inside your Segment snippet, even before the ready method returns.


Use this method if you are queueing calls before ready returns and they require a custom anonymousId. Keep in mind that setting the anonymousId in Analytics.js does not overwrite the anonymous tracking IDs for any destinations you’re using.

Device-mode destinations that load their code on your site might also set their own anonymous ID for the user that is separate and different from the Segment generated one. Some destinations use the Segment anonymousId. Read the documentation for each Destination to find out if a Destination sets its own ID.

Override the default Anonymous ID with a call

If the default generated UUID does not meet your needs, you can override it anonymousId for the current user using either of the following methods.


These methods behave exactly the same.

Override the Anonymous ID using the options object

Or in the options object of identify, page, or track calls, like this:

Set the anonymousId in the Options object using the format in the following examples.

The custom anonymousId persists when you use these methods, even if you do not explicitly specify the anonymousId in the calls.

For example, after the Track call below sets the anonId, any later track calls from this user will have the anonymousId of ABC-123-XYZ, even if it is not explicitly specified in the track call.

Override anonymousId in an Identify call

analytics.identify('user_123', {
  name: 'Jane Kim'
}, {
  anonymousId: 'ABC-123-XYZ'

Override anonymousId on a Page call{}, { anonymousId: 'ABC-123-XYZ' });

Override anonymousId on a Track call

analytics.track('Email Clicked', {
  callToAction: 'Signup'
}, {
  anonymousId: 'ABC-123-XYZ'

Saving traits to the context object

Traits are individual pieces of information that you know about a user or a group, and which can change over time.

The options dictionary contains a sub-dictionary called context which automatically captures data depending on the event- and source-type. See the Context documentation to learn more.

The context object contains an optional traits dictionary that contains traits about the current user. You can use this to store information about a user that you got from previous Identify calls, and that you want to add to Track or Page events.

The information you pass in context.traits does not appear in your downstream tools (such as Salesforce, Mixpanel, or Google Analytics); however, this data does appear in your warehouses and storage destinations.

The options object described in the previous section behaves differently from the options.context.traits object discussed here. The traits object described here does not cause anonymousId to persist across different calls.

Consider this Identify event:

analytics.identify('12091906-01011992', {
    plan_id: 'Paid, Tier 2',
    email: ''

The “trait” on this event is plan_id. You can pass these traits into context.traits, so you can use them in Track or Page events that the user triggers later.

The example below shows how you could pass the plan_id as a trait so you can use it later.

analytics.track('Clicked Email', {
  	emailCampaign: 'First Touch'
    traits: {
      plan_id: 'Paid, Tier 2'

This appends the plan_id trait to this Track event. This does not add the name or email, since those traits were not added to the context object. You must do this for every following event you want these traits to appear on, as the traits object does not persist between calls.

Clearing Traits

You can pass an empty object to the traits object to clear all cached traits for a User or Group.

Traits are cached by default when you call the Identify and Group methods. You can clear the traits object for the user or group by passing traits an empty object:


Using analytics.user() and

You can use the user or group method as soon as the Analytics.js library loads, to return information about the currently identified user or group. This information is retrieved from the user’s cookie.

Tip: You can wrap any reference to user() or group() in a ready function block to ensure that Analytics.js has fully loaded so these methods are available.


analytics.ready(function() {

  var user = analytics.user();
  var id =;
  var traits = user.traits();

analytics.ready(function() {

  var group =;
  var id =;
  var traits = group.traits();


Anonymizing IP

Segment automatically collects the user’s IP address for device-based (iOS, Android, Analytics.js and Xamarin) events.


At the moment, Segment doesn’t support automatically collecting IPv6 addresses.

You can manually set the IP by passing a value for options.context.ip to prevent the Segment systems from recording the IP address for the request, as in the example below.

  analytics.track("Order Completed", {}, { context: { ip: "" }});

You must add this override to every Track call to explicitly override IP collection. If you reset this trait in the context object, Segment defaults to the normal IP collection behavior.

This page was last modified: 24 May 2024

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