Analytics.js Source

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Flagship ✓

Flagship libraries offer the most up-to-date functionality on Segment’s most popular platforms. Segment actively maintains flagship libraries, which benefit from new feature releases and ongoing development and support.

Analytics.js enables you to send your data to hundreds of destination tools without having to learn, test, or use a new API every time.

Segment’s Analytics.js library is fully open-source and can be viewed on GitHub.

Getting started

Use the Analytics.js QuickStart Guide to learn how to add Analytics.js to your site. Once you’ve installed the library, read on for the detailed API reference.

Benefits of Analytics.js

Analytics.js provides two key benefits over the previous version.


Analytics.js reduces page load time and improves site performance. Its package size is ~70% smaller than its predecessor, the classic version of Analytics.js.

Many factors impact page load time, including network conditions, hosting locations, and page weight. Page weight for each customer integration varies based on the number of device-mode destinations that are enabled for each source. The more device-mode destinations that are enabled, the more data gets added to the library, which will impact the weight of the library.

Developer experience

Analytics.js improves developer experience by introducing new ways for developers to augment events throughout the event timeline. For example, developers can augment events either before or after an event occurs, or while the event is in-flight.

For example, you can use Analytics.js to build features that:

  • Ensure you have user consent to track before an event fires
  • Enrich events with customer or page context while in-flight with middleware
  • Check an event for errors after the event is sent to Segment

Basic tracking methods

The basic tracking methods below serve as the building blocks of your Segment tracking. They include Identify, Track, Page, Group, and Alias.

These methods correspond with those used in the Segment Spec. The documentation on this page explains how to use these methods in Analytics.js.

Good to know

For any of the methods described in this page, you can replace the properties in the code samples with variables that represent the data collected.


Use the identify method to link your users and their actions, to a recognizable userId and traits. You can see an identify example in the Quickstart guide or find details on the identify method payload.

`identify` and anonymous visitors

Segment recommends against using identify for anonymous visitors to your site. Analytics.js automatically retrieves an anonymousId from localStorage or assigns one for new visitors, and then attaches it to all page and track events both before and after an identify.

The Identify method follows the format below:

analytics.identify([userId], [traits], [options], [callback]);

The Identify call has the following fields:

Field   Type Description
userId optional String The database ID for the user. If you don’t know who the user is yet, you can omit the userId and just record traits. You can read more about identities in the identify reference.
traits optional Object A dictionary of traits you know about the user, like email or name. You can read more about traits in the identify reference.
options optional Object A dictionary of options. For example, enable or disable specific destinations for the call. Note: If you do not pass a traits object, pass an empty object (as an ‘{}’) before options.
callback optional Function A function executed after a timeout of 300 ms, giving the browser time to make outbound requests first.

By default, Analytics.js caches traits in the browser’s localStorage and attaches them to each Identify call.

For example, you might call Identify when someone signs up for a newsletter but hasn’t yet created an account on your site. The example below shows an Identify call (using hard-coded traits) that you might send in this case.

  nickname: 'Amazing Grace',
  favoriteCompiler: 'A-0',
  industry: 'Computer Science'

Then, when the user completes the sign up process, you might see the following:

analytics.identify('12091906-01011992', {
  name: 'Grace Hopper',
  email: ''

The traits object for the second call also includes nickname, favoriteCompiler, and industry.

You may omit both traits and options, and pass the callback as the second argument.

analytics.identify('12091906-01011992', function(){
  // Do something after the identify request has been sent
  // Note: site-critical functionality should not depend on your analytics provider


The Track method lets you record actions your users perform. You can see a track example in the Quickstart guide or find details on the track method payload.

The Track method follows the format below:

analytics.track(event, [properties], [options], [callback]);

The track call has the following fields:

Field Type Description
event String The name of the event you’re tracking. You can read more about the track method and recommended event names.
properties Object Optional. A dictionary of properties for the event. If the event was 'Added to Cart', it might have properties like price and productType.
options Object Optional. A dictionary of options. For example, enable or disable specific destinations for the call. Note: If you do not pass a properties object, pass an empty object (like ‘{}’) before options.
callback Function Optional. A function that runs after a timeout of 300 ms, giving the browser time to make outbound requests first.

The only required argument in Analytics.js is an event name string. You can read more about how Segment recommends you name events.

Example Track call:

analytics.track('Article Completed', {
  title: 'How to Create a Tracking Plan',
  course: 'Intro to Analytics',

For more information about choosing which events to track, event naming, and more, check out Analytics Academy.

The only required argument on Track calls in Analytics.js is an event name string. Read more about how Segment recommends naming your events.

trackLink is a helper method that attaches the track call as a handler to a link. With trackLink, Analytics.js inserts a timeout of 300 ms to give the track call more time. This is useful when a page would redirect before the track method could complete all requests.

The trackLink method follows the format below.

analytics.trackLink(element, event, [properties])
Field   Type Description
element(s)   Element or Array DOM element to bind with track method. You may pass an array of elements or jQuery objects. Note: This must be an element, not a CSS selector.
event   String or Function The name of the event, passed to the track method. Or a function that returns a string to use as the name of the track event.
properties optional Object or Function A dictionary of properties to pass with the track method or a function that returns an object to use as the properties of the event.


var link = document.getElementById('free-trial-link');

analytics.trackLink(link, 'Clicked Free-Trial Link', {
  plan: 'Enterprise'

Track form

trackForm is a helper method that binds a track call to a form submission. The trackForm method inserts a timeout of 300 ms to give the track call more time to complete. This is useful to prevent a page from redirecting before the track method could complete all requests.

The trackForm method follows the format below.

analytics.trackForm(form, event, [properties])
Field   Type Description
form(s)   Element or Array The form element to track or an array of form elements or jQuery objects. Note: trackForm takes an element, not a CSS selector.
event   String or Function The name of the event, passed to the track method. Or a function that returns a string to use as the name of the track event.
properties optional Object or Function A dictionary of properties to pass with the track method. Or a function that returns an object to use as the properties of the event.


var form = document.getElementById('signup-form');

analytics.trackForm(form, 'Signed Up', {
  plan: 'Premium',
  revenue: 99.00


The Page method lets you record page views on your website, along with optional extra information about the page viewed by the user.

Because some Destinations require a page call to instantiate their libraries, you must call page at least once per page load. You can call it more than once if needed, for example, on virtual page changes in a single page app.

See the implementation guide for more information about calling the Page method.

Analytics.js includes a Page call by default as the final line in the Analytics.js snippet. You can update this page call within the guidelines below.

The page method follows the format below.[category], [name], [properties], [options], [callback]);

The page call has the following fields:

Field   Type Description
category optional String The category of the page. Useful for cases like ecommerce where many pages might live under a single category. Note: if you pass only one string to page it is assumed to be name. You must include a name to send a category.
name optional String The name of the page.
properties optional Object A dictionary of properties of the page. Note: Analytics.js collects url, title, referrer and path are automatically. This defaults to a canonical url, if available, and falls back to document.location.href.
options optional Object A dictionary of options. For example, enable or disable specific destinations for the call. Note: If you do not pass a properties object, pass an empty object (like ‘{}’) before options.
callback optional Function A function that runs after a timeout of 300 ms, giving the browser time to make outbound requests first.

Default page properties

Analytics.js adds properties to each page call.'Pricing');

Segment adds the following information:'Pricing', {
  title: 'Segment Pricing',
  url: '',
  path: '/pricing',
  referrer: ''

You can override these values by explicitly setting them in your calls. For example:'Pricing', {
  title: 'My Overridden Title',
  path: '/pricing/view'

Translates to:'Pricing', {
  title: 'My Overridden Title',
  url: '',
  path: '/pricing/view',
  referrer: ''

Segment sets the path and url property to the value of the canonical element on your page. If a canonical element is not set, the values will be set from the browser.


The Group method associates an identified user with a company, organization, project, workspace, team, tribe, platoon, assemblage, cluster, troop, gang, party, society or any other collective noun you come up with for the same concept.

This is useful for tools like Intercom, Preact, and Totango, as it ties the user to a group of other users.

The Group method follows the format below., [traits], [options], [callback]);

The Group call has the following fields:

Field   Type Description
groupId   String The Group ID to associate with the current user.
traits optional Object A dictionary of traits for the group. Example traits for a group include address, website, and employees.
options optional Object A dictionary of options. For example, enable or disable specific destinations for the call. Note: If you do not pass a properties object, pass an empty object (like ‘{}’) before options.
callback optional Function A function that runs after a timeout of 300 ms, giving the browser time to make outbound requests first.

Example group call:'UNIVAC Working Group', {
  principles: ['Eckert', 'Mauchly'],
  site: 'Eckert–Mauchly Computer Corporation',
  statedGoals: 'Develop the first commercial computer',
  industry: 'Technology'

By default, Analytics.js caches group traits in the browser’s local storage and attaches them to each group call, similar to how the identify method works.

Find more details about group, including the group payload, in the Group Spec.


The Alias method combines two unassociated user identities. Segment usually handles aliasing automatically when you call identify on a user, however some tools require an explicit alias call.

This is an advanced method, but it’s required to manage user identities successfully in some Segment destinations like Kissmetrics and Mixpanel.

The Alias method follows the format below:

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

The Alias call has the following fields:

Field   Type Description
userId   String The new user ID you want to associate with the user.
previousId optional String The previous ID that the user was recognized by. This defaults to the currently identified user’s ID.
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.

For more details about Alias, including the alias call payload, check out the Segment Spec.

Utility methods

The Analytics.js utility methods help you change how Segment loads on your page. They include:


The load method is also available when you load analytics.js through the NPM package.

You can load a buffered version of analytics.js that requires you to call load explicitly before analytics.js initiates any network activity. This is useful if you want to, for example, wait for user consent before you fetch tracking destinations or send buffered events to Segment.

Call load one time only.

export const analytics = new AnalyticsBrowser()

analytics.identify("hello world")

if (userConsentsToBeingTracked) {
    analytics.load({ writeKey: '<YOUR_WRITE_KEY>' }) // destinations loaded, enqueued events are flushed

You can also use load if you fetch some settings asynchronously.

const analytics = new AnalyticsBrowser()
fetchWriteKey().then(writeKey => analytics.load({ writeKey }))

analytics.identify("hello world")


The ready method allows you to pass in a method that is called once Analytics.js finishes initializing, and once all enabled device-mode destinations load. It’s like jQuery’s ready method, except for Destinations.

The ready method isn’t invoked if any Destination throws an error (for example, for an expired API key, incorrect settings configuration, or when a Destination is blocked by the browser) during initialization.

The code in the ready function only executes after ready is emitted.

If you want to access end-tool library methods that do not match any Analytics.js methods, like adding an extra setting to Mixpanel, you can use a ready callback so that you’re guaranteed to have access to the Mixpanel object, like so:

analytics.ready(function() {
  window.mixpanel.set_config({ verbose: true });

The ready method uses the following format:


The ready method has the following fields:

Field Type Description
callback Function A function to be executed after all enabled destinations have loaded.


Calling the debug method turns on debug mode, which logs helpful messages to the console. Subsequent Segment events generate messages in the developer console after you invoke debug.






The global analytics object emits events whenever you call alias, group, identify, track, or page.

Use the on method to set listeners for these events and run your own custom code. This can be useful if you want to send data to a service for which Segment doesn’t have a destination.

analytics.on(method, callback);
Field Type Description
method String Name of the method to listen for.
callback Function A function to execute after each emitted method, taking three arguments: event, properties, options.


analytics.on('track', function(event, properties, options) {

  bigdataTool.push(['recordEvent', event]);


This method emits events before they are processed by the Segment integration, and may not include some of the normalization Segment performs on the client before sending the data to the Segment servers.


Page event properties are stored in the options object.

Extending timeout

The timeout method sets the length (in milliseconds) of callbacks and helper functions. This is useful if you have multiple scripts that need to fire in your callback or trackLink, trackForm helper function.

The example below sets the timeout to 500 ms.


If you’re triggering ad network conversion pixels, Segment recommends extending timeout to 500 ms to account for slow load times.

Reset or log out

Calling reset resets the id, including anonymousId, and clears traits for the currently identified user and group.


The reset method only clears the cookies and localStorage created by Segment. It doesn’t clear data from other integrated tools, as those native libraries might set their own cookies to manage user tracking, sessions, and manage state. To completely clear out the user session, see the documentation provided by those tools.

Segment doesn’t share localStorage across subdomains. If you use Segment tracking on multiple subdomains, you must call analytics.reset() for each subdomain to completely clear out the user session.


You can utilize this in instances where an API call fires on a hard redirect, and are missed from getting captured in Segment. If you set this flag to true, it enables firing the event before the redirect. This is available for all events. You can read more about this in the Github PR.

Managing data flow with the Integrations object

Tip: You can change how your data flows in several different ways without having to change your code. See Filtering Data to learn more.

You can pass an integrations object in the options of Alias, Group, Identify, Page, and Track methods to send data to only the selected destinations. By default, all Destinations are enabled.

The example below sends a message only to Intercom and Google Analytics.

analytics.identify('user_123', {
  email: '',
  name: 'Jane Kim'
}, {
  integrations: {
    'All': false,
    'Intercom': true,
    'Google Analytics': true

'All': false tells Segment not to send data to any Destinations by default, unless they’re explicitly listed as true in the next lines.

As an opposite example, the snippet below sends a message to all integrations except Intercom and Google Analytics.

analytics.identify('user_123', {
  email: '',
  name: 'Jane Kim'
}, {
  integrations: {
    'Intercom': false,
    'Google Analytics': false

You don’t need to include 'All': true in this call because it’s implied as the default behavior. Instead, only list the destinations that you want to exclude, with a false flag for each.

Destination flags are case sensitive and match the destination’s name in the docs (for example, “AdLearn Open Platform”, “”, “Mailchimp”, etc). If a Destination has more than one acceptable name, this appears in the documentation for that destination.

Business tier customers can filter Track calls from the Source Schema page in the Segment UI. Segment recommends that you use the UI to simplify filter management and make updates without changing your site’s code.

Load options

Note: To use this feature, you must be on snippet version 4.1.0 or later. You can get the latest version of the snippet here.

You can modify the .load method in Analytics.js (the second line of the snippet) to take a second argument. If you pass an object with an integrations dictionary, then Segment only loads the integrations in that dictionary that are marked as enabled with the boolean value true.

You can only call .load on page load, or reload (refresh). If you modify the .load method between page loads, it doesn’t have any effect until the page is reloaded.

For example:

analytics.load('writekey', { integrations: { All: false, 'Google Analytics': true, '': true } })

This way, you can conditionally load integrations based on what customers opt into on your site. The example below shows how you might load only the tools that the user agreed to use.

  analytics.load('writekey', { integrations: consentedTools })

Bundle obfuscation

You can also add an obfuscate property to the object in the second parameter, which obscures the URL from which your integrations and destination actions are loaded. This helps prevent words that are flagged by ad blockers to not be detected in your URL, enabling the integration to properly load.

For example:

analytics.load('writekey', { obfuscate: true })

The obfuscate value is false by default.

ISO string conversion

By default, the Analytics.js library will convert ISO8061 strings to a Date object before passing it to downstream device-mode integrations. If you would like to disable this functionality and send those strings as they are passed to the event, you can use the load method to pass in the disableAutoISOConversion option.

For example:

analytics.load('writekey', { disableAutoISOConversion: true })

Client hints

Some userAgent strings are frozen and contain less information. If you would like to request more information when it’s available, you can pass an array of strings with fields you would like to request to the highEntropyValuesClientHints option. The example array below contains all possible values.

For example:

analytics.load('writekey', { highEntropyValuesClientHints: ['architecture', 'bitness', 'model', 'platformVersion', 'uaFullVersion', 'fullVersionList', 'wow64'] })


For testing or staging environments, it can be useful to disable your SDK to ensure no events send.

If disable: true is passed, all analytics method calls will be a no-op, and no network calls will be initiated.

analytics.load('writekey', { disable: true })

For wrapper/plugin authors: if you have a use case where you need special access to the CDN Settings (for example, consent management), you can also pass a function. This API waits for cdnSettings to be fetched. Keep in mind that cdnSettings is an unstable object.

analytics.load('writekey', { disable: (cdnSettings) => true })


When enabled, Analytics.js automatically retries network and server errors. With persistent retries, Analytics.js can:

  • Support offline tracking. Analytics.js queues your events and delivers them when the user comes back online.
  • Better handle network issues. When your application can’t connect to the Segment API, Segment continues to store the events on the browser to prevent data loss.

Analytics.js stores events in localStorage and falls back to in-memory storage when localStorage is unavailable. It retries up to 10 times with an incrementally increasing back-off time between each retry. Analytics.js queues up to 100 events at a time to avoid using too much of the device’s local storage. See the destination Retries documentation to learn more.


Batching is the ability to group multiple requests or calls into one request or API call. All requests sent within the same batch have the same receivedAt time. With Analytics.js, you can send events to Segment in batches. Sending events in batches enables you to have:

  • Delivery of multiple events with fewer API calls
  • Fewer errors if a connection is lost because an entire batch will retry at once rather than multiple calls retrying at random times.


You can start batching by changing the strategy to "batching" and the parameters for size and timeout within the load method in the analytics object. Batching requires both parameters.

analytics.load("<write_key>", {
    integrations: {
      "": {
        deliveryStrategy: {
          strategy: "batching",
          config: {
            size: 10,
            timeout: 5000

You can check to see if batching works by checking your source’s debugger in Sources > Debugger. When you select an event and view the Raw code, the receivedAt time of all the events in the batch should be the same.

Batch size

The batch size is the threshold that forces all batched events to be sent once it’s reached. For example, size: 10 means that after triggering 10 events, Analytics.js sends those 10 events together as a batch to Segment.

Your total batched events can’t exceed the maximum payload size of 500 KB, with a limit of 32 KB for each event in the batch. If the 500 KB limit is reached, the batch will be split.


timeout is the number of milliseconds that forces all events queued for batching to be sent, regardless of the batch size, once it’s reached. For example, timeout: 5000 sends every event in the batch to Segment once 5 seconds passes.

Batching FAQs

Will Analytics.js deliver events that are in the queue when a user closes the browser?

Analytics.js does its best to deliver the queued events before the browser closes, but the delivery isn’t guaranteed.

Upon receiving the beforeunload browser event, Analytics.js attempts to flush the queue using fetch requests with keepalive set to true. Since the max size of keepalive payloads is limited to 64 KB, if the queue size is bigger than 64 KB at the time the browser closes, then there is a chance of losing a subset of the queued events. Reducing the batch size or timeout will alleviate this issue, but that will be a trade-off decision.

Can other destinations receive batched events?

No, this batching only impacts events sent to Segment. Once the batch reaches Segment, it’s split up and follows the normal path of an event.

Will batching impact billing or throughput?

No, batching won’t impact billing or throughput.

Can I use batching with partner integrations?

Partner integrations don’t support batching as all other partner integrations run one event at a time. Only events support batched delivery.

Does batching work on all browsers?

Batching won’t work on Internet Explorer.

If a source has retry enabled, does the retry behavior change when using batching?

Batching delays retries, as events that are queued for batching aren’t retried until a batch delivery fails.

When using Middlewares as a source and destination, is there a change in behavior when using batching?

No, there is no change in behavior to Middlewares.

When using Segment features (Schema filtering, integrations object, Protocols) to filter events from going to destinations (device and cloud-mode), will batching impact the filtering of events?

No, there is no impact to how events filter.

Plugins and source middleware

When you develop against Analytics 2.0, the plugins you write can augment functionality, enrich data, and control the flow and delivery of events. From modifying event payloads to changing analytics functionality, plugins and middleware help to speed up the process of getting things done.

Plugins and source middleware accomplish the same thing, but plugins are significantly more powerful (but more verbose to implement).

For basic use cases like adding event fields or dropping specific events, use source middleware. If you need more granular control of the lifecycle, or want to be able to abort the Segment initialization, you should use plugins.

Source Middleware

Source middleware runs before any other plugins. You can use this to enrich or drop an event.

Example usage of addSourceMiddleware

Here are some examples of using addSourceMiddleware for enrichment and validation.

  • Enrichment
      analytics.addSourceMiddleware(({ payload, next }) => {
         const { event } = payload.obj.context
         if (event.type === 'track') {
  • Validation
      analytics.addSourceMiddleware(({ payload, next }) => {
        const { event } = payload.obj.context
        if (!isValid(event)) {
          return null // event is dropped

Plugin categories

Plugins are bound by Analytics 2.0 which handles operations such as observability, retries, and error handling. There are two different categories of plugins:

  • Critical Plugins:
    • Errors thrown in load() will completely stop segment from initializing.
Type Details
before Executes before event processing begins. These are plugins that run before any other plugins run.

See the example of how Analytics.js uses the Event Validation plugin to verify that every event has the correct shape.

Source middleware added via addSourceMiddleware is treated as a before plugin.
  • Non-critical Plugins:
    • This plugin can throw an error on load(), and all other segment plugins, including destinations, will load as usual, without blocking the delivery pipeline.

Non-critical plugins are only non-critical from a loading standpoint. For example, if the before plugin crashes, this can still halt the event delivery pipeline.

Non-critical plugins run through a timeline that executes in order of insertion based on the entry type. Segment has these four entry types of non-critical plugins:

Type Details
enrichment Executes as the first level of event processing. These plugins modify an event.
destination Executes as events begin to pass off to destinations.

This doesn’t modify the event outside of the specific destination, and failure doesn’t halt the execution.
after Executes after all event processing completes. You can use this to perform cleanup operations.
utility Executes once during the bootstrap, to give you an outlet to make any modifications as to how Analytics.js works internally. This allows you to augment Analytics.js functionality.

Example plugins

Here’s an example of a plugin that converts all track event names to lowercase before the event goes through the rest of the pipeline:

export const lowercase: Plugin = {
  name: 'Lowercase events',
  type: 'enrichment',
  version: '1.0.0',

  isLoaded: () => true,
  load: () => Promise.resolve(),

  track: (ctx) => {
    ctx.updateEvent('event', ctx.event.event.toLowerCase())
    return ctx

const identityStitching = () => {
  let user

  const identity = {
    // Identifies your plugin in the Plugins stack.
    // Access `` to see the full list of plugins
    name: 'Identity Stitching',
    // Defines where in the event timeline a plugin should run
    type: 'enrichment',
    version: '0.1.0',

    // use the `load` hook to bootstrap your plugin
    // The load hook will receive a context object as its first argument
    // followed by a reference to the analytics.js instance from the page
    load: async (_ctx, ajs) => {
      user = ajs.user()

    // Used to signal that a plugin has been property loaded
    isLoaded: () => user !== undefined,

    // Applies the plugin code to every `identify` call in Analytics.js
    // You can override any of the existing types in the Segment Spec.
    async identify(ctx) {
      // Request some extra info to enrich your `identify` events from
      // an external API.
      const req = await fetch(
      const userReq = await req.json()

      // ctx.updateEvent can be used to update deeply nested properties
      // in your events. It's a safe way to change events as it'll
      //  create any missing objects and properties you may require.
      ctx.updateEvent('traits.custom', userReq)

      // Every plugin must return a `ctx` object, so that the event
      // timeline can continue processing.
      return ctx

  return identity

// Registers Segment's new plugin into Analytics.js

Here’s an example of a utility plugin that allows you to change the format of the anonymous_id cookie: => {{
        name: 'Cookie Compatibility',
        version: '0.1.0',
        type: 'utility',
        load: (_ctx, ajs) => {
          const user = ajs.user()
          const cookieJar = user.cookies
          const cookieSetter = cookieJar.set.bind(cookieJar)

          // blindly convert any values into JSON strings
          cookieJar.set = (key, value, opts) => cookieSetter(key, JSON.stringify(value), opts)

          // stringify any existing IDs

        isLoaded: () => true

You can view Segment’s existing plugins to see more examples.

Register a plugin

Registering plugins enable you to modify your analytics implementation to best fit your needs. You can register a plugin using this:

// A promise will resolve once the plugins have been successfully loaded into Analytics.js
// You can register multiple plugins at once by using the variable args interface in Analytics.js
await, pluginB, pluginN)

Video player plugins

Segment offers video player ‘plugins’ so you can quickly collect video events using Analytics.js. See the specific documentation below to learn more:

Cross-subdomain analytics

Analytics.js tracks across subdomains out of the box. All Segment destinations fully support this feature.

To track activity on your subdomains, include the Segment Analytics.js snippet on each subdomain. Segment sets users’ anonymousId on the top-level domain, so that users are tracked across any subdomain.

Because Segment tracks across subdomains, you can either use the same Segment source, or use separate sources for each subdomain. What you decide depends on your team’s goals for tracking each subdomain.

Segment doesn’t offer tracking across top-level domains out of the box. If you want to track across top-level domains, you can utilize Segment’s Querystring API to pass the anonymousId from Website A to Website B in the query string. When a user moves from Website A to Website B with the anonymousId in the query string, Analytics.js reads that value and sets the anonymousId to it, rather than generating a new one.

UTM Tracking

UTM parameters are only used when linking to your site from outside your domain. When a visitor arrives using a link containing UTM parameters, Segment’s analytics.js library will parse the URL query string and add the information to the event payload. For more information about UTM tracking, see the Tracking Customers Across Channels and Devices documentation.

UTM parameters contain three essential components (utm_source, utm_medium, utm_campaign) and two optional (utm_content, utm_term). For example, if you include the following three parameters in your URL: ?utm_source=mysource&utm_medium=email&utm_campaign=mytestcampaign, once a visitor arrives using a link containing the above, Segment automatically grabs the UTM parameters and subsequent events will contain these parameters within the ‘context’ object (visible in the raw view of your Source Debugger.)

So, for example, if somebody follows the link with above query string to your site, the subsequent ‘page’ call in your Debugger should contain the below and will be passed to any enabled destinations:

“context”: { “campaign”: { “medium”: “email”, “name”: “mytestcampaign”, “source”: “mysource”, },

Whenever the UTM parameters are no longer a part of the URL, Segment no longer includes them. For example, if the user goes to a new page within your website which does not contain these parameters, they will not be included in subsequent events. UTM parameters are non-persistent by default as they could potentially cause data accuracy problems. Here’s an example of why: Say a user clicks on an ad and lands on your site. He navigates around and bookmarks an internal page - or maybe shares a link with a friend, who shares it with another friend. All those links would then point back to the same test utm_source as the initial referrer for any purchase.

Analytics.js performance

The Analytics.js library and all Destination libraries are loaded with the HTML script async tag. This also means that Segment fires methods asynchronously, so you should adjust your code accordingly if you require that events be sent from the browser in a specific order.

While many tools require access to the DOM or cookies, for the Zendesk, Salesforce, and Mailchimp destinations, Segment doesn’t need to load a native JavaScript library. Instead, Segment’s servers send data to the end-tools.

Segment loads the libraries required for your enabled Destinations. When you disable a destination, the custom version of Analytics.js loaded on your site stops requesting that library.

Using Analytics.js doesn’t offer a large performance benefit, but is more performant than installing each of the destinations individually. And as more destinations move to accept data directly from Segment, you’ll receive more performance benefits automatically.

One option, if you don’t want to use any bundled third-party tools, is to use the Analytics-Node package.

Analytics.js doesn’t set third-party cookies and only sets first-party cookies.

Bundle size

Segment’s Analytics.js JavaScript snippet increases the page size by about 1.1KB.

The snippet asynchronously requests and loads a customized JavaScript bundle (analytics.min.js), which contains the code and settings needed to load your device-mode destinations. The size of this file changes depending on the number of and which destinations you enable.

Without any destinations enabled, the analytics.min.js file is about 62KB. Each time you enable a destination, the file’s size may increase slightly.

Cookies set by Analytics.js

Segment sets three cookies in general:

Cookie Description
ajs_anonymous_id An anonymous ID generated by Analytics.js, used for Segment calls.
ajs_group_id A group ID that can be specified by making a group() call with Analytics.js.
ajs_user_id A user ID that can be specified by making an identify() call with Analytics.js.

For Google Chrome, these cookies expire by default one year after the date created. Other supported browsers might have a different expiration time.

Some user/group traits are also stored in localStorage:

Cookie Description
ajs_user_traits The traits that are passed in an identify() call.
ajs_group_properties The properties that are passed in a group() call.

Note that localStorage variables don’t expire because the browser defines that functionality.

Local storage cookies used by Analytics.js

Analytics.js uses localstorage cookies if you have retries enabled, to keep track of retry timing.

  • The ack cookie is a timer used to see if another tab should claim the retry queue.
  • The reclaimStart and reclaimEnd cookies determine if a tab takes over the queue from another tab.
  • The inProgress and queue cookies track events in progress, and events queued for retry.

For more information, visit the Segment localstorage-retry library.

You can set the debug cookie to analytics.js to log debug messages from Analytics.js to the console.

Tracking Blockers and Browser Privacy Settings

Segment does not endorse bypassing tracking blockers or browser privacy settings for client-side tracking. Your users have control over what gets loaded on their pages and can use plugins or browser settings to block third-party scripts, including Segment. To minimize client-side data loss, Segment recommends you choose from the following routes:

  1. Respect the user’s decision to implement tracking blockers or use privacy settings, knowing that, unfortunately, some data will be lost.
  2. Ask the customer to disable the tracking blockers or adjust their privacy settings (for example, in the case of large, corporate customers).
  3. Move as many events and tracking actions as possible to a server-side library, which won’t encounter the same limitations.

To minimize client-side data loss, Segment provides a few workarounds. However, it’s important to note that Segment cannot guarantee their effectiveness.

  • Use the bundle obfuscation feature. You can add an obfuscate property to the object in the second parameter, which obscures the URL from which your integrations and destination actions are loaded. This helps prevent words that are flagged by ad blockers to not be detected in your URL, enabling the integration to properly load.

  • Create a custom proxy. This changes the URL that Segment loads from ( and the outgoing requests generated when events are triggered (

  • Consider implementing the Segment Edge SDK. The Segment Edge SDK leverages Cloudflare Workers to facilitate first-party data collection and real-time user profiling for app personalization. It integrates Segment’s library into web apps, manages user identity via HttpOnly cookies, and employs an internal router for efficient data processing and user experience customization. This innovative approach simplifies tracking and personalization for Segment customers. More information is available in the Edge SDK README.

  • Consider using one of Segment’s server-side libraries. Using a server-side library eliminates concerns about tracking blockers and privacy browsers that can prevent Segment from loading. This option may require additional code to track actions like a Page call, as you now need to manually pass contextual information that would have been automatically collected by Analytics.js, like url, path, and referrer. Note that some destinations are device-mode only.

Installing the library under a custom global namespace

When you load Analytics.js through snippet code, by default, the SDK installs on global variable. If this causes a conflict with another library on your page, you can change the global variable used by Analytics.js if you use snippet version 5.2.1 or later.

Change the global variable in the beginning of your snippet code as shown below. In this case, Analytics.js uses window.custom_key to load instead of

  - !function(){var i="analytics", ...
  + !function(){var i="custom_key", ...

Add destinations from npm

Bundle the destinations you want loaded from npm instead of having them loaded from a remote CDN. This enables you to have fewer network requests when adding destinations.

  • To add actions-based destinations from npm:

    import vwo from '@segment/analytics-browser-actions-vwo'
    import braze from '@segment/analytics-browser-actions-braze'
    const analytics = AnalyticsBrowser.load({
      writeKey: '<WRITE_KEY>',
      plugins: [vwo, braze],

    Pass in the destination plugin to the added config option called plugins. A list of all action destination packages can be found here.

  • To add classic destinations from npm:

    import { AnalyticsBrowser } from '@segment/analytics-next'
    import GoogleAnalyticsIntegration from '@segment/analytics.js-integration-google-analytics'
    // The following example assumes configuration for Google Analytics will be available in the fetched settings
    const analytics = AnalyticsBrowser.load({
      writeKey: '<WRITE_KEY>',
      classicIntegrations: [ GoogleAnalyticsIntegration ]

Segment Inspector

The Segment Inspector is a Chrome web extension that enables you to debug your Segment integration on web applications instrumented with Analytics.js. Analytics.js sends data to the extension so that you can see how events change before they’re sent to your destinations and so that you can verify that the event details are correct. The Segment Inspector also lets you analyze and confirm that API calls made from your website arrive to your Analytics.js source.

For the Segment inspector to work, you must enable the Analytics.js source.

To add the Segment Inspector as a Chrome extension:

  1. Go to the Segment Inspector in the Chrome web store.
  2. Click Add to Chrome.
  3. Click Add Extension in the pop-up window.

Once installed, use the Inspect Elements developer tool in Chrome to use the Segment Inspector. To access the Inspector, go to the top menu bar of Chrome and navigate to View > Developer > Developer Tools and go to the Segment tab. On the Segment tab, you can:

  • Filter the different calls by type
  • Search based off of the content in the calls
  • Identify users

Components of the Segment Inspector

The Segment Inspector is composed of these three components:

  1. The Diagnostics tab
    • This tab shows the library versions and the list of active integrations that are running.
    • When you select an integration, you can see the options that passed while the integration loads. If you made any local overrides within the integration or on the page itself, they appear highlighted in the code.
  2. The Events tab
    • This tab enables you to select an event and see the specific details of the event. You can view the time the event occurred, the status of the event (whether it sent or failed), what plugins were added, and how the context object changed. Any changes made to the payload appear highlighted.
    • Select the double-checked icon to see the payload at the delivery stage.
    • Select the fx icon to see the payloads after plugins ran.
    • Select the single-checked icon to see the payload as it was when the event triggered.
  3. The Identity tab
    • This tab enables you to see the information of a user if you’re using the identify feature. You can associate the data to an individual and measure their activity across multiple sessions and devices. This tab only shows the user’s traits that are on the client.
    • If you’re not using the identify feature, the user remains anonymous.

Example uses

Here are some examples of using Analytics.js. Note that the examples assume Analytics.js is installed through npm.

External dependencies

Analytics.js includes the following open source dependencies:

uuid v2.0.0 ( Copyright Luke Edwards <> ( License: MIT License, available here:

dset v2.0.1 ( Copyright (c) Luke Edwards <> ( License: MIT License, available here:

js-cookie v2.2.1 Copyright (c) 2018 Copyright 2018 Klaus Hartl, Fagner Brack, GitHub Contributors   License: MIT License, available here:

md5 v2.3.0 ( Copyright (c) 2011-2012, Paul Vorbach. Copyright (c) 2009, Jeff Mott. License: BSD-3-Clause “New” or “Revised” License, available at:

unfetch v4.1.0 ( Copyright (c) 2017 Jason Miller License: MIT License, available at:

This page was last modified: 17 Apr 2024

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