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Initialization options

Tracker initialization is started by calling the "newTracker" function and takes three arguments:

  1. The tracker namespace
  2. The collector endpoint
  3. An optional configuration object containing other settings

Here is a simple example of how to initialise a tracker:

snowplow('newTracker', 'sp', '{{collector_url_here}}', {
appId: 'my-app-id',
discoverRootDomain: true,
cookieSameSite: 'Lax', // Recommended
contexts: {
webPage: true // default, can be omitted

The tracker will be named “sp” and will send events to the a collector url you specify by replacing {{collector_url_here}}. The final argument is the configuration object. Here it is just used to set the app ID and the common webPage context for each event. Each event the tracker sends will have an app ID field set to “my-app-id”.

Here is a longer example in which every tracker configuration parameter is set:

snowplow('newTracker', 'sp', '{{collector_url_here}}', {
appId: 'my-app-id',
platform: 'web',
cookieDomain: null,
discoverRootDomain: true,
cookieName: '_sp_',
cookieSameSite: 'Lax', // Recommended
cookieSecure: true,
encodeBase64: true,
respectDoNotTrack: false,
eventMethod: 'post',
bufferSize: 1,
maxPostBytes: 40000,
maxGetBytes: 1000, // available in v3.4+
postPath: '/custom/path', // Collector must be configured
crossDomainLinker: function (linkElement) {
return (linkElement.href === '' || === 'crossDomainLink');
cookieLifetime: 63072000,
stateStorageStrategy: 'cookieAndLocalStorage',
maxLocalStorageQueueSize: 1000,
resetActivityTrackingOnPageView: true,
connectionTimeout: 5000,
anonymousTracking: false,
// anonymousTracking: { withSessionTracking: true },
// anonymousTracking: { withSessionTracking: true, withServerAnonymisation: true },
customHeaders: {}, // Use with caution. Available from v3.2.0+
withCredentials: true, // Available from v3.2.0+
contexts: {
webPage: true, // Default
session: false, // Adds client session context entity to events, off by default. Available in v3.5+.
browser: false, // Adds browser context entity to events, off by default. Available in v3.9+.
performanceTiming: true,
gaCookies: true,
geolocation: false,
clientHints: true,
// clientHints: { includeHighEntropy: true }, // Optional
retryStatusCodes: [],
dontRetryStatusCodes: [],
onSessionUpdateCallback: function(clientSession) { }, // Allows the addition of a callback, whenever a new session is generated. Available in v3.11+.

We will now go through the various configuration parameters. Note that these are all optional. In fact, you aren’t required to provide any configuration object at all.

Setting the application ID

Set the application ID using the appId field of the configuration object. This will be attached to every event the tracker fires. You can set different application IDs on different parts of your site. You can then distinguish events that occur on different applications by grouping results based on application_id.

Setting the platform

Set the application platform using the platform field of the configuration object. This will be attached to every event the tracker fires. Its default value is “web”. For a list of supported platforms, please see the Snowplow Tracker Protocol.

If your website spans multiple subdomains e.g.

You will want to track user behavior across all those subdomains, rather than within each individually. As a result, it is important that the domain for your first party cookies is set to ‘’ rather than ‘’. By doing so, any values that are stored on the cookie on one of subdomain will be accessible on all the others.

It is recommended that you enable automatic discovery and setting of the root domain.

Otherwise, set the cookie domain for the tracker instance using the cookieDomain field of the configuration object. If this field is not set, the cookies will not be given a domain.

WARNINGChanging the cookie domain will reset all existing cookies. As a result, it might be a major one-time disruption to data analytics because all visitors to the website will receive a new domain_userid.

Set the cookie name for the tracker instance using the cookieName field of the configuration object. The default is “sp“. Snowplow uses two cookies, a domain cookie and a session cookie. In the default case, their names are “_sp_id” and “_sp_ses” respectively. If you are upgrading from an earlier version of Snowplow, you should use the default cookie name so that the cookies set by the earlier version are still remembered. Otherwise you should provide a new name to prevent clashes with other Snowplow users on the same page.

Once set, you can retrieve a cookie name thanks to the getCookieName(basename) method where basename is id or ses for the domain and session cookie respectively. As an example, you can retrieve the complete name of the domain cookie with getCookieName('id').

Set the cookie samesite attribute for the tracker instance using the cookieSameSite field of the configuration object. The default is “None” for backward compatibility reasons, however ‘Lax’ is likely a better option for most use cases given the reasons below. Valid values are "Strict", "Lax", "None" or null. null will not set the SameSite attribute.

It is recommended to set either "None" or "Lax". You must use "None" if using the tracker in a third party iframe. "Lax" is good in all other cases and must be used if not setting Secure to true.

Safari 12 Issue with SameSite cookies

It's been noted that Safari 12 doesn't persist cookies with SameSite: None as expected which can lead to rotation of the domain_userid from users using this browser. You should switch to cookieSameSite: 'Lax' in your tracker configuration to solve this, unless you are tracking inside a third party iframe.

Set the cookie secure attribute for the tracker instance using the cookieSecure field of the configuration object. The default is "true". Valid values are "true" or "false".

It is recommended to set this to "true". This must be set to "false" if using the tracker on non-secure HTTP.

Configuring base 64 encoding

By default, self-describing events and custom contexts are encoded into Base64 to ensure that no data is lost or corrupted. You can turn encoding on or off using the encodeBase64 field of the configuration object.

Respecting Do Not Track

Most browsers have a Do Not Track option which allows users to express a preference not to be tracked. You can respect that preference by setting the respectDoNotTrack field of the configuration object to true. This prevents cookies from being sent and events from being fired.

It is possible to set an opt-out cookie in order not to track anything similarly to Do Not Track through window.snowplow('setOptOutCookie', 'opt-out'); where ‘opt-out’ is the name of your opt-out cookie. If this cookie is set, cookies won’t be stored and events won’t be fired.

Anonymous Tracking

The Snowplow JavaScript tracker offers two techniques where tracking can be done anonymously. This means that no user identifiers are sent to the Snowplow Collector. By default anonymousTracking: false.

Recommended configurations when using anonymousTracking:

anonymousTracking: true, 
stateStorageStrategy: 'cookieAndLocalStorage'


anonymousTracking: { withSessionTracking: true },
stateStorageStrategy: 'cookieAndLocalStorage'

or for a completely cookieless experience (from JavaScript Tracker 2.17.0+)

anonymousTracking: { withServerAnonymisation: true },
stateStorageStrategy: 'none',
eventMethod: 'post'
Client Anonymous Tracking

anonymousTracking: true

This mode will no longer track any user identifiers or session information. Similar in behavior to setting stateStorageStrategy: 'none', as it will store no values in cookies or localStorage, however by using anonymousTracking you can toggle this behavior on and off (useful for allowing events to be sent without user identifiers until cookie banners have been accepted).

Setting stateStorageStrategy to cookieAndLocalStorage or localStorage also allows for event buffering to continue working whilst not sending user information when anonymousTracking is enabled.

Anonymous tracking can be toggled on and off. The methods to control this behavior are described here.

Full Anonymous/Cookieless Tracking

anonymousTracking: { withServerAnonymisation: true }

Server Anonymisation requires the Snowplow Stream Collector v2.1.0+. Using a lower version will cause events to fail to send until Server Anonymisatoin is disabled.

Server Anonymisation will not work when the tracker is initialized with eventMethod: 'beacon' as it requires additional custom headers which beacon does not support.

This mode will no longer track any user identifiers or session information, and will additionally prevent the Snowplow Collector from generating a network_userid cookie and capturing the users IP address. The same behavior described for above for Client side Anonymous tracking also applies.

Setting stateStorageStrategy to cookieAndLocalStorage or localStorage also allows for event buffering to continue working whilst not sending user information when anonymousTracking is enabled. However for an experience that doesn't use any browser storage (cookieless), set stateStorageStrategy to none. This can be later toggled on, once a user accepts a cookie policy.

Anonymous tracking can be toggled on and off. The methods to control this behavior are described here.

Anonymous Session Tracking

anonymousTracking: { withSessionTracking: true }

This mode will continue to track session information in the client side but will track no user identifiers. To achieve this, the tracker will use Cookies or Local Storage. For session tracking, stateStorageStrategy must be either cookieAndLocalStorage (default), localStorage or cookie. If this feature is enabled and the storage strategy is not appropriate, then full anonymous tracking will occur.

The Snowplow JavaScript Tracker performs sessionization client side. This allows anonymous session tracking to be done using client side storage without sending any user identifier fields to the collector.

Setting the event request protocol

Normally the protocol (http or https) used by the Tracker to send events to a collector is the same as the protocol of the current page. You can force the tracker to use https by prefixing the collector endpoint with the protocol. For example:

newTracker('sp', 'https://{{collector_url_here}}', {
appId: 'my-app-id'

Whenever an event fires, the Tracker creates a session cookie. If the cookie didn’t previously exist, the Tracker interprets this as the start of a new session.

By default the session cookie expires after 30 minutes. This means that a user leaving the site and returning in under 30 minutes does not change the session. You can override this default by setting sessionCookieTimeout to a duration (in seconds) in the configuration object. For example,

sessionCookieTimeout: 3600

would set the session cookie lifespan to an hour.

Configuring the storage strategy

Three strategies are made available to store the Tracker’s state: cookies, local storage or no storage at all. You can set the strategy with the help of the stateStorageStrategy parameter in the configuration object to “cookieAndLocalStorage” (the default), “cookie”, “localStorage” or “none” respectively.

When choosing local storage, the Tracker will additionally store events in local storage before sending them so that they can be recovered if the user leaves the page before they are sent.

See also How the Tracker uses localStorage for an explanation of how the tracker can later recover and send unsent events.

Adding predefined contexts

The JavaScript Tracker comes with many predefined contexts which you can automatically add to every event you send. To enable them, simply add them to the contexts field of the configuration object as above.

webPage context

When the JavaScript Tracker loads on a page, it generates a new page view UUID. If the webPage context is enabled, then a context containing this UUID is attached to every page view.

Enabled by default

From v3 of the JavaScript Tracker, the webPage context is enabled by default. You can disable it if you don't require it but we advise you leave this enabled so you can use the Snowplow Web Data Model.

session context

If this context is enabled, the JavaScript tracker will add a context entity to events with information about the current session. The context entity repeats some of the session information stored in canonical event properties (e.g., domain_useriddomain_sessionid), but also adds new information. It adds a reference to the previous session (previousSessionId) and first event in the current session (firstEventIdfirstEventTimestamp). It also adds an index of the event in the session useful for ordering events as they were tracked (eventIndex).

Anonymous tracking has to be disabled for the session context entities to be added to events.

The client_session context entity consists of the following properties:

userIdAn identifier for the user of the session (same as domain_userid).Yes
sessionIdAn identifier (UUID) for the session (same as domain_sessionid).Yes
sessionIndexThe index of the current session for this user (same as domain_sessionidx).Yes
eventIndexOptional index of the current event in the session. Signifies the order of events in which they were tracked.No
previousSessionIdThe previous session identifier (UUID) for this user.No
storageMechanismThe mechanism that the session information has been stored on the device.Yes
firstEventIdThe optional identifier (UUID) of the first event id for this session.No
firstEventTimestampOptional date-time timestamp of when the first event in the session was tracked.No

Please note that the session context entity is only available since version 3.5 of the tracker.

browser context

The browser context entity consists of the following properties:

viewportViewport dimensions of the browser. Arrives in the form of WidthxHeight e.g. 1200x900.Yes
documentSizeDocument dimensions. Arrives in the form of WidthxHeight e.g. 1200x900.Yes
resolutionDevice native resolution. Arrives in the form of WidthxHeight e.g. 1200x900.Yes
colorDepthThe number of bits allocated to colors for a pixel in the output device, excluding the alpha channel.Yes
devicePixelRatioRatio of the resolution in physical pixels to the resolution in CSS pixels for the current display device.No
cookiesEnabledIndicates whether cookies are enabled or not. More info and caveats at the official documentation.Yes
onlineReturns the online status of the browser. Important caveats are described in documentation.Yes
browserLanguageThe preferred language of the user, usually the language of the browser UI. Defined in RFC 5646.No
documentLanguageThe language of the HTML document. Defined in RFC 5646.No
webdriverIndicates whether the user agent is controlled by automation.No
deviceMemoryApproximate amount of device memory in gigabytes.No
hardwareConcurrencyNumber of logical processors available to run threads on the user's computer.No
tabIdA UUID identifier for the client browser tab the event is sent from.No

Please note that the browser context entity is only available since version 3.9 of the tracker.

performanceTiming context

If this context is enabled, the JavaScript Tracker will use the create a context JSON from the window.performance.timing object, along with the Chrome firstPaintTime field (renamed to "chromeFirstPaint") if it exists. This data can be used to calculate page performance metrics.

Note that if you fire a page view event as soon as the page loads, the domCompleteloadEventStartloadEventEnd, and chromeFirstPaint metrics in the Navigation Timing API may be set to zero. This is because those properties are only known once all scripts on the page have finished executing. See the Advanced Usage page for more information on circumventing this limitation. Additionally the redirectStartredirectEnd, and secureConnectionStart are set to 0 if there is no redirect or a secure connection is not requested.

For more information on the Navigation Timing API, see the specification.

gaCookies context

If this context is enabled, the JavaScript Tracker will look for Google Analytics cookies (specifically the “utma”, “utmb”, “utmc”, “utmv”, “__utmz”, and “_ga” cookies) and combine their values into a JSON which gets sent with every event.

clientHints context

If this context is enabled, the JavaScript Tracker will capture the Client Hints data made available in the browser. See here for browser support.

This is useful data to capture as browsers are moving away from high entropy User Agent strings. Client Hints offer useful information to understand browser usage without the potential to infringe on a users privacy as is often the case with the User Agent string.

This context be enabled in two ways:

  1. clientHints: true
    • This will capture the "basic" client hints
  2. clientHints: { includeHighEntropy: true }
    • This will capture the "basic" client hints as well as hints that are deemed "High Entropy" and could be used to fingerprint users. Browsers may choose to prompt the user before making this data available.

To see what will be captured please see the JsonSchema file org.ietf/http_client_hints/jsonschema/1-0-0.

geolocation context

If this context is enabled, the JavaScript Tracker will attempt to create a context from the visitor’s geolocation information. If the visitor has not already given or denied the website permission to use their geolocation information, a prompt will appear. If they give permission, then all events from that moment on will include their geolocation information.


If the geolocation context isn't enabled at tracker initialization, then it can be enabled at a later time by calling enableGeolocationContext. This is useful if you have other areas of your site where you require requesting geolocation access, as you can defer enabling this on your Snowplow events until you have permission to read the users geolocation for your other use case.

For more information on the geolocation API, see the specification.

optimizelyXSummary context

Support for OptimizelyX has been introduced in the tracker, you can have a look at the JsonSchema in com.optimizely.optimizelyx/summary/jsonschema/1-0-0 to see what is being captured.

If you’re planning on leveraging the context’s variation names, you’ll have to untick ‘Mask descriptive names in project code and third-party integrations’ in the OptimizelyX menu -> Settings -> Privacy. Otherwise, all variation names will be null.

POST support

If you set the eventMethod field of the configuration object to post, the tracker will send events using POST requests rather than GET requests. In browsers which do not support cross-origin XMLHttpRequests (e.g. IE9), the tracker will fall back to using GET.

eventMethod defaults to post, other options available are get for GET requests and beacon for using the Beacon API (Note: Beacon support is not available and/or unreliable in some browsers, in these cases the tracker will fallback to POST).

The main advantage of POST requests is that they circumvent Internet Explorer’s maximum URL length of 2083 characters by storing the event data in the body of the request rather than the querystring.

You can also batch events sent by POST by setting a numeric bufferSize field in the configuration object. This is the number of events to buffer before sending them all in a single POST. If the user navigates away from the page while the buffer is only partially full, the tracker will attempt to send all stored events immediately, but this often doesn’t happen before the page unloads. Normally the tracker will store unsent events in localStorage, meaning that unsent events will be resent when the user next visits a page on the same domain. The bufferSize defaults to 1, meaning events are sent as soon as they are created.

We recommend leaving the bufferSize as the default value of 1. This ensure that events are sent as they are created, and reduces the chance of events being unsent and left in local storage, if a user closes their browser before a flush can occur (which happens on page visibility changing).

If you have set bufferSize to greater than 1, you can flush the buffer using the flushBuffer method:


For instance, if you wish to send several events at once, you might make the API calls to create the events and store them and then and call flushBuffer afterwards to ensure they are all sent before the user leaves the page.

Note that if localStorage is inaccessible or you are not using it to store data, the buffer size will always be 1 to prevent losing events when the user leaves the page.

Beacon API support

The Beacon interface is used to schedule asynchronous and non-blocking requests to a web server. This will allow events to be sent even after a webpage is closed. This browser interface can be used to send events by setting the eventMethod field in the configuration object to beacon.

Using Beacon will store a Session Cookie in the users browser for reliability reasons, and will always send the first request as a standard POST. This prevents data loss in a number of older browsers with broken Beacon implementations.

Note: the Beacon API makes POST requests.

More information and documentation about the Beacon API can be found here.

POST path

The POST path that is used to send POST requests to a collector can be change with the configuration object value postPath.

postPath defaults to the standard path: /com.snowplowanalytics.snowplow/tp2


Changing postPath is non-standard behavior that does not conform to the default Collector protocol.

You must update the Collector configuration to support the new path before you send events to it with this setting. Otherwise, events will not be received by the Collector, or in some cases will be collected but will fail validation.

Make sure that requests are supported by your Collector configuration or redirected to the Collector at the correct endpoint (normally this is /com.snowplowanalytics.snowplow/tp2).

Configuring cross-domain tracking

The JavaScript Tracker can add an additional parameter named “_sp” to the querystring of outbound links. This process is called “link decoration”. The _sp value includes the domain user ID for the current page and the time at which the link was clicked (according to the device's clock). This makes these values visible in the “url” field of events sent by an instance of the JavaScript Tracker on the destination page. The enrichment process will use these values to populate the refr_domain_userid and refr_dvce_tstamp fields for all events fired on the destination page where the URL includes the _sp parameter.

You can configure which links get decorated this way using the crossDomainLinker field of the configuration object. The value should be a function taking one argument (the link element) and return true if the link element should be decorated and false otherwise. For example, this function would only decorate those links whose destination is “” or whose HTML id is “crossDomainLink”:

crossDomainLinker: function (linkElement) {
return (linkElement.href === '' || === 'crossDomainLink');

If you want to decorate every link to the domain

crossDomainLinker: function (linkElement) {
return /^https:\/\/github\.com/.test(linkElement.href);

If you want to decorate every link, regardless of its destination:

crossDomainLinker: function (linkElement) {
return true;

Note that the above will decorate “links” which are actually just JavaScript actions (with an href of "javascript:void(0)"), or links to email addresses or telephone numbers (mailto: or tel: schemes). To exclude these links, check them explicitly:

snowplow('crossDomainLinker', function(linkElement) {
return linkElement.href.indexOf('javascript:') < 0;

Alternatively, only count links that parse as web URLs by checking the link's hostname. This should automatically exclude links that don't lead simply to other web pages.

snowplow('crossDomainLinker', function(linkElement) {
return linkElement.hostname !== "";
Opt-in vs opt-out

Some of the above examples will also decorate any links to the same site the user is currently on. For most users we recommend explicitly allowing the list of domains you want to decorate, as well as ensuring that you only decorate links that are on a different domain to the current page.

crossDomainLinker: function (linkElement) {
return linkElement.hostname !== location.hostname;

If you have a shared configuration deployed across multiple sites, you may want to combine the approaches:

crossDomainLinker: function (linkElement) {
var networkSites = [
return linkElement.hostname !== location.hostname && networkSites.indexOf(linkElement.hostname) > -1;

This allows the same list of sites to be shared between each configuration, but link decoration will only occur when links are to different sites within the list.

Note that when the tracker loads, it does not immediately decorate links. Instead, it adds event listeners to links which decorate them as soon as a user clicks on them or navigates to them using the keyboard. This ensures that the timestamp added to the querystring is fresh.

If further links get added to the page after the tracker has loaded, you can use the tracker’s crossDomainLinker method to add listeners again. (Listeners won’t be added to links which already have them.)

snowplow('crossDomainLinker', function (linkElement) {
return (linkElement.href === '' || === 'crossDomainLink');

If you enable link decoration, you should also make sure that at least one event is fired on the page. Firing an event causes the tracker to write the domain_userid to a cookie. If the cookie doesn’t exist when the user leaves the page, the tracker will generate a new ID for them when they return, rather than keeping the old ID.

Reduce Shared Link Decoration URLs

If a user clicks a cross-site link and the URL is decorated with their domain_userid, and then they share that URL, other users will also have the parameters when they visit the shared page.

Because enrichment populates dedicated fields with the parameter, and it doesn't get persisted through the user's session (it is essentially only on the landing page), you can choose to hide the parameter after the first event (e.g. Page view) is fired on the destination page without needing additional modeling to stitch the IDs. This means after the next page is tracked, if the user copies the URL after the tracking code has fired, it may not include the _sp parameter and get copied to other users they share it with.

You can do this using the History API.

The URL updating code runs in a Tracker Callback to ensure it does not run before the page view event has a chance to capture the original URL.

snowplow('trackPageView'); // page URL is
if (/[?&]_sp=/.test(window.location.href)) {
history.replaceState(history.state, "", window.location.replace(/&?_sp=[^&]+/, "")); // page URL is now

This approach can work with other parameters (e.g. utm_source) but it is not recommended unless you are sure other systems have also already captured the parameter values for their own systems. Some systems may consider the URL update a single page application page change, and automatically fire additional page view events with this implementation, we suggest careful testing of this technique to ensure compatibility with your existing vendors.

Configuring the maximum payload size in bytes

POST requests

Because the Snowplow Stream Collector can have a maximum request size, the Tracker limits POST requests to 40000 bytes. If the combined size of the events in localStorage is greater than this limit, they will be split into multiple POST requests. You can override this default using a maxPostBytes in the configuration object.

GET requests

By default, there is no limit on the maximum size of GET requests – the tracker will add to queue and try to emit all GET requests irrespective of their size. However (since version 3.4), there is an optional maxGetBytes parameter which serves two purposes:

  1. It prevents requests over the threshold in bytes to be added to event queue and retried in case sending them is not successful.
  2. It sends events over the threshold as individual POST requests (same as for maxPostBytes).

The size of GET requests is calculated for their full GET request URL.

Collector limit

The Snowplow Stream Collector cannot process requests bigger than 1MB because that is the maximum size of a Kinesis record.

Automatically discover and set the root domain

If the optional discoverRootDomain field of the configuration object is set to true, the Tracker automatically discovers and sets the configCookieDomain value to the root domain.

NOTE: If you have been setting this manually please note that the automatic detection does not prepend a ‘.’ to the domain. For example a root domain of “” would become “”. This is because the library we use for setting cookies doesn’t care about the difference.

This will then result in a different domain hash, so we recommend that if you have been setting this manually with a leading ‘.’ to continue to do so manually.

Configuring the cookies lifetime

Whenever tracker initialized on your domain – it will set domain-specific visitor’s cookies. By default, these cookies will be active for 2 years. You can change this duration using cookieLifetime configuration object parameter or setVisitorCookieTimeout method.

snowplow('newTracker', 'cf', '{{COLLECTOR_URL}}', {
cookieLifetime: 86400 * 31,


snowplow('setVisitorCookieTimeout', 86400 * 30);  // 30 days

If cookieLifetime is set to 0, the cookie will expire at the end of the session (when the browser closes). If set to -1, the first-party cookies will be disabled.

Limiting Local Storage queue size

Because most browsers limit Local Storage to around 5mb per site, you may want to limit the number of events the tracker will queue in local storage if they fail to send. The default is a max queue size of 1000, but you may wish to reduce this if your web application also makes use local storage. To do so, you should set the optional maxLocalStorageQueueSize field of the configuration object is set to your desired value (e.g. 500).

Reset Page Ping on Page View

By default the tracker will reset the Page Ping timers, which were configured when enableActivityTracking is called, as well as reset the attached Page View contexts on all future Page Pings when a new trackPageView event occurs. This is enabled by default as of 2.13.0 and is particularly useful for Single Page Applications (SPA), if you previously relied on this behavior, you can disable this functionality by specifying resetActivityTrackingOnPageView: false in the configuration object on tracker initialisation.

Set connection timeout

When events are sent using POST or GET, they are given 5 seconds to complete by default. GET requests having a timeout is only available in 2.15.0.

_connectionTimeout_: 5000

This value is configurable when initialising the tracker and is specified in milliseconds. The value specified here will effect both POST and GET requests.

Warning: Setting this value too low may prevent events from successfully sending to your collector or the tracker may retry to send events that have already arrived at the collector, as the tracker will assume the request failed on timeout, leading to duplicate events in the warehouse. We recommend 5000 milliseconds as the minimum value and 10000 as the maximum value.

Setting Custom Header values

From v3.2.0, you are able to set custom headers with an eventMethod: "post" and eventMethod: "get" (Except for IE9). This functionality should only be used in the case where a Proxy or other Collector type is being used which allows for custom headers to be set on the request. CAUTION: Adding additional headers without returning the appropriate CORS Headers on the OPTIONS request will cause events to fail to send.

customHeaders: {
'Content-Language': 'de-DE, en-CA',

Disabling withCredentials flag

From v3.2.0, it's now possible to turn off the withCredentials flag on all requests to the collector. The default value is true which sets withCredentials to true on requests. Disabling this flag will have impact when using eventMethod: "post" and eventMethod: "get". This flag has no effect on same site requests, but disabling it will prevent cookies being sent with requests to a Snowplow Collector running on a different domain. You can read more about this flag at MDN.

withCredentials: false

Setting custom retry HTTP status codes

The tracker provides a retry functionality that sends the same events repeatedly in case GET or POST requests to the Collector fail. This may happen due to connection issues or a non-successful HTTP status code (>= 300) in Collector response.

Prior to version 3.5 of the tracker, requests receiving all 4xx and 5xx HTTP status codes in Collector response were retried. Since version 3.5, the behavior changed and became customizable:

By default, the tracker retries on all 3xx, 4xx, and 5xx status codes except for 400, 401, 403, 410, and 422. The set of status codes for which events should be retried or not is customizable. You can make use of the retryStatusCodes and dontRetryStatusCodes lists to specify them. Retry behavior can only be configured for non-successful status codes (i.e., >= 300).

retryStatusCodes: [403], // override default behavior and retry on 403
dontRetryStatusCodes: [418] // force retry on 418

Please note that not retrying sending events to the Collector means that the events will be dropped when they fail to be sent. Take caution when choosing the dontRetryStatusCodes.

On session update callback

The onSessionUpdateCallback option, allows you to supply a callback function to be executed whenever a new session is generated on the tracker.

The callback's signature is: (clientSession: ClientSession) => void where clientSession includes the same values as you would expect on the client_session context.

Note: The callback is not called whenever a session is expired, but only when a new one is generated.


Please note that the session context entity is only available since version 3.11 of the tracker.

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