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1 General parameters for the Javascript tracker v2.6

Anton Parkhomenko edited this page Mar 16, 2017 · 2 revisions

HOME » SNOWPLOW TECHNICAL DOCUMENTATION » Trackers » JavaScript Tracker » General parameters

This page refers to version 2.6.2 of the Snowplow JavaScript Tracker.
Click here for the corresponding documentation for version 1.
Click here for the corresponding documentation for version 2.1.1.
Click here for the corresponding documentation for version 2.2.2.
Click here for the corresponding documentation for version 2.3.0.
Click here for the corresponding documentation for version 2.4.3.
Click here for the corresponding documentation for version 2.5.3.
Click [here][general-parameters-v2.7] for the corresponding documentation for version 2.7.0.

2. General parameters

2.1 Loading Snowplow.js

Use the following tag to your page to load Snowplow.js:

<script type="text/javascript" async=1>

Important note regarding testing: "//" is the protocol-relative URL used to fetch sp.js. It will work if the your web page is using the "http" or "https" protocol. But if you are testing locally and loading your page from your filesystem using the "file" protocol (so its URI looks something like "file:///home/joe/snowplow_test.html"), the protocol-relative URL will also use that protocol, preventing the script from loading. To avoid this, change the URL to "" when testing locally.

As well as loading Snowplow, this tag creates a global function called "snowplow_name_here" which you use to access the Tracker. You can replace the string "snowplow_name_here" with the function name of your choice. This is encouraged: if there are two Snowplow users on the same page, there won't be any conflict between them as long as they have chosen different function names. The rest of the documentation will assume that the function is called "snowplow_name_here".

Once the function snowplow_name_here is created, the syntax for using Snowplow methods is as follows:

snowplow_name_here({{"methodName"}}, {{first method argument}}, {{second method argument}}, ...);

For example, the method trackStructEvent has this signature:

function trackStructEvent(category, action, label, property, value, context)

where only the first two arguments are required. You would use it like this:

snowplow_name_here('trackStructEvent', 'Mixes', 'Play', '', '', 20);

Empty strings are provided for the label and value arguments to pad them out. (Null would also work.) They won't be added to the event. Neither will the context argument, which isn't provided at all.

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2.2 Initialising a tracker

Tracker initialization is indicated with the "newTracker" string and takes three arguments:

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

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

snowplow_name_here("newTracker", "cf", "", {
  appId: "cfe23a",
  platform: "mob"

The tracker will be named "cf" and will send events to, the cloudfront collector specified. The final argument is called the argmap. Here it is just used to set the app ID and platform for the tracker. Each event the tracker sends will have an app ID field set to "cfe23a" and a platform field set to "mob".

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

snowplow_name_here("newTracker", "cf", "", {
  appId: "cfe23a",
  platform: "mob"
  cookieDomain: null,
  discoverRootDomain: true,
  cookieName: "_sp534_",
  encodeBase64: false,
  respectDoNotTrack: false,
  userFingerprint: true,
  userFingerprintSeed: 6385926734,
  pageUnloadTimer: 0,
  forceSecureTracker: true,
  useCookies: true,
  post: true,
  bufferSize: 5,
  maxPostBytes: 45000,
  crossDomainLinker: function (linkElement) {
    return (linkElement.href === "" || === "crossDomainLink"); 
  contexts: {
    webPage: true,
    performanceTiming: true,
    gaCookies: true,
    geolocation: false

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

2.2.1 Setting the application ID

Set the application ID using the appId field of the argmap. 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.

2.2.2 Setting the platform

Set the application platform using the platform field of the argmap. 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.

2.2.3 Configuring the cookie domain

If your website spans multiple subdomains e.g.

You will want to track user behaviour 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.

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

2.2.4 Configuring the cookie name

Set the cookie name for the tracker instance using the cookieName field of the argmap. 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.

2.2.5 Configuring base 64 encoding

By default, unstructured 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 argmap.

2.2.6 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 argmap to true. This prevents cookies from being sent and events from being fired.

2.2.7 User fingerprinting

By default, the tracker generates a user fingerprint based on various browser features. This fingerprint is likely to be unique and so can be used to track anonymous users. You can turn user fingerprinting off by setting the userFingerprint field of the argmap to false.

2.2.8 Setting the user fingerprint seed

The userFingerprintSeed field of the the argmap lets you choose the hash seed used to generate the user fingerprint. If this is not specified, the default is 123412414.

2.2.9 Setting the page unload pause

Whenever the Snowplow Javascript Tracker fires an event, it automatically starts a 500 millisecond timer running. If the user clicks on a link or refreshes the page during this period (or, more likely, if the event was triggered by the user clicking a link), the page will wait until either the event is sent or the timer is finished before unloading. 500 milliseconds is usually enough to ensure the event has time to be sent.

You can change the pause length (in milliseconds) using the pageUnloadTimer of the argmap. The above example completely eliminates the pause. This does make it unlikely that events triggered by link clicks will be sent.

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

2.2.10 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 it to use https by setting the forceSecureTracker field of the argmap to true.

2.2.11 Setting an unsecure 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 it to use http by setting the forceUnsecureTracker field of the argmap to true. If forceSecureTracker is activated this argument is ignored.

NOTE: This argument should only be used for testing purposes as it creates security vulnerabilities.

2.2.12 Configuring the session cookie duration

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 argmap. For example,

  sessionCookieTimeout: 3600

would set the session cookie lifespan to an hour.

2.2.13 Configuring localStorage

By default the Tracker will store events in localStorage before sending them so that they can be recovered if the user leaves the page before they are sent. You can disable this feature by setting a useLocalStorage: false field in the argmap.

2.2.14 Adding predefined contexts

The JavaScript Tracker comes with four predefined contexts which you can automatically add to every event you send. To enable them, simply add them to the contexts field of the argmap 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. 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 domComplete, loadEventStart, loadEventEnd, 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 redirectStart, redirectEnd, 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. 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.

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

If this context is enabled the JavaScript Tracker will use the window['augur'] object to create a context JSON.

To see what will be captured please see the JsonSchema file io.augur.snowplow/identity_lite/jsonschema/1-0-0. optimizelyExperiments context

If this context is enabled the JavaScript Tracker will use the window['optimizely'].data.experiments object to create an array of context JSONs; one for each sub-object.

To see what will be captured please see the JsonSchema file com.optimizely/experiment/jsonschema/1-0-0. optimizelyStates context

If this context is enabled the JavaScript Tracker will use the window['optimizely'].data.state object to create an array of context JSONs; one for each sub-object.

To see what will be captured please see the JsonSchema file com.optimizely/state/jsonschema/1-0-0. optimizelyVariations context

If this context is enabled the JavaScript Tracker will use the window['optimizely'].data.variations object to create an array of context JSONs; one for each sub-object.

To see what will be captured please see the JsonSchema file com.optimizely/variation/jsonschema/1-0-0. optimizelyVisitor context

If this context is enabled the JavaScript Tracker will use the window['optimizely'].data.visitor object to create a context JSON.

To see what will be captured please see the JsonSchema file com.optimizely/visitor/jsonschema/1-0-0. optimizelyAudiences context

If this context is enabled the JavaScript Tracker will use the window['optimizely'].data.visitor.audiences object to create an array of context JSONs; one for each sub-object.

To see what will be captured please see the JsonSchema file com.optimizely/visitor_audience/jsonschema/1-0-0. optimizelyDimensions context

If this context is enabled the JavaScript Tracker will use the window['optimizely'].data.visitor.dimensions object to create an array of context JSONs; one for each sub-object.

To see what will be captured please see the JsonSchema file com.optimizely/visitor_dimension/jsonschema/1-0-0.

2.2.15 POST support

If you set the post field of the argmap to true, the tracker will send events using POST requests rather than GET requests. In browsers such as Internet Explorer 9 which do not support cross-origin XMLHttpRequests, the tracker will fall back to using GET.

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.

The Clojure Collector and Scala Stream Collector accept events sent by POST; the Cloudfront Collector does not..

You can also batch events sent by POST by setting a numeric bufferSize field in the argmap. 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.

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.

2.2.16 Disabling cookies

You can prevent the Tracker from setting or reading first-party cookies by adding useCookies: false to the argmap.

2.2.17 Configuring cross-domain tracking

The JavaScript Tracker can add an additional parameter named "_sp" to the querystring of outbound links. Its value includes the domain user ID for the current page and the time at which the link was clicked. 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 in Redshift for all events fired on the destination page.

You can configure which links get decorated this way using the crossDomainLinker field of the argmap. This field 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)"). To exclude these links:

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

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

Warning: 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.

2.2.18 Configuring the maximum payload size in bytes

Because the Clojure Collector and the Scala Stream Collector both 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 argmap.

The Clojure Collector can't handle requests bigger than 64kB. The Scala Stream Collector cannot process requests bigger than 50kB because that is the maximum size of a Kinesis record.

2.2.19 Automatically discover and set the root domain

If the optional discoverRootDomain field of the argmap 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.

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2.3 Other parameters

2.3.1 Setting the user ID

The JavaScript Tracker automatically sets a domain_userid based on a first party cookie.

There are many situations, however, when you will want to identify a specific user using an ID generated by one of your business systems. To do this, you use one of the methods described in this section: setUserId, setUserIdFromLocation, setUserIdFromReferrer, and setUserIdFromCookie.

Typically, companies do this at points in the customer journey when the user identifies him / herself e.g. if he / she logs in.

Note: this will only set the user ID on further events fired while the user is on this page; if you want events on another page to record this user ID too, you must call setUserId on the other page as well. setUserId

setUserId is the simplest of the four methods. It sets the business user ID to a string of your choice:

snowplow_name_here('setUserId', ''); setUserIdFromLocation

setUserIdFromLocation lets you set the user ID based on a querystring field of your choice. For example, if the URL is, then the following code would set the user ID to "user345":

snowplow_name_here('setUserIdFromLocation', 'id'); setUserIdFromReferrer

setUserIdFromReferrer functions in the same way as setUserIdFromLocation, except that it uses the referrer querystring rather than the querystring of the current page.

snowplow_name_here('setUserIdFromReferrer', 'id'); setUserIdFromCookie

Use setUserIdFromCookie to set the value of a cookie as the user ID. For example, if you have a cookie called "cookieid" whose value is "user123", the following code would set the user ID to "user123":

snowplow_name_here('setUserIdFromCookie', 'cookieid');

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2.3.2 Setting a custom page URL and referrer URL

The Snowplow JavaScript Tracker automatically tracks the page URL and referrerURL on any event tracked. However, in certain situations, you may want to override the one or both of these URLs with a custom value. (For example, this might be desirable if your CMS spits out particularly ugly URLs that are hard to unpick at analysis time.)

To set a custom page URL, use the setCustomUrl method:

snowplow_name_here('setCustomUrl', '');

To set a custom referrer, use the setReferrerUrl method:

snowplow_name_here('setReferrerUrl', '');

On a single-page app, the page URL might change without the page being reloaded. Whenever an event is fired, the Tracker checks whether the page URL has changed since the last event. If it has, the page URL is updated and the URL at the time of the last event is used as the referrer. If you use setCustomUrl, the page URL will no longer be updated in this way. If you use setReferrerUrl, the referrer URL will no longer be updated in this way.

If you want to ensure that the original referrer is preserved even though your page URL can change without the page being reloaded, use setReferrerUrl like this before sending any events:

snowplow_name_here('setReferrerUrl', document.referrer);

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2.4 Setting onload callbacks

If you call snowplow_name_here with a function as the argument, the function will be executed when sp.js loads:

snowplow_name_here(function () {
  console.log("sp.js has loaded");

Or equivalently:

snowplow_name_here(function (x) {
}, "sp.js has loaded");

The callback function should not be a method:

// TypeError: Illegal invocation
snowplow_name_here(console.log, "sp.js has loaded");

will not work, because the value of this in the console.log function will be window rather than console.

You can get around this problem using Function.prototoype.bind as follows:

snowplow_name_here(console.log.bind(console), "sp.js has loaded");

2.5 Managing multiple trackers

You have more than one tracker instance running on the same page at once. This may be useful if you want to log events to different collectors. By default, any Snowplow method you call will be executed by every tracker you have created so far:

snowplow_name_here("newTracker", "cf1", "", {
  appId: "cfe23a",
  platform: "mob"

snowplow_name_here("newTracker", "cf2", "", {
  appId: "cfe23a",
  platform: "mob"

// Both trackers will use this custom title
snowplow_name_here('setCustomUrl', '');

// Both trackers will fire a structured event
snowplow_name_here('trackStructEvent', 'Mixes', 'Play', 'MrC/fabric-0503-mix', '', '0.0');

You can override this behaviour and specify which trackers will execute a Snowplow method. To do this, change the method name by adding a colon followed by a list of tracker names separated by semicolons:

// Only the first tracker will fire this structured event
snowplow_name_here('trackStructEvent:cf1', 'Mixes', 'Play', 'MrC/fabric-0503-mix', '', '0.0');

// Only the second tracker will fire this unstructured event
snowplow_name_here('trackUnstructEvent:cf2', 'com.acme_company' 'Viewed Product',
        product_id: 'ASO01043',
        category: 'Dresses',
        brand: 'ACME',
        returning: true,
        price: 49.95,
        sizes: ['xs', 's', 'l', 'xl', 'xxl'],
        available_since$dt: new Date(2013,3,7)

// Both trackers will fire a page view event

2.6 How the Tracker uses cookies

Unless you have enabled respectDoNotTrack in the configuration argmap, the tracker will use cookies to persist information. There are two first party cookies: the session cookie and the ID cookie. By default their names are prefixed with "sp", but you can change this using the "cookieName" field in the argmap. Their names are suffixed with a hash of the current domain, so the full cookie names might look something like _sp_ses.4209 and _sp_id.4209.

The session cookie

Called _sp_ses.{{DOMAIN HASH}} by default, the only purpose of this cookie is to differentiate between different visits. Whenever an event is fired, the session cookie is set to expire in 30 minutes. (This value can be altered using setSessionCookieTimeout.)

If no session cookie is already present when an event fires, the tracker treats this as an indication that long enough has passed since the user last visited that this session should be treated as a new session rather than a continuation of the previous session. The visitCount (how many times the user has visited) is increased by one and the lastVisitTs (the timestamp for the last session) is updated.

The ID cookie

This cookie is called _sp_id.{{DOMAIN HASH}} by default. It is used to persist information about a user's activity on the domain between sessions. It contains the following information:

  • An ID for the user based on a hash of various browser attributes
  • How many times the user has visited the domain
  • The timestamp of the user's first visit
  • The timestamp of the current visit
  • The timestamp of the last visit
  • The ID of the current session

It expires after 2 years.

The Clojure Collector cookie

There is a third sort of Snowplow-related cookie: the cookie set by the Clojure Collector, independently of the JavaScript Tracker. If you are using another type of collector, this cookie will not be set. The Clojure Collector cookie is called "sp". It is a third-party cookie used to track users over multiple domains. It expires after one year.

2.7 Getting the user ID from the Snowplow cookie

You can use the following function to extract the user ID from the ID cookie:

* Function to extract the Snowplow user ID from the first-party cookie set by the Snowplow JavaScript Tracker
* @param string cookieName (optional) The value used for "cookieName" in the tracker constructor argmap
* (leave blank if you did not set a custom cookie name)
* @return string or bool The ID string if the cookie exists or false if the cookie has not been set yet
function getSnowplowDuid(cookieName) {
  cookieName = cookieName || '_sp_';
  var matcher = new RegExp(cookieName + 'id\\.[a-f0-9]+=([^;]+);?');
  var match = document.cookie.match(matcher);
  if (match && match[1]) {
    return match[1].split('.')[0];
  } else {
    return false;

If you set a custom cookieName field in the argmap, pass that name into the function; otherwise call the function without arguments. Note that if the function is called before the cookie exists (i.e. when the user is visiting the page for the first time and sp.js has not yet loaded) if will return false.

2.8 How the Tracker uses localStorage

The Snowplow JavaScript Tracker uses window.localStorage to store events in case the user goes offline. Whenever the Tracker tries to fire an event, it first appends it to the queue in localStorage, and then sends events from the front of the queue until the queue is empty or an event fails to send.

localStorage is only shared between pages with the exact same domain. So if a user clicks on an internal link to another page in the same domain but the link click event fails to send before the page unloads, the event will be available in localStorage to the destination page, and if sp.js is also loaded on that page, it will send the request. Note that the tracker on the second page must have the same Snowplow function name (e.g. "snowplow_name_here") and the same tracker namespace (e.g. "cf") as the tracker on the first page for this to work.

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