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boomerang always comes back, except when it hits something.


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boomerang is a JavaScript library that measures the page load time experienced by real users, commonly called RUM (Real User Measurement). It has the ability to send this data back to your server for further analysis. With boomerang, you find out exactly how fast your users think your site is.

Apart from page load time, boomerang measures performance timings, metrics and characteristics of your user's web browsing experience. All you have to do is include it in your web pages and call the BOOMR.init() method. Once the performance data is captured, it will be beaconed to your chosen URL.

boomerang is designed to be a performant and flexible library that can be adapted to your site's needs. It has an extensive plugin architecture, and works with both traditional and modern websites (including Single Page Apps).

boomerang's goal is to not affect the load time of the page (avoiding the Observer Effect). It can be loaded in an asynchronous way that will not delay the page load even if boomerang.js is unavailable.


  • Supports:
    • IE 6+, Edge, all major versions of Firefox, Chrome, Opera, and Safari
    • Desktop and mobile devices
  • Captures (all optional):
    • Page characteristics such as the URL and Referrer
    • Overall page load times (via NavigationTiming if available)
    • DNS, TCP, Request and Response timings (via NavigationTiming)
    • Browser characteristics such as screen size, orientation, memory usage, visibility state
    • DOM characteristics such as the number of nodes, HTML length, number of images, scripts, etc
    • ResourceTiming data (to reconstruct the page's Waterfall)
    • Bandwidth
    • Mobile connection data
    • DNS latency
    • JavaScript Errors
    • XMLHttpRequest instrumentation
    • Third-Party analytics providers IDs
    • Single Page App interactions


boomerang can be included on your page in one of two ways: synchronously or asynchronously.

The asynchronous method is recommended.

The simple synchronous way

<script src="boomerang.js"></script>
<script src="plugins/rt.js"></script>
<!-- any other plugins you want to include -->
    beacon_url: ""

Note: You must include at least one plugin (it doesn't have to be RT) or else the beacon will never fire.

Each plugin has its own configuration as well -- these configuration options should be included in the BOOMR.init() call:

  beacon_url: "",
  ResourceTiming: {
    enabled: true,
    clearOnBeacon: true

The faster, more involved, asynchronous way

Loading boomerang asynchronously ensures that even if boomerang.js is unavailable (or loads slowly), your host page will not be affected.

1. Add a plugin to init your code

Create a plugin (or use the sample zzz-last-plugin.js) with a call to BOOMR.init:

  config: parameters,
BOOMR.t_end = new Date().getTime();

You could also include any other code you need. For example, you could include a timer to measure when boomerang has finished loading (as above).

2. Build boomerang

The build process bundles boomerang.js and all of the plugins listed in plugins.json (in that order).

To build boomerang with all of your desired plugins, you would run:

grunt clean build

This creates a deployable boomerang in the build directory, e.g. build/boomerang-<version>.min.js.

Install this file on your web server or origin server where your CDN can pick it up. Set a far future max-age header for it. This file will never change.

3. Asynchronously include the script on your page

There are two methods of asynchronously including boomerang on your page: by adding it to your main document, or via the IFRAME/Preload method.

The former method could block your onload event (affecting the measured performance of your page), so the later method is recommended.

3.1. Adding it to the main document

Include the following code at the top of your HTML document:

(function(d, s) {
  var js = d.createElement(s),
      sc = d.getElementsByTagName(s)[0];

  sc.parentNode.insertBefore(js, sc);
}(document, "script"));

Best practices will suggest including all scripts at the bottom of your page. However, that only applies to scripts that block downloading of other resources.

Including a script this way will not block other resources, however it will block onload.

Including the script at the top of your page gives it a good chance of loading before the rest of your page does, thereby reducing the probability of it blocking the onload event.

If you don't want to block onload either, use the following IFRAME/Preload method:

3.2. Adding it via an IFRAME/Preload

The method described in 3.1 will still block onload on most browsers.

To avoid blocking onload, we can load boomerang in an asynchronous IFRAME or via LINK preload (for browsers that support it). The general process is documented on in this blog post.

For boomerang, the asynchronous loader snippet you'll use is:

(function() {
  // Boomerang Loader Snippet version 15
  if (window.BOOMR && (window.BOOMR.version || window.BOOMR.snippetExecuted)) {

  window.BOOMR = window.BOOMR || {};
  window.BOOMR.snippetStart = new Date().getTime();
  window.BOOMR.snippetExecuted = true;
  window.BOOMR.snippetVersion = 15;

  // NOTE: Set Boomerang URL here
  window.BOOMR.url = "";

  // document.currentScript is supported in all browsers other than IE
  var where = document.currentScript || document.getElementsByTagName("script")[0],
      // Parent element of the script we inject
      parentNode = where.parentNode,
      // Whether or not Preload method has worked
      promoted = false,
      // How long to wait for Preload to work before falling back to iframe method
      LOADER_TIMEOUT = 3000;

  // Tells the browser to execute the Preloaded script by adding it to the DOM
  function promote() {
    if (promoted) {

    var script = document.createElement("script"); = "boomr-scr-as";
    script.src = window.BOOMR.url;

    // Not really needed since dynamic scripts are async by default and the script is already in cache at this point,
    // but some naive parsers will see a missing async attribute and think we're not async
    script.async = true;


    promoted = true;

  // Non-blocking iframe loader (fallback for non-Preload scenarios) for all recent browsers.
  // For IE 6/7/8, falls back to dynamic script node.
  function iframeLoader(wasFallback) {
    promoted = true;

    var dom,
        doc = document,
        bootstrap, iframe, iframeStyle,
        win = window;

    window.BOOMR.snippetMethod = wasFallback ? "if" : "i";

    // Adds Boomerang within the iframe
    bootstrap = function(parent, scriptId) {
      var script = doc.createElement("script"); = scriptId || "boomr-if-as";
      script.src = window.BOOMR.url;

      BOOMR_lstart = new Date().getTime();

      parent = parent || doc.body;

    // For IE 6/7/8, we'll just load the script in the current frame:
    // * IE 6/7 don't support 'about:blank' for an iframe src (it triggers warnings on secure sites)
    // * IE 8 required a doc write call for it to work, which is bad practice
    // This means loading on IE 6/7/8 may cause SPoF.
    if (!window.addEventListener && window.attachEvent && navigator.userAgent.match(/MSIE [678]\./)) {
      window.BOOMR.snippetMethod = "s";

      bootstrap(parentNode, "boomr-async");


    // The rest of this function is for browsers that don't support Preload hints but will work with CSP & iframes
    iframe = document.createElement("IFRAME");

    // An empty frame
    iframe.src = "about:blank";

    // We set title and role appropriately to play nicely with screen readers and other assistive technologies
    iframe.title = "";
    iframe.role = "presentation";

    // Ensure we're not loaded lazily
    iframe.loading = "eager";

    // Hide the iframe
    iframeStyle = (iframe.frameElement || iframe).style;
    iframeStyle.width = 0;
    iframeStyle.height = 0;
    iframeStyle.border = 0;
    iframeStyle.display = "none";

    // Append to the end of the current block

    // Try to get the iframe's document object
    try {
      win = iframe.contentWindow;
      doc =;
    catch (e) {
      // document.domain has been changed and we're on an old version of IE, so we got an access denied.
      // Note: the only browsers that have this problem also do not have CSP support.

      // Get document.domain of the parent window
      dom = document.domain;

      // Set the src of the iframe to a JavaScript URL that will immediately set its document.domain
      // to match the parent.
      // This lets us access the iframe document long enough to inject our script.
      // Our script may need to do more domain massaging later.
      iframe.src = "javascript:var;d.domain='" + dom + "';void 0;";
      win = iframe.contentWindow;

      doc =;

    // document.domain hasn't changed, regular method should be OK
    win._boomrl = function() {

    if (win.addEventListener) {
      win.addEventListener("load", win._boomrl, false);
    else if (win.attachEvent) {
      win.attachEvent("onload", win._boomrl);

    // Finish the document

  // See if Preload is supported or not
  var link = document.createElement("link");

  if (link.relList &&
      typeof link.relList.supports === "function" &&
      link.relList.supports("preload") &&
      ("as" in link)) {
    window.BOOMR.snippetMethod = "p";

    // Set attributes to trigger a Preload
    link.href = window.BOOMR.url;
    link.rel  = "preload";   = "script";

    // Add our script tag if successful, fallback to iframe if not
    link.addEventListener("load", promote);
    link.addEventListener("error", function() {

    // Have a fallback in case Preload does nothing or is slow
    setTimeout(function() {
      if (!promoted) {

    // Note the timestamp we started trying to Preload
    BOOMR_lstart = new Date().getTime();

    // Append our link tag
  else {
    // No Preload support, use iframe loader

  // Save when the onload event happened, in case this is a non-NavigationTiming browser
  function boomerangSaveLoadTime(e) {
    window.BOOMR_onload = (e && e.timeStamp) || new Date().getTime();

  if (window.addEventListener) {
    window.addEventListener("load", boomerangSaveLoadTime, false);
  else if (window.attachEvent) {
    window.attachEvent("onload", boomerangSaveLoadTime);


<script>(function(){if(window.BOOMR&&(window.BOOMR.version||window.BOOMR.snippetExecuted)){return}window.BOOMR=window.BOOMR||{};window.BOOMR.snippetStart=(new Date).getTime();window.BOOMR.snippetExecuted=true;window.BOOMR.snippetVersion=15;window.BOOMR.url="";var e=document.currentScript||document.getElementsByTagName("script")[0],a=e.parentNode,s=false,t=3e3;function n(){if(s){return}var e=document.createElement("script");"boomr-scr-as";e.src=window.BOOMR.url;e.async=true;a.appendChild(e);s=true}function i(e){s=true;var t,i=document,n,o,d,r=window;window.BOOMR.snippetMethod=e?"if":"i";n=function(e,t){var n=i.createElement("script");||"boomr-if-as";n.src=window.BOOMR.url;BOOMR_lstart=(new Date).getTime();e=e||i.body;e.appendChild(n)};if(!window.addEventListener&&window.attachEvent&&navigator.userAgent.match(/MSIE [678]\./)){window.BOOMR.snippetMethod="s";n(a,"boomr-async");return}o=document.createElement("IFRAME");o.src="about:blank";o.title="";o.role="presentation";o.loading="eager";d=(o.frameElement||o).style;d.width=0;d.height=0;d.border=0;d.display="none";a.appendChild(o);try{r=o.contentWindow;}catch(e){t=document.domain;o.src="javascript:var;d.domain='"+t+"';void 0;";r=o.contentWindow;}r._boomrl=function(){n()};if(r.addEventListener){r.addEventListener("load",r._boomrl,false)}else if(r.attachEvent){r.attachEvent("onload",r._boomrl)}i.close()}var o=document.createElement("link");if(o.relList&&typeof o.relList.supports==="function"&&o.relList.supports("preload")&&"as"in o){window.BOOMR.snippetMethod="p";o.href=window.BOOMR.url;o.rel="preload";"script";o.addEventListener("load",n);o.addEventListener("error",function(){i(true)});setTimeout(function(){if(!s){i(true)}},t);BOOMR_lstart=(new Date).getTime();a.appendChild(o)}else{i(false)}function d(e){window.BOOMR_onload=e&&e.timeStamp||(new Date).getTime()}if(window.addEventListener){window.addEventListener("load",d,false)}else if(window.attachEvent){window.attachEvent("onload",d)}})();</script>

Change the boomerangUrl to the location of Boomerang on your server.

The id of the script node created by this code MUST be boomr-if-as (for IFRAME mode) or boomr-scr-as (for Preload mode) as boomerang looks for those ids to determine if it's running within an IFRAME and to determine the URL of the script.

boomerang will still export the BOOMR object to the parent window if running inside an IFRAME, so the rest of your code should remain unchanged.

3.3. Identifying when boomerang has loaded

If you load boomerang asynchronously, there's some uncertainty in when boomerang has completed loading. To get around this, you can subscribe to the onBoomerangLoaded Custom Event on the document object:

// Modern browsers
if (document.addEventListener) {
  document.addEventListener("onBoomerangLoaded", function(e) {
    // e.detail.BOOMR is a reference to the BOOMR global object
// IE 6, 7, 8 we use onPropertyChange and look for propertyName === "onBoomerangLoaded"
else if (document.attachEvent) {
  document.attachEvent("onpropertychange", function(e) {
    if (!e) e=event;
    if (e.propertyName === "onBoomerangLoaded") {
      // e.detail.BOOMR is a reference to the BOOMR global object

Note that this only works on browsers that support the CustomEvent interface, which is Chrome (including Android), Firefox 6+ (including Android), Opera (including Android, but not Opera Mini), Safari (including iOS), IE 6+ (but see the code above for the special way to listen for the event on IE6, 7 & 8).

boomerang also fires the onBeforeBoomerangBeacon and onBoomerangBeacon events just before and during beaconing.


There are several ways of including Boomerang in your project:

  1. Boomerang can be downloaded from the official Boomerang Github repository.

  2. NPM: npm install boomerangjs

  3. Bower: bower install boomerang

Once fetched, see Building Boomerang for more details on how to include the plugins you require.


Documentation is in the docs/ directory. Boomerang documentation is written in Markdown and is built via JSDoc.

You can build the current documentation by running Grunt:

grunt jsdoc

HTML files will be built under build/docs.

Open-source Boomerang Documentation is currently published at

The team at Akamai works on mPulse Boomerang, which contains a few mPulse-specific plugins and may have additional changes being tested before being backported to the open-source Boomerang. mPulse Boomerang usage documentation is available at and mPulse Boomerang API documentation is at

Additional documentation:

Source code

The boomerang source code is primarily on GitHub at

Feel free to fork it and contribute to it.

You can also get a check out the releases or download a tarball or zip of the code.


We use GitHub Issues for discussions, feature requests and bug reports.

Get in touch at

boomerang is supported by the developers at Akamai, and the awesome community of open-source developers that use and hack it. That's you. Thank you!


Boomerang is brought to you by:

To help out, please read our contributing page.


  • Copyright (c) 2011, Yahoo! Inc. All rights reserved.
  • Copyright (c) 2011-2012, Log-Normal Inc. All rights reserved.
  • Copyright (c) 2012-2017 SOASTA, Inc. All rights reserved.
  • Copyright (c) 2017-2023, Akamai Technologies, Inc. All rights reserved.
  • Copyrights licensed under the BSD License. See the accompanying LICENSE.txt file for terms.