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Some code Copyright (c) 2011, Yahoo! Inc. All rights reserved. Some code Copyright (c) 2011-2012, Log-Normal Inc. All rights reserved. Most code Copyright (c) 2012-2016 SOASTA, Inc. All rights reserved.

Copyrights licensed under the BSD License. See the accompanying LICENSE.txt file for terms.

boomerang always comes back, except when it hits something.


Join the chat at

boomerang is a JavaScript library that measures the page load time experienced by real users, commonly called RUM.

Apart from page load time, boomerang measures a whole bunch of performance characteristics of your user's web browsing experience. All you have to do is stick it into your web pages and call the init() method.


The simple synchronous way

<script src="boomerang/boomerang.js"></script>
<script src="boomerang/plugins/rt.js"></script>
       beacon_url: "/boomerang_handler"

Note - you must include at least one plugin (it doesn't have to be rt) or else the beacon will never actually be called.

The faster, more involved, asynchronous way

This is what I like to do for sites I control.

1. Add a plugin to init your code

Create a plugin (call it zzz_init.js or whatever you like) with your init code in there:

	config: parameters,

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

I call my plugin zzz_init.js to remind me to include it last in the plugin list

2. Build boomerang

The build process picks up all the plugins referenced in the plugins.json file. To change the plugins included in the boomerang build, change the contents of the file to your needs.

grunt clean build

This creates deployable boomerang versions 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

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"));

Yes, the best practices say to include scripts at the bottom. That's different. That's for 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, follow Stoyan's advice from the Meebo team.

3.2. Adding it via an iframe

The method described in 3.1 will still block onload on most browsers (Internet Explorer not included). To avoid blocking onload, we could load boomerang in an iframe. Stoyan's documented the technique on his blog. We've modified it to work across browsers with different configurations, documented on the lognormal blog.

For boomerang, this is the code you'll include:

  var dom,doc,where,iframe = document.createElement('iframe');
  iframe.src = "javascript:void(0)";
  (iframe.frameElement || iframe).style.cssText = "width: 0; height: 0; border: 0";
  var where = document.getElementsByTagName('script')[0];
  where.parentNode.insertBefore(iframe, where);

  try {
    doc = iframe.contentWindow.document;
  } catch(e) {
    dom = document.domain;
    doc = iframe.contentWindow.document;
  } = function() {
    var js = this.createElement("script");
    if(dom) this.domain = dom; = "boomr-if-as";
    js.src = '<version>.js';
  doc.write('<body onload="document._l();">');

The id of the script node created by this code MUST be boomr-if-as as boomerang looks for that id to determine if it's running within an iframe or not.

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 at this time 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.

3.4. Method queue pattern

If you want to call a public method that lives on BOOMR, but either don't know if Boomerang has loaded or don't want to wait, you can use the method queue pattern!

Instead of:

BOOMR.addVar('myVarName', 'myVarValue')

... you can write:

BOOMR_mq = window.BOOMR_mq || [];
BOOMR_mq.push(['addVar', 'myVarName', 'myVarValue']);

Or, if you care about the return value, instead of:

var hasMyVar = BOOMR.hasVar('myVarName');

... you can write:

var hasMyVar;
BOOMR_mq = window.BOOMR_mq || [];
   arguments: ['hasVar', 'myVarName'],
   callback: function(returnValue) {
     hasMyVar = returnValue;


Documentation is in the docs/ sub directory, and is written in HTML. Your best bet is to check it out and view it locally, though it works best through a web server (you'll need cookies). Thanks to github's awesome gh-pages feature, we're able to host the boomerang docs right here on github. Visit for a browsable version where all the examples work.

In case you're browsing this elsewhere, the latest development version of the code and docs are available at, while the latest stable version is at


We use github issues for discussions, feature requests and bug reports. Get in touch at You'll need a github account to participate, but then you'll need one to check out the code as well :)

Thanks for dropping by, and please leave us a message telling us if you use boomerang.

boomerang is supported by the devs at SOASTA, and the awesome community of opensource developers that use and hack it. That's you. Thank you!


End user oriented web performance testing and beaconing




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