If you are loading boomerang.js separately from your main application bundle, i.e. from a CDN or a third-party service (such as mPulse), we recommend loading boomerang.js using the CSP-compliant non-blocking script loader pattern. The methodology, [developed by Philip Tellis](http://www.lognormal.com/blog/2012/12/12/the-script-loader-pattern/) and others, and further [improved in 2018](https://calendar.perfplanet.com/2018/a-csp-compliant-non-blocking-script-loader/), ensures Boomerang (or any third-party JavaScript) loads asynchronously and non-blocking. This means that the browser will not pause while the JavaScript is loading, nor will it block the `onload` event. The Boomerang Loader Snippet is currently around 210 lines of code (with comments), and minifies to around 2500 bytes. The snippet does the following: 1. It checks whether or not the snippet has already been run or Boomerang has already been loaded. If so, it exits. 2. It adds a `window` `load` event handler, to ensure that Boomerang can measure the Page Load time in non-NavigationTiming browsers, even if boomerang.js loads after the `load` event. 3. For browsers that [support Preload](https://caniuse.com/#feat=link-rel-preload) ``, Boomerang will add a `` node to tell the browser to fetch Boomerang.js. * Once the Preload has finished, Boomerang adds a regular ` ``` Minified: ```javascript ``` ## Delaying the Snippet You may want to delay loading Boomerang until after the `onload` event. This would ensure that no Boomerang code is executed in the critical-path of the page load. The main downside to doing this is that you are more likely to lose beacons from some users. The longer it takes Boomerang to load on the page, the higher chance that the user will have navigated away, or closed the browser, before boomerang.js is loaded. Here is a modification of the Boomerang Loader Snippet to delay until after `onload`: ```javascript ``` Minified: ```javascript ``` ## Known Issues * Websites using Google Tag Manager (GTM) to inject the Loader Snippet may not see beacons from Firefox <= 74 * These versions of Firefox do not support Preload, so fallback to using the IFRAME loader * boomerang.js is not fetched due to a Firefox bug with setting the `iframe.src = "about:blank"`, which is done for Content Security Policies (CSP) compatibility * Websites that are not using Content Security Policies can change: ```javascript // An empty frame iframe.src = "about:blank"; ``` to ```javascript // An empty frame iframe.src = "javascript:void(0)"; ``` * Websites that are using Content Security Policies should use a `