UX Capture JavaScript Library
Browser instrumentation JavaScript library that makes it easier to capture UX performance metrics using UX Capture approach
The intent of this library is to help developers instrument technical events (marks) on their pages and group them into "zones" that represent "phases" of page load, with each phase representing distinct stages of user experience.
React bindings for this library exists as a separate module @meetup/react-ux-capture
Usage
NOTE: this version of the library relies on UserTiming API to be available
in the browser, but should not break if it doesn't. You can
use a polyfill if you want to support older browsers.
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Load the library by inlining the contents of ux-capture.min.js in a
<script>tag in the HTML document<head>. Here's an example using server-side React:const uxCaptureFilename = require.resolve('ux-capture/lib/ux-capture.min.js'); const uxCaptureJS = fs.readFileSync(uxCaptureFilename, 'utf8'); ... render() { <head> <title>My Page</title> <script dangerouslySetInnerHTML={{ __html: uxCaptureJS }} /> ... </head> ... }
NOTE: The script must be inlined. Do not use a script tag with a
srcattribute. Waiting for network requests might artifically skew event timings on the page and lead to race conditions.NOTE: It is important to have this code available very early on the page since we need to instrument events that happen as early as HTML parsing, so ideally in the
<head>. -
Initialize UXCapture using
UXCapture.create(), optionally with mark and measure event handlers, e.g.<script> UXCapture.create({ onMark: name => console.log('marked', name), onMeasure: name => console.log('measured', name), }); </script>
Custom event handlers are useful in cases where the monitoring solution you use (e.g., NewRelic) does not support the W3C UserTiming API natively. You can then provide a custom method of recording the results.
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onMark- provides a custom handler to be called every time a mark is recorded with the name of the mark as the only argument -
onMeasure- provides a custom handler to be called every time a measure is recorded with the name of the measure as the only argument
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At the top of the view markup, define the expected zones and corresponding marks with
UXCapture.startView(), e.g.<script> UXCapture.startView([ { name: 'ux-destination-verified', marks: ['ux-1', 'ux-2'] },{ name: 'ux-primary-content-available', marks: ['ux-3', 'ux-4'] } ... ]); </script>
NOTE:
UXCapture.startView()will throw an error if called while previous view is active, so be careful to only call it once.Each individual zone configuration object contains of zone's
namethat will be used as a name of corresponding W3C UserTiming APImeasureandmarksarray of individual event name strings that zone groups together, each individual name will be used when recording corresponding events as W3C UserTiming APImark. -
You can optionally update a view that has already been started and add more zones by calling
UXCapture.updateView(). -
Call UXCapture.mark in the HTML markup for each ‘mark’ name passed into
UXCapture.startView()/updateView().<script>UXCapture.mark('ux-1')</script> <img onload="UXCapture.mark('ux-2')" … /> ...
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(SPA support) For 'interactive' view changes (usually associated with a route change), the client app must imperatively indicate when the current view is no longer valid using
UXCapture.startTransition, and clear any marks that should not be considered valid for the subsequent view usingUXCapture.clearMarks(name). To clear all marks, omit the name argument. Do not clear marks that are associated with elements that do not change between views.The call to
UXCapture.startTransitiondoes not need to be in the markup (and generally shouldn’t be).history.push(‘/foo’) UXCapture.startTransition();
or, a little less controlled, using History API:
window.onpopstate = UXCapture.startTransition; const pushState = window.history.pushState; window.history.pushState = (...args) => { UXCapture.startTransition(); return pushState.apply(window.history, args); }; const replaceState = window.history.replaceState; window.history.replaceState = (...args) => { UXCapture.startTransition(); return replaceState.apply(window.history, args); };
Summary: A SPA view transition is comprised of the following calls:
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UXCapture.startTransition()– required -
UXCapture.clearMarks(name)– optional, but should be called for each existing mark that is no longer valid
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Repeat from step 3.
Sample page
This repository contains a sample page that implements basic instrumentation for your reference: https://www.ux-capture.org/examples/
Instrumentation
This documentation shows snippets of code using UX Capture JavaScript library, for more information on methods of individual element instrumentation, see project page.
Image elements
Image tracking requires two measurements, one within the onload callback of the image itself and another within inline <script> tag directly after the image.
<img src="hero.jpg" onload="UXCapture.mark('ux-image-onload-logo')">
<script>UXCapture.mark('ux-image-inline-logo')</script>Text without custom font
Text that does not use a custom font can be instrumented by supplying one inline
<script> tag directly after the text:
<h1>Headline</h1>
<script>UXCapture.mark("ux-text-headline");</script>Text with custom font
Many pages use custom fonts to display text and often experience Flash of Invisible Text or FOIT. It is important to take into account time to load custom fonts. You can do it using font loaders provided by using event tracking in Web Font Loader used by Typekit and Google.
You can inline the library in HTML and then use the following code to fire a mark when font loaded.
<script>
WebFont.load({
custom: {
families: ["Montserrat:n4"]
},
active: function() {
UXCapture.mark("ux-font-montserrat-normal");
}
});
</script>NOTE: See Font Variation Description format used by Web Font Loader for specifying particular font variation to track.
Similarly to tracking text without custom font, inject a mark inline after text that uses custom font in question.
<h2>Title with font</h2>
<script>UXCapture.mark("ux-text-title-using-montserrat-normal");</script>Event handler attachment
Some user activity requires custom JavaScript handler code to be attached to an
event on the page, e.g. click of the button (e.g. it's only "available" when
visible AND clickable). Instrumenting handler attachment is straightforward, just
include the call right after handler attachment in JavaScript code.
var button_element = document.getElementById('mybutton');
button_element.addEventListener('click', myActionHandler);
UXCapture.mark('ux-handler-myaction');