Aaron Peters

Optimized JS snippet for Firebase Performance Monitoring

At Google I/O 2019, Google announced the beta release of Firebase Performance Monitoring for Web, a free service that helps you gain insight into the performance characteristics of your website / web app.

Firebase Performance Monitoring for Web collects performance data from within the browser, so you have to add a small JavaScript snippet to your app / pages.

Although the snippet provided by Google works as intended, it is not as good as it can and should be.

In this article I'll outline the problems with Google's snippet and present the optimized JS snippet for Firebase Performance Monitoring for Web.

The default Firebase JS snippet

The Firebase Get started with Performance Monitoring for web page outlines the steps to take to get up & running with Firebase Performance Monitoring for Web. If you're not using other Firebase products in your app you'll need to grab the JS snippet that loads the standalone SDK.

Here's that snippet:

(function(sa, fbc) {
function load(f, c) {
var a = document.createElement('script');
a.async = 1;
a.src = f;
var s = document.getElementsByTagName('script')[0];
s.parentNode.insertBefore(a, s);
window.addEventListener('load', function() {
})(performance_standalone, firebaseConfig);


  • performance_standalone is the URL of the standalone SDK (the JS file), currently https://www.gstatic.com/firebasejs/7.5.0/firebase-performance-standalone.js
  • firebaseConfig is your app's Firebase config object

The Firebase documentation states "Add the following script to the header of your index file". In other words: you must add the JS snippet to the <head> of the HTML.

Problems with this snippet

SDK loads in browsers where it will do nothing

The SDK for Firebase Performance Monitoring for Web uses the Fetch API, a modern replacement for XMLHttpRequest.

Fetch is not supported in Internet Explorer, so it's pointless to load the SDK in IE.

Download of the SDK starts too soon

The browsers starts downloading the SDK as soon as it executes the snippet: the browser evaluates load(sa) and the download starts right away.

By placing the default snippet high up in the HTML (in the <head>), the SDK download starts early in the page load process and consumes some of the available bandwidth, leaving less bandwidth to download other, more important content and scripts.

Consequently, those other more important content and scripts will finish downloading later and this delays the loading and rendering of the page.

My advice is to defer the downloading of third party scripts until after the page has completely loaded (the load event has fired).

Poor man's hook into the 'page is done loading' event

What if the snippet is not placed in the <head> of the HTML directly but instead injected into the page with a tag manager, maybe even after the user clicks OK in the cookie wall?

This will result in the snippet executing late, and if that is after the load event fires, the performance monitoring by Firebase will not happen.

Optimized Firebase JS snippet

The optimized snippet below fixes all the problems mentioned above:

  • do nothing in unsupported browsers
  • don't load the JS file before the page has finished loading: even though it loads async, it is best to delay it's loading until after the load event because a) the browser can use available bandwidth to load more important resources, and b) async loaded scripts delay the load event
  • ensure the snippet works in case it executes not just before but also after the page has finished loading
  • pass window and document into the function (a micro optimization for just a tiny bit faster parsing)
(function(win, doc, sa, fbc) {

if(!win.fetch){ // do nothing if browser does not support the Fetch API

function load(f, c, a, s) {
a = doc.createElement('script');
a.async = 1;
a.src = f;
a.addEventListener('load', (function(){
s = doc.getElementsByTagName('script')[0];
s.parentNode.insertBefore(a, s);

if (doc.readyState !== 'complete') { // page has not finished loading yet, so add a listener for the 'load' event
win.addEventListener('load', (function(){
}), false)
} else { // page finished loading, so fetch the script immediately

})(window, document, performance_standalone, firebaseConfig)


  • performance_standalone is https://www.gstatic.com/firebasejs/7.5.0/firebase-performance-standalone.js
  • firebaseConfig is your app's Firebase config object

Place the snippet in the top of the HEAD

Where to best place the Firebase JS snippet in your HTML, exactly?

Sure, Firebase states you should add the snippet to the <head> but that is not specific enough.

From a performance perspective, it matters greatly where in the <head> you place the JS snippet. Inline JS in the <head> that comes after CSS (inline or external) blocks parsing of the document until all the CSS has been downloaded and processed.

So, put the Firebase JS snippet before any inline or external CSS.


  • Do not assume a snippet provided by a third party is optimized for performance
  • Defer the loading of third party scripts until after the load event (even if the script loads async)
  • Inline JS should go in the top of <head> (before CSS)
  • Ensure your snippet always references the latest version of the standalone SDK: check the Firebase Performance Monitoring web site regularly


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