JavaScript: Asynchronous Script Loading and Lazy Loading

Published: July 12, 2011 9:04 pm

Most of the time remote scripts are included at the end of an HTML document, right before the closing body tag. This is because browsers are single threaded and when they encounter a script tag, they halt any other processes until they download and parse the script. By including scripts at the end, you allow the browser to download and render all page elements, style sheets and images without any unnecessary delay. Also, if the browser renders the page before executing any script, you know that all page elements are already available to retrieve.

However, websites like Facebook for example, use a more advanced technique. They include scripts dynamically via DOM methods. This technique, which I’ll briefly explain here, is known as “Asynchronous Script Loading”.

Lets take a look at the script that Facebook uses to download its JS library:

(function () {
    var e = document.createElement('script');
    e.src = '';
    e.async = true;

When you dynamically append a script to a page, the browser does not halt other processes, so it continues rendering page elements and downloading resources. The best place to put this code is right after the opening body tag. This allows Facebook initialization to happen in parallel with the initialization on the rest of the page.

Facebook also makes non-blocking loading of the script easy to use by providing the fbAsyncInit hook. If this global function is defined, it will be executed when the library is loaded.

window.fbAsyncInit = function () {
        appId: 'YOUR APP ID',
        status: true,
        cookie: true,
        xfbml: true

Once the library has loaded, Facebook checks the value of window.fbAsyncInit.hasRun and if it’s false it makes a call to the fbAsyncInit function:

if (window.fbAsyncInit && !window.fbAsyncInit.hasRun) {
    window.fbAsyncInit.hasRun = true;

Now, what if you want to load multiple files asynchronously, or you need to include a small amount of code at page load and then download other scripts only when needed? Loading scripts on demand is called “Lazy Loading”. There are many libraries that exist specifically for this purpose, however, you only need a few lines of JavaScript to do this.

Here is an example:

$L = function (c, d) {
    for (var b = c.length, e = b, f = function () {
            if (!(this.readyState
            		&& this.readyState !== "complete"
            		&& this.readyState !== "loaded")) {
                this.onload = this.onreadystatechange = null;
                --e || d()
        }, g = document.getElementsByTagName("head")[0], i = function (h) {
            var a = document.createElement("script");
            a.async = true;
            a.src = h;
            a.onload = a.onreadystatechange = f;
        }; b;) i(c[--b])

The best place to put this code is inside the head tag. You can then use the $L function to asynchronously load your scripts on demand. $L takes two arguments: an array (c) and a callback function (d).

var scripts = [];
scripts[0] = '';
scripts[1] = '';

$L(scripts, function () {
    console.log("ga and jquery scripts loaded");

$L([''], function () {
    console.log("facebook script loaded");
    window.fbAsyncInit.hasRun = true;
        appId: 'YOUR APP ID',
        status: true,
        cookie: true,
        xfbml: true

You can see this script in action here (right click -> view page source).



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6 responses to “JavaScript: Asynchronous Script Loading and Lazy Loading”

  1. And noone remembers, that script tags already support asyncronus loading on pageload. At least in some browsers.

  2. This will work only as long as the remote scripts do not use document.write to output HTML content to the current page position. Using document.write after the body is loaded will clear the current page content,

    If you need to execute arbitrary JavaScript code delayed at the end of the page, you need to use better tricks. That is what the Fast Page Content Loader component does.

  3. Manual, I checked out your js. Does it require that you clean up the script first, e.g. I see in your example html that you have hardcoded some script code in the, including manually breaking up the tags etc. Doesn’t your script automatically prepare the remote code?

  4. There’s a trick which FB is doing in the code samples and can be important if you are on https:

    e.src = document.location.protocol + ‘//’;

    Essentially making sure that the protocol is matching the current URL’s.

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