Browser Script Loading

In this post I will discuss how scripts are loaded and executed by the browser.

It’s common to have scripts loaded by the browser by using the script tag (<script>) with a src (source) attribute. In the old days, the early versions of IE, FireFox, and Chrome, the script tags would load and execute synchronously. Today newer browsers support the ability to download scripts in parallel. The scripts are still executed in order they are declared.

Here’s a list of browser support for asynchronous downloading of scripts:


The previous image is from BrowserScope: http://www.browserscope.org/?category=network&v=2&ua=Chrome%204,Firefox%203.0,Firefox%203.5,Firefox%203.6,IE%206,IE%208,Opera%2010.10,Safari%203.2,Safari%204.0

In the following example we will load 6 scripts (note the script at the bottom). The order of the scripts does matter. Since backbone.js depends on underscore.js, underscore.js will be declared before backbone.js. Here’s the code that will be used to help us understand script loading.
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