Async JavaScript

Programmatic Loading

While <script> tags are appealingly simple, there are some situations that require a more sophisticated approach to script loading. Perhaps we want a certain script to load only for users who meet certain requirements, such as premium subscribers or gamers who’ve reached a certain level. Or we may want a certain feature, like a chat widget, to load only when the user clicks to activate it.

In this section, we’ll look at how scripts can load other scripts. After a brief look at low-level approaches, we’ll look at two popular libraries that make script loading a breeze: yepnope and Require.js.

Loading Scripts Directly

At the browser API level, there are two (reasonable) ways to fetch a script from a server and run it. ...