Load CSS or JS Files Simply & Asynchronously
Posted by Scott on 07/14/2014
Well, we happen to think about these things a lot, and over the years we’ve been using and improving a couple of basic functions, which we call loadCSS and loadJS, that do just that. Here’s how they work, in brief.
Loading CSS asynchronously is tricky (NOTE: see our updated post on this here!). Browsers tend to handle all CSS requests as if they’re essential, halting page rendering until the full CSS finishes loading and parsing. But we frequently have cases where we know that a CSS file can be parsed after a page has rendered.
For these cases, there’s a workaround to ensure we don’t hold anything up: Many modern browsers will evaluate the
media attribute of a stylesheet before requesting it; if the
media doesn’t match the browser’s current conditions, they will deprioritize that stylesheet and load it in parallel while the page is rendered. Our loadCSS function takes advantage of this behavior to ensure a stylesheet is requested asynchronously, and after the request goes out, it gives the stylesheet
media type of “all” that will ensure it still applies when it loads.
To use loadCSS, just call the
loadCSS function and pass it a URL of a stylesheet to load.
loadCSS( "/path/to/my/stylesheet.css" );
There are more usage options listed at the project website: loadCSS
A common issue comes from loader functions that insert scripts into the page using the
appendChild method , as it can cause errors in old versions of IE when appending to elements that haven’t finished loading. Instead, it’s recommended to use the
insertBefore method (Paul Irish’s post, Surefire DOM Insertion explains this situation and the rather than the
insertBefore workaround well.) Also, if you’re loading multiple scripts with dependencies, you’ll need to manage their execution order. We typically recommend concatenating scripts into a single file and loading them via a single http request, so for our needs, a simple script is best.
script tag into a reliable spot in the
head of the page. We wrap a little function around that pattern that you can find at our loadJS project. Here’s how it works:
loadJS( "/path/to/my/script.js" );
Once again, you can find more info on
loadJS at the project website: loadJS