Lazy Load Resources Based on Element Presence

By  on  

Fans of AMD JavaScript will probably tell you that they love loading only what they need, when they need them.  I am one of those people.  Let's take a site like mine for example: some pages require a syntax highlighter, some do not.  Why make the effort to load the syntax highlighter CSS and JavaScript if there are no pre elements that would require it?

The following is an example of how I occasionally load resources based on DOM contents:

$('article pre').length && (function() {
        var mediaPath = '/assets/';

        $('').attr({
            type: 'text/css',
            rel: 'stylesheet',
            href: mediaPath + 'css/syntax.css'
        }).appendTo(document.head);

        var syntaxScript = document.createElement('script');
        syntaxScript.async = 'true';
        syntaxScript.src = mediaPath + 'js/syntax.js';
        document.body.appendChild(syntaxScript);
    })();

The arguments against this practice will be (1) concatenating into existing JS and CSS to save on the number of requests and (2) flash of content style changes.  The first argument needs to be judged on a per-case basis;  if the required CSS and JS is small, it should be concatenated to a file used throughout the site or site subsection.  The second argument can always be hushed with a bit of transition magic!

Recent Features

  • By
    fetch API

    One of the worst kept secrets about AJAX on the web is that the underlying API for it, XMLHttpRequest, wasn't really made for what we've been using it for.  We've done well to create elegant APIs around XHR but we know we can do better.  Our effort to...

  • By
    CSS 3D Folding Animation

    Google Plus provides loads of inspiration for front-end developers, especially when it comes to the CSS and JavaScript wonders they create. Last year I duplicated their incredible PhotoStack effect with both MooTools and pure CSS; this time I'm going to duplicate...

Incredible Demos

  • By
    Drag. Drop. Lock.

    I've received dozens of emails about my Six Degrees of Kevin Bacon Using MooTools article. The MooTools in my article contained a lot of conditional code to require correct dropping per the game and many people requested that I simplify the process and just...

  • By
    MooTools: Set Style Per Media

    I'd bet one of the most used MooTools methods is the setStyle() method, which allows you to set CSS style declarations for an element. One of the limitations of MooTools' setStyle() method is that it sets the specific style for all medias.

Discussion

  1. Really interesting and surprisingly simple, I need to look into this for an upcoming build, thanks for sharing.

  2. Does async make a difference when the script is being injected dynamically (like in the code above)? I thought it only applies on “pre-existing” script elements (i.e. scripts that appear in the HTML source of the page).

    • Hm, that question is quite old, but it looks like that async = true is not needed.

  3. The first argument is why I would concatenate this; the difference in load time for this particular functionality would be imperceptible and would ensure the functionality is available whether syntax highlighting is used down the page or on a separate one.

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!