Question

The following are the first lines of code in a <script> tag just above the closing body tag in my document (it specifies that a locally-served copy of jQuery is run in the event that Google’s CDN fails):

if(!window.jQuery){
    var script = document.createElement('script');
    script.type = 'text/javascript';
    script.src = '/js/jquery.js';
    var scriptHook = document.getElementsByTagName('script')[0];
    scriptHook.parentNode.insertBefore(script, scriptHook);
}


jQuery(document).ready(function($){
// page behaviors
});

It does execute successfully, in the sense that if my computer is not connected to the Internet (this is a locally-served page), the local copy of jQuery is inserted. However, the document.ready() section below does not execute. I’m guessing this is because it is invoked before the fallback copy of jQuery takes effect. What’s the proper practice for somehow “delaying” its execution so that either copy of jQuery will work properly?

Answer 1

You need to be sure that the script you are appending to the dom has finished loading before calling jQuery. You can do this with the technique described here:

if(!window.jQuery){
    var script = document.createElement('script');
    script.type = 'text/javascript';
    script.src = '/js/jquery.js';
    script.onreadystatechange= function () {
      if (this.readyState == 'complete') jQueryLoaded();
    }
    script.onload = jQueryLoaded;
    var scriptHook = document.getElementsByTagName('script')[0];
    scriptHook.parentNode.insertBefore(script, scriptHook);
}

function jQueryLoaded() {  };

You can also fetch the jQuery contents as an Ajax request, create a script tag with those as the body of the script and append it. That would also work.