How can I delay running some JS code until ALL of my asynchronous JS files downloaded?

Question

UPDATE:

I have the following code:

<script type="text/javascript">
function addScript(url) {
    var script = document.createElement('script');
    script.src = url;
    document.getElementsByTagName('head')[0].appendChild(script);
}   
addScript('http://google.com/google-maps.js');
addScript('http://jquery.com/jquery.js');

...

// run code below this point once both google-maps.js & jquery.js has been downloaded and excuted

</script>

How can I prevent code from executing until all required JS have been downloaded and executed? In my example above, those required files being google-maps.js and jquery.js.

Answer 1

You can use the onload event of the script element for most browsers, and use a callback argument:

Edit: You can't really stop the execution of the code when you load scripts in this way (and making synchronous Ajax requests is a bad idea most of the times).

But you can chain callbacks, so if you have some code that depends on both, Two.js and Three.js, you can chain the loading actions, for example:

loadScript('http://example.com/Two.js', function () {
  // Two.js is already loaded here, get Three.js...
  loadScript('http://example.com/Three.js', function () {
    // Both, Two.js and Three.js loaded...
    // you can place dependent code here...
  });
});

Implementation:

function loadScript(url, callback) {
  var head = document.getElementsByTagName("head")[0],
      script = document.createElement("script"),
      done = false;

  script.src = url;

  // Attach event handlers for all browsers
  script.onload = script.onreadystatechange = function(){
    if ( !done && (!this.readyState || // IE stuff...
      this.readyState == "loaded" || this.readyState == "complete") ) {
      done = true;
      callback(); // execute callback function

      // Prevent memory leaks in IE
      script.onload = script.onreadystatechange = null;
      head.removeChild( script );
    }
  };
  head.appendChild(script);
}

For IE, the onreadystatechange event has to be bound.

Answer 2

I just read CMS's answer, and decided that from his "most browsers" comment, I might have a crack at getting it work for ones that do not have this functionality natively.

Basically, it's an interval that polls for a variable.

var poll = window.setInterval(function() {

    if (typeof myVar !== 'undefined') {
        clearInterval(poll);
        doSomething();
    };

}, 100);