dynamic script loading synchronization

You need to attach an event handler to either the onload method, in browsers compliant with Web standards, or the onreadystatechange, checking for the script.readyState property getting equal to “loaded” or “complete”, in Internet Explorer.

Before you get notified that the script was loaded, you are probably trying to access objects, functions and properties that have not been declared or created yet.

Here is an example function, extracted from the module bezen.dom.js in my Javascript library, bezen.org:

var appendScript = function(parent, scriptElt, listener) {
    // append a script element as last child in parent and configure 
    // provided listener function for the script load event
    //
    // params:
    //   parent - (DOM element) (!nil) the parent node to append the script to
    //   scriptElt - (DOM element) (!nil) a new script element 
    //   listener - (function) (!nil) listener function for script load event
    //
    // Notes:
    //   - in IE, the load event is simulated by setting an intermediate 
    //     listener to onreadystate which filters events and fires the
    //     callback just once when the state is "loaded" or "complete"
    //
    //   - Opera supports both readyState and onload, but does not behave in
    //     the exact same way as IE for readyState, e.g. "loaded" may be
    //     reached before the script runs.

    var safelistener = catchError(listener,'script.onload');

    // Opera has readyState too, but does not behave in a consistent way
    if (scriptElt.readyState && scriptElt.onload!==null) {
      // IE only (onload===undefined) not Opera (onload===null)
      scriptElt.onreadystatechange = function() {
        if ( scriptElt.readyState === "loaded" || 
             scriptElt.readyState === "complete" ) {
          // Avoid memory leaks (and duplicate call to callback) in IE
          scriptElt.onreadystatechange = null;
          safelistener();
        }
      };
    } else {
      // other browsers (DOM Level 0)
      scriptElt.onload = safelistener;
    }
    parent.appendChild( scriptElt );
};

To adapt it to your needs, you may replace the call to catchError, which wraps the listener to catch and log errors, and use the modified function:

var appendScript = function(parent, scriptElt, listener) {
    // append a script element as last child in parent and configure 
    // provided listener function for the script load event
    //
    // params:
    //   parent - (DOM element) (!nil) the parent node to append the script to
    //   scriptElt - (DOM element) (!nil) a new script element 
    //   listener - (function) (!nil) listener function for script load event
    //
    // Notes:
    //   - in IE, the load event is simulated by setting an intermediate 
    //     listener to onreadystate which filters events and fires the
    //     callback just once when the state is "loaded" or "complete"
    //
    //   - Opera supports both readyState and onload, but does not behave in
    //     the exact same way as IE for readyState, e.g. "loaded" may be
    //     reached before the script runs.

    var safelistener = function(){
      try {
        listener();
      } catch(e) {
        // do something with the error
      }
    };

    // Opera has readyState too, but does not behave in a consistent way
    if (scriptElt.readyState && scriptElt.onload!==null) {
      // IE only (onload===undefined) not Opera (onload===null)
      scriptElt.onreadystatechange = function() {
        if ( scriptElt.readyState === "loaded" || 
             scriptElt.readyState === "complete" ) {
          // Avoid memory leaks (and duplicate call to callback) in IE
          scriptElt.onreadystatechange = null;
          safelistener();
        }
      };
    } else {
      // other browsers (DOM Level 0)
      scriptElt.onload = safelistener;
    }
    parent.appendChild( scriptElt );
};