Can I register external JS files on a page via Javascript?

You can add a script tag to your page using the following code:

var head  = document.documentElement.childNodes[0];
var sTag  = document.createElement("script");
sTag.src  = "/path/to/script.js";
sTag.type = "text/javascript";
head.appendChild(sTag);

You could also use document.getElementsByTagName("head")[0] for the head var. Alternatively, you could use document.write, like so:

document.write(
    '<script src="path/to/script.js" type="text/javascript"><\/script>'
);

I made the following function, inspired by the jQuery's $.getScript method, it takes an url and a callback argument.

The callback is very useful, it is executed when the script has been loaded successfully, and you are ready to use it.

This function also takes care of removing the script elements from the DOM to avoid well known memory leaks:

loadScript("myLib.js", function () {
  // myLib is loaded
  //..
});

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 ||
      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);
}