Highlighting 'visited' links in Twine

visitedLinks.js

/*
 The first thing we are doing here is to create a new
 display function, oldDisplay, as a reference to the
 the normal History.display function.

 (Note: History.display does most of the heavy lifting
  in Twine. When a link is clicked, it is what
  grabs the content of the passage from HTML and 
  adds it to the screen as text.)
*/
History.prototype.oldDisplay = History.prototype.display;

/*
 Once we have saved a reference to the History.display function,
  we can overwrite it like the following.
*/
History.prototype.display = function(name, source, type, callback) {
  
  /*
    We need to make sure the 'source' element is not null here.
    Because, when this is called for the Start passage, it will be.
    (Start doesn't have a 'source'; it isn't called 
     by any other passage.)
  */
  if(source != null) {

        /*
          Source is the element that did the calling.
          In other words, an 'a' element when the user clicks on it.
          By using its style property, we can change 
          its CSS properties. 
        */
	source.style.color = "green";
        source.style.fontSize = "larger";
  }

  /*
   The final step is to call the oldDisplay within the same function
   scope as it would have normally run in before we interrupted the process.
   
   This is done using the 'apply' and 'this' keywords, telling the code
   to run from the current this (History.display) scope and to apply the same
   to History.oldDisplay using the same arguments passed to the current function scope.
  */
  this.oldDisplay.apply(this, arguments);
};