"this" doesn't work when binding functions to events inside a Javascript class

Question

First off, I know I can copy "this" on instantiation, but that doesn't work here.

Basically I'm writing something to track people interacting with Youtube videos.

I got this working fine for one video at a time. But I want it to work on pages with multiple Youtube videos as well, so I converted the code to a class so I can create a new instance of it for each video on the page.

The problem is when trying to bind to the Youtube event listener for state changes. For "non-class" code, it looks like this:

var o = document.getElementById( id );
o.addEventListener("onStateChange", "onPlayerStateChange" );

(onPlayerStateChange being the function I wrote to track state changes in the video)

(I'm also aware that addEventListener won't work with MSIE but I'm not worrying about that yet)

But when I'm inside a class, I have to use "this" to refer to another function in that class. Here's what the code looks like:

this.o = document.getElementById( id );
this.o.addEventListener("onStateChange", "this.onPlayerStateChange" );

When it's written like this, this.onPlayerStateChange is never called. I've tried copying "this" into another variable, e.g. "me", but that doesn't work either. The onPlayerStateChange function is defined within the "this" scope before I do this:

var me = this;
this.o = document.getElementById( id );
this.o.addEventListener("onStateChange", "me.onPlayerStateChange" );

Any insights?

Looking through other similar questions here, all of them are using jQuery, and I think doing it that way might work if I did it that way. But I don't want to use jQuery, because this is going to be deployed on random third party sites. I love jQuery but I don't want it to be a requirement to use this.

Answer 1

TheCloudlessSky was partly right and Sean was partly right. You can continue to use "onStateChange" as the event name, but don't put this.onPlayerStateChange in quotations - doing so removes the special meaning of this and javascript will look for a function named "this.onPlayerStateChange" rather than looking for a function "onPlayerStateChange" within this object.

this.o.addEventListener("onStateChange", this.onPlayerStateChange);

Answer 2

You need a global way to access the onPlayerStateChange method of your object. When you assign me as var me = this;, the variable me is only valid inside the object method where it is created. However, the Youtube player API requires a function that is accessible globally, since the actual call is coming from Flash and it has no direct reference to your JavaScript function.

I found a very helpful blog post by James Coglan in which he discussed a nice way to communicate with the Youtube's JavaScript API and manage events for multiple videos.

I have released a JavaScript wrapper library using his ideas at http://github.com/AnuragMishra/YoutubePlayer. Feel free to checkout the code. The underlying idea is simple - store all instances of the player object on the constructor. For example:

function Player(id) {
    // id of the placeholder div that gets replaced
    // the <object> element in which the flash video resides will
    // replace the placeholder div and take over its id
    this.id = id;

    Player.instances.push(this);
}

Player.instances = [];

When passing a string as a callback, use a string of the form:

"Player.dispatchEvent('playerId')"

When the flash player evals this string, it should return a function. That function is the callback that will ultimately receive the playback event id.

Player.dispatchEvent = function(id) {
    var player = ..; // search player object using id in "instances"
    return function(eventId) { // this is the callback that Flash talks to
        player.notify(eventId);
    };
};

When the flash player has loaded the video, the global onYoutubePlayerReady function is called. Inside that method, setup the event handlers for listening to playback events.

function onYouTubePlayerReady(id) {
    var player = ..; // find player in "instances"

    // replace <id> with player.id
    var callback = "YoutubePlayer.dispatchEvent({id})";
    callback = callback.replace("{id}", player.id);

    player.addEventListener('onStateChange', callback);
}

See a working example here..

Answer 3

After looking at the Youtube Api, it looks like the addEventListener only accepts a String for the event handler function. That means there's no clean way to register a unique event handler for each object.

An alternative is to register a global handler for all youtube state changes, and then let that handler pass the state change onto all your objects. Assuming you have an array of "tracker" objects:

function globalOnPlayerStateChange() {
    for (tracker in myTrackerObjects) {
        tracker.playerStateChange();
    }
}

Each tracker object can then figure out by itself whether or not a state change actually occured (using the API's getPlayerState function):

function MyYoutubeTracker() {
    this.currentState = ...

    // Determine if state changed happened or not
    this.playerStateChange = function() {
        var newState = this.o.getPlayerState();
        if (newState != this.currentState) {
            // State has changed
            this.currentState = newState;
        }
    }

    // Register global event handler for this youtube object
    this.o.addEventListener("onStateChange", "globalOnPlayerStateChange");
}

Answer 4

You can use a technique called currying to achieve this. For that you need a currying function. Here's one I wrote some time back

     /**
      * Changes the scope of function "fn" to the "scope" parameter specified or
      * if not, defaults to window scope. The scope of the function determines what
      * "this" inside "fn" evaluates to, inside the function "fn". Any additional arguments
      * specified in this are passed to the underlying "curried" function. If the underlying
      * function is already passed some arguments, the optional arguments are appended
      * to the argument array of the underlying function. To explain this lets take
      * the example below:
      *
      * You can pass any number of arguments that are passed to the underlying (curried)
      * function
      * @param {Function} fn The function to curry
      * @param {Object} scope The scope to be set inside the curried function, if
      * not specified, defaults to window
      * @param arguments {...} Any other optional arguments ot be passed to the curried function
      *
      */
     var curry = function(fn, scope /*, arguments */) {
        scope = scope || window;
        var actualArgs = arguments;

        return function() {
           var args = [];
           for(var j = 0; j < arguments.length; j++) {
              args.push(arguments[j]);
           }

           for(var i = 2; i < actualArgs.length; i++) {
              args.push(actualArgs[i]);
           }

           return fn.apply(scope, args);
        };
     };

You can use it to curry other functions and maintain the 'this' scope inside the functions. Check out this article on currying

     this.o.addEventListener("onStateChange", curry(onPlayerStateChange, this));

Edit:

var curriedFunc = curry(onPlayerStateChange, this);
this.o.addEventListener("onStateChange", "curriedFunc");

Edit: Okay lets say this is your custom class you create:

function MyCustomClass() {
   var privateVar = "x"; // some variables;
   this.onPlayerStateChange = function() {  //instance method on your custom class
       // do something important
   }
}

On a global level you create an instance of MyCustomClass

   var myCustom = new MyCustomClass(); // create a new instance of your custom class
   var curriedFunc = curry(myCustom.onplayerStageChange, myCustom); // curry its onplayerstateChange
   // now add it to your event handler
   o.addEventListener("onStateChange", "curriedFunc");

Answer 5

Ok, I got this all working. It's a bit of an ugly hack but it works. Basically I'm storing each new instance of the class in an array, and I'm passing the array key (1, 2, etc) into the class, so it can refer to itself externally as needed in a few key places.

The places I need the class to refer to itself externally are the string I pass to addEventListener, and within a few setTimeout functions, where "this" apparently loses its context (as far as I can tell anyways, because the only way I could them working was changing "this" to use external references instead.

Here's the full code.

On the page that has Youtube videos, they are injected using swfobject. The _ytmeta object stores the titles for each video. It's optional, but it's the only way to log the title of a video, because Youtube's API does not give it to you. This means you have to know the title up front, but the point is simply that if you want the title to show up in our reports, you have to create this object:

<div id='yt1'></div>

<script src='youtube.js'></script>
<script src='swfobject.js'></script>
<script>
var _ytmeta = {}
_ytmeta.yt1 = { 'title': 'Moonwalking in Walmart' };

var params = { allowScriptAccess: "always" };
swfobject.embedSWF("http://www.youtube.com/v/gE1ZvCnwkYk?enablejsapi=1&amp;playerapiid=yt1", "yt1", "425", "356", "8", null, null, params );
</script>

So we're including the swfobject javascript code, as well as the youtube.js file, which will be hosted on our server and included on the pages you want to track videos.

Here are the contents of youtube.js:

// we're storing each youtube object (video) in an array, and passing the array key into the class, so the class instance can refer to itself externally
// this is necessary for two reasons
// first, the event listener function we pass to Youtube has to be globally accessible, so passing "this.blah" doesn't work
// it has to be passed as a string also, so putting "this" in quotes makes it lose its special meaning
// second, when we create timeout functions, the meaning of "this" inside that function loses its scope, so we have to refer to the class externally from there too.

// _yt is the global youtube array that stores each youtube object. yti is the array key, incremented automatically for each new object created
var _yt = [], _yti = 0;

// this is the function the youtube player calls once it's loaded. 
// each time it's called, it creates a new object in the global array, and passes the array key into the class so the class can refer to itself externally
function onYouTubePlayerReady( id ) {
  _yti++;
  _yt[ _yti ] = new _yta( id, _yti );
}

function _yta( id, i ) {

  if( !id || !i ) return;

  this.id = id;
  this.mytime;
  this.scrubTimer;
  this.startTimer;
  this.last = 'none';
  this.scrubbing = false;

  this.o = document.getElementById( this.id );
  this.o.addEventListener("onStateChange", "_yt["+i+"].onPlayerStateChange" );

  this.onPlayerStateChange = function( newState ) {

    // some events rely on a timer to determine what action was performed, we clear it on every state change.
    if( this.myTime != undefined ) clearTimeout( this.myTime );

    // pause - happens when clicking pause, or seeking
    // that's why a timeout is used, so if we're seeking, once it starts playing again, we log it as a seek and kill the timer that would have logged the pause
    // we're only giving it 2 seconds to start playing again though. that should be enough for most users.
    // if we happen to log a pause during the seek - so be it.
    if( newState == '2' ) {
      this.myTime = setTimeout( function() {
        _yt[i].videoLog('pause');
        _yt[i].last = 'pause';
        _yt[i].scrubbing = false;
        }, 2000 );
      if( this.scrubbing == false ){
        this.last = 'pre-scrub';
        this.scrubbing = true;
      }
    }

    // play
    else if( newState == '1' ) {

      switch( this.last ) {

        case 'none':
          this.killTimers();
          this.startTimer = setInterval( this.startRun, 200 );
          break;

        case 'pause':
          this.myTime = setTimeout( function() {
            _yt[i].videoLog('play');
            _yt[i].last = 'play';
          }, 2000 );
          break;

        case 'pre-scrub':
          this.killTimers();
          this.scrubTimer = setInterval( this.scrubRun, 200 );
          break;
      }
    }

    // end
    else if( newState == '0' ) {
      this.last = 'none';
      this.videoLog('end');
    }
  }


  // have to use external calls here because these are set as timeouts, which makes "this" change context (apparently)
  this.scrubRun = function() {
    _yt[i].videoLog('seek');
    _yt[i].killTimers();
    _yt[i].last = 'scrub';
    _yt[i].scrubbing = false;
  }
  this.startRun = function() {
    _yt[i].videoLog('play');
    _yt[i].killTimers();
    _yt[i].last = 'start';
  }

  this.killTimers = function() {
    if( this.startTimer ) {
      clearInterval( this.startTimer );
      this.startTimer = null;
    }
    if( this.scrubTimer ){
      clearInterval( this.scrubTimer );
      this.scrubTimer = null;
    }
  }

  this.videoLog = function( action ) {
    clicky.video( action, this.videoTime(), this.videoURL(), this.videoTitle());
  }

  this.videoTime = function() {
    return Math.round( this.o.getCurrentTime() );
  }

  this.videoURL = function() {
    return this.o.getVideoUrl().split('&')[0]; // remove any extra parameters - we just want the first one, which is the video ID.
  }

  this.videoTitle = function() {
    // titles have to be defined in an external object
    if( window['_ytmeta'] ) return window['_ytmeta'][ this.id ].title || '';
  }
}

Hopefully, someone in the future will find this helpful, because it was a serious pain in the ass to get it working!

Thank you everyone who posted their ideas here. :)