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Introduction

In Higher Order Programming in Javascript I discussed the various ways of using functions as values. One particular trick in that document caught the attention of Dan Shappir, who pointed out to me: "your technique shows how JavaScript supported delegates all along, so this is not some great C#/.NET invention." While searching for some more information about these delegates, I found out that Microsoft has added them to Visual J++ too, invoking an interesting response from Sun, where it’s discussed how delegates can much better be implemented with Inner Classes. As this is aparently a hot issue in other languages, and because event handlers and callbacks are useful in webpages too, I decided to write a new article to discuss this.

The Trick

The problem is as follows: Methods use this to refer to the object it is a method of. But, when using a method as event handler or callback function, this does no longer point to that object. The trick is to use the closure like behaviour of functions, so that the method will always have access to it’s object.

Let’s look at an example: We want to use a method of an Alerter class as an event handler.

function Alerter(text) {

  this.text=text;

  var me=this;

  this.invoke=function () {

    alert(me.text);

  }

}

 

var sayHi=new Alerter('Hello, world!');

div.attachEvent('onclick', sayHi.invoke);

So, instead of using this, we use a variable me, that equals this when the invoke method is created. And, in contrast to this, me will keep refering to the correct Alerter object, even when it’s passed as a function to the attachEvent method of an HTML element.

Higher Order Programming again

var sayHi=new Alerter('Hello, world!');  

var sayBye=new Alerter('Bye, world!');

setTimeout(function() {sayHi.invoke();sayBye.invoke();},2000);

Function.prototype.andThen=function(g) {

  var f=this;

  return function() {

    f();g();

  }

};

setTimeout((sayHi.invoke).andThen(sayBye.invoke),2000);

function Manager() {

  this.callback=function () {}; // do nothing

  this.registerCallback=function(callbackFunction) {

    this.callback=(this.callback).andThen(callbackFunction);

  }

}

 

var manager=new Manager();

manager.registerCallback(sayHi.invoke);

manager.registerCallback(sayBye.invoke);

manager.callback();

Conclusion

With a simple technique event handlers and callback functions become a lot more interesting in Javascript. As Dan Shappir pointed this out to me, just this morning (after which I started to write the article), I can’t wait to try more of this. I hope you can’t too, and if you’ve got something to show, don’t hesitate to contact me.