Question

I am trying to make a simple event handler.

(Note, I’m not trying to implement a full-blown publish/subscribe model; I’m just interested in why my example doesn’t work as I think it should)

var myObj = (function () {

  var private = "X";

  function triggerEvent(eventName) {
    if (this[eventName]) {
      this[eventName]();
    }
  }

  // Setter / Getter
  function getProp() {
    return private;
  }

  function setProp(value) {
    private = value;
    triggerEvent("onPropChange");
  }

  // Public API
  return {
    // Events
    "onPropChange": null,    // Fires when prop value is changed

    // Methods
    "getProp": getProp,
    "setProp": setProp
  };
})();

// Now set event handler
myObj.onPropChange = function () {
  alert("You changed the property!");
};

myObj.setProp("Z");  // --> Nothing happens. Wrong
                     // Why doesn't my alert show?

I set the onPropChange property of my object to a simpler handler function but it is not being fired. I have debugged this and it seems that in triggerEvent the variable this is referencing the global window object. I thought it should reference myObj (which is what I need).

Can someone explain the error in my thinking and how I correct this? Help much appreciated.

jsFiddle here

Answer 1

Change your code so you create the obj and then return it like this:

var myObj = (function () {

  var private = "X";
  var obj = {
    "onPropChange": null,    // Fires when prop value is changed

    // Methods
    "getProp": getProp,
    "setProp": setProp
  };

  function triggerEvent(eventName) {
    if (obj[eventName]) {
      obj[eventName]();
    }
  }

  // Setter / Getter
  function getProp() {
    return private;
  }

  function setProp(value) {
    private = value;
    triggerEvent("onPropChange");
  }

  return obj;          
})();

This way you have a reference to the object for the private functions.

Fixed fiddle – http://jsfiddle.net/infernalbadger/dkqUW/2/

Another way to fix it that you might prefer is to use the .call function:

function setProp(value) {
    private = value;
    triggerEvent.call(this, "onPropChange");
}

This specifies what this will be in the method being called (triggerEvent).

http://jsfiddle.net/infernalbadger/dkqUW/3/