Question

I can use immediate invocation to create an object with methods that have access to private members.

var myObj = (function () {
  // Private member variables
  var privateX;

  function setPropX (value) {
    privateX = value;
  }

  function getPropX () {
    return privateX;
  }

  function init() {
    privateX = "Initial value privateX";
  }

  init();

  // Public API
  return {
    // Methods
    "setPropX": setPropX,
    "getPropX": getPropX
  };
}());

After this I can work with the object just fine using the public API but my private members are correctly hidden:

myObj.setPropX("Hello world");
alert(myObj.getPropX());           // --> Hello world
alert(myObj.privateX);             // --> Undefined

Now for my question:

Is it possible to add public properties to the object which can be accessed both by the caller and from within my code? They would then be used like this:

myObj.publicY = "ABC";
myObj.sayPublicY();           // Should alert --> ABC

I tried to do it like this:

var myObj = (function () {
  // Private member variables
  var self = this;
  var privateX;

  function setPropX (value) {
    privateX = value;
  }

  function getPropX () {
    return privateX;
  }

  function sayPublicY () {
    alert(self.publicY);
  }

  function init() {
    privateX = "Initial value privateX";
    self.publicY = "Initial value publicY";
  }

  init();

  // Public API
  return {
    // Methods
    "setPropX": setPropX,
    "getPropX": getPropX,
    "sayPublicY": sayPublicY
  };
}());

I thought this would work but it doesn’t. The selfreference saved in the closure is referencing the window object. Why does it not reference myObj ?

I must be missing something obvious but I couldn’t find anything on Google or SO.

What is the correct way to do this?

EDIT: Added details of init() method which is still not working

Answer 1

Because inside your closure this (var self = this;) equals the window rather than the instance you’ve created. This declaration of self is useless; as it’ll always equal window, and should be removed (and not used).

The resolution of this is delayed until execution time, however because you’re specifically using self instead of this, you’re always using window. Change self to this inside sayPublicY, and it’ll work.

The use of an init function using this form of construction is not valid. The constructor in this code is the following few lines:

return {
  // Methods
  "setPropX": setPropX,
  "getPropX": getPropX,
  "sayPublicY": sayPublicY
};

Up until those few lines, you do not have an instance of myObj. You merely have the definitions and declarations of private variables/ methods which could be added to the myObj object. The return statement constructs your myObj, and assigns it the methosd setPropX, getPropX and sayPublicY. This is also where you should add your publicY attribute:

return {
  // Methods
  "setPropX": setPropX,
  "getPropX": getPropX,
  "sayPublicY": sayPublicY,
  "publicY": "Initial Value of Public Y"
};