Question

Suppose I create a custom object/javascript “class” (airquotes) as follows:

// Constructor
function CustomObject(stringParam) {
  var privateProperty = stringParam;

  // Accessor
  this.privilegedGetMethod = function() {
    return privateProperty;
  }

  // Mutator
  this.privilegedSetMethod = function(newStringParam) {
    privateProperty = newStringParam;
  }
}

Then I want to make a list of those custom objects where I can easily add or remove things from that list. I decide to use objects as a way to store the list of custom objects, so I can add custom objects to the list with

var customObjectInstance1 = new CustomObject('someString');
var customObjectInstance2 = new CustomObject('someOtherString');
var customObjectInstance3 = new CustomObject('yetAnotherString');

myListOfCustomObjects[customObjectInstance1] = true;
myListOfCustomObjects[customObjectInstance2] = true;
myListOfCustomObjects[customObjectInstance3] = true;

and remove custom objects from the list with

delete myListOfCustomObjects[customObjectInstance1];

but if i try to iterate through the list with

for (i in myListOfCustomObjects) {
  alert(i.privilegedGetMethod());
}

I would get an error in the FireBug console that says “i.privilegedGetMethod() is not a function”. Is there a way to fix this problem or an idiom in javascript to do what I want? Sorry if this is a dumb question, but I’m new to javascript and have scoured the internet for solutions to my problem with no avail. Any help would be appreciated!

P.S. I realize that my example is super simplified, and I can just make the privateProperty public using this.property or something, but then i would still get undefined in the alert, and I would like to keep it encapsulated.

Answer 1

i won’t be the original object as you were expecting:

for (i in myListOfCustomObjects) {
    alert(typeof i); // "string"
}

This is because all keys in JavaScript are Strings. Any attempt to use another type as a key will first be serialized by toString().

If the result of toString() isn’t somehow unique for each instance, they will all be the same key:

function MyClass() { }
var obj = {};

var k1 = new MyClass();
var k2 = new MyClass();

obj[k1] = {};
obj[k2] = {};

// only 1 "[object Object]" key was created, not 2 object keys
for (var key in obj) {
    alert(key);
}

To make them unique, define a custom toString:

function CustomObject(stringParam) {
    /* snip */

    this.toString = function () {
        return 'CustomObject ' + stringParam;
    };
}

var obj = {};
var k1 = new CustomObject('key1');
var k2 = new CustomObject('key2');

obj[k1] = {};
obj[k2] = {};

// "CustomObject key1" then "CustomObject key2"
for (var key in obj) {
    alert(key);
}

[Edit]

With a custom toString, you can set the object as the serialized key and the value to keep them organized and still continue to access them:

var customObjectInstance1 = new CustomObject('someString');
var customObjectInstance2 = new CustomObject('someOtherString');
var customObjectInstance3 = new CustomObject('yetAnotherString');

myListOfCustomObjects[customObjectInstance1] = customObjectInstance1;
myListOfCustomObjects[customObjectInstance2] = customObjectInstance2;
myListOfCustomObjects[customObjectInstance3] = customObjectInstance3;

for (i in myListOfCustomObjects) {
  alert(myListOfCustomObjects[i].privilegedGetMethod());
}