I'm attempting to use a TypeScript-like structure to emulate an enumeration in a self-executing function for the 'value:' property. I checked all over for examples of ECMAScript5 / Crockford Object.defineProperty() use in the constructor (using 'this') but could not find much. As far as I can tell, the self-executing function should fill up both objects with 10 properties, and the closure is correct ('this' is not the window object). Here's my design pattern:
var player = {};
var Foo = function () {
this.levelIndex;
Object.defineProperty(this, 'levelIndex', {
value: (function (levelIndex) {
levelIndex[levelIndex.ONE = 0] = 'ONE';
levelIndex[levelIndex.TWO = 1] = 'TWO';
levelIndex[levelIndex.THREE = 2] = 'THREE';
levelIndex[levelIndex.FOUR = 3] = 'FOUR';
levelIndex[levelIndex.FIVE = 4] = 'FIVE';
// setup the player object with the properties of the levelIndex
// before it becomes non-enumerable:
for (var i in levelIndex) {
Object.defineProperty(player, i, {
value: isNaN(levelIndex[i]) ? 'LEVEL_' + levelIndex[i] : levelIndex[i],
enumerable: false
});
alert(player[i]); //<-- LEVEL_ONE, LEVEL_TWO, LEVEL_THREE, LEVEL_FOUR, LEVEL_FIVE, 0, 1, 2, 3, 4
}
alert('window? : ' + this === window); //<-- false
})(this.levelIndex || (this.levelIndex = {})),
writable: false,
enumerable: false,
configurable: false
});
};
var foo = new Foo();
alert(player.ONE); //<-- 0
alert(foo.levelIndex); //<-- undefined
alert(foo.levelIndex.ONE); //<-- Uncaught TypeError: Cannot read property 'ONE' of undefined
Why is foo.levelIndex
undefined here?
EDIT: Fixed with return levelIndex;
added to anonymous function call.
Any comments on my design pattern or suggestions for improvement welcome!
You're calling the anonymous function with a plain function call, so the value of this
will be undefined
in strict mode, or the global object ( window
) otherwise. What it definitely won't be is a reference to the object literal you're defining.
The first line of your "Foo" function looks a little suspicious:
this.levelIndex;
That will have no effect on anything; specifically, it will not cause there to be a property named "levelIndex" created on the object referenced by this
.
edit — also, as I look more at what you're doing, it's pretty clear why the "levelIndex" property ends up undefined
: that object literal has a "value" property that's set to the return value of that anonymous function call. The anonymous function, however, has no return
statement, so the call to Object.defineProperty
involves a property object with the "value" property set to undefined
. If you add
return levelIndex;
to the end of that anonymous function, it ( might ) work. (I think it would.)
Based on help from @Pointy and @Bergi, here is the updated working code:
var player = {};
var Foo = function () {
Object.defineProperty(this, 'levelIndex', {
value: (function (levelIndex) {
levelIndex[levelIndex.ONE = 0] = 'ONE';
levelIndex[levelIndex.TWO = 1] = 'TWO';
levelIndex[levelIndex.THREE = 2] = 'THREE';
levelIndex[levelIndex.FOUR = 3] = 'FOUR';
levelIndex[levelIndex.FIVE = 4] = 'FIVE';
return levelIndex;
})(this.levelIndex || (this.levelIndex = {})),
writable: false,
enumerable: false,
configurable: false
});
// setup the player object with the properties of the levelIndex:
for (var i in this.levelIndex) {
Object.defineProperty(player, i, {
value: isNaN(this.levelIndex[i]) ? 'LEVEL_' + this.levelIndex[i] : this.levelIndex[i],
enumerable: false
});
}
};
var foo = new Foo();
alert(player[0]); //<-- LEVEL_ONE
alert(foo.levelIndex.FIVE); //<-- 4
for (var i in player) {
alert(i + ' ' + player[i]); //<-- no output (properties are not enumerable here)
}
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