Dynamic JavaScript object property value

I'm trying to figure out if there is a way to have an object property defined in its prototype as a dynamic value that can change every time an instance of the object is created. That's kind of the best way I can describe it; I have made a fiddle to show what I'm trying to do (though it doesn't work).

var Response = {
LCID: 321
};


Date.prototype.LCID = Number(0);
Date.prototype.LCID.valueOf = function() { return Response.LCID; };


document.write((new Date()).LCID);
Response.LCID = 456;
document.write((new Date()).LCID);

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What I'm trying to achieve is that `Response.LCID` can change over the lifetime of the code as you can see from me changing its value in the code later on. Whenever I create a `Date` object, I want the value of `(new Date()).LCID` to be the current value of `Response.LCID`, not just the value that it had when I first create `Date.prototype.LCID`.

Is there any way this can be done? The biggest limitation is that it has to work in JavaScript 1.5... though I would be curious to see if it could be done in recent versions.

`Number(0) === 0`. Modifying `.valueOf` and `.toString` on primitive values have no effect.

The correct way to do this is to pass an **instance of** the `Number` constructor, using the [`new` operator][1]:

var Response = {
LCID: 321
};
Date.prototype.LCID = new Number(); // <-- Use the "new" operator
Date.prototype.LCID.valueOf = function() { return Response.LCID; };

##Annotated demo and notes:

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* `LCID` is an object. **`typeof new Date().LCID === 'object'`**.
* `LCID` is a true instance of `Number`. **`new Date().LCID instanceof Number === true`**
* `LCID` is equal to 321: **`(new Date().LCID == 321) === true`**
* `LCID` is not identical to 321: **`(new Date().LCID === 321) === false`**.<br>
(because LCID is an object, and 321 is a primitive number value).

PS. If you're not familiar with `==` versus `===`, see

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[1]:

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After some more research and experimenting, I was able to actually solve the issue. I had tried playing with the Date constructor and such, but I didn't have much luck in my initial trials - apparently because I overlooked the fact that the Date object is unique in that it differs in functionality depending on how it is called (as a function or object constructor). This means that you can't just do `Date.prototype.constructor.apply(this, arguments)` since all you'll ever get back is a string (the Date object is being called as a function).

After having found [this thread][1] and reading it, I came up with the following code that creates an actual Date object (or string if called as a function) and mimics the built-in Date object perfectly (as far as my tests show anyways). Every time a new Date object is created, it gets the LCID property which is dynamically generated during object creation which is exactly what I needed.

Date = (function(orig) {
var date = function(a, b, c, d, e, f, g) {
var object = (this instanceof Object ? (arguments.length < 1 ? new orig() : (arguments.length < 2 ? new orig(a) : (arguments.length < 4 ? new orig(a, b || 0, c || 1) : new orig(a, b, c, d || 0, e || 0, f || 0, g || 0)))) : orig());
object.LCID = Response.LCID;

return object;
};
date.prototype = orig.prototype;

return date;
})(Date);

I also created a bunch of test cases to ensure that there is no difference from a built-in Date object, or using this code (comment out this code to see the results using the built-in Date object and compare).

var Response = { 'LCID': 123 };


Date = (function(orig) {
var date = function(a, b, c, d, e, f, g) {
var object = (this instanceof Object ? (arguments.length < 1 ? new orig() : (arguments.length < 2 ? new orig(a) : (arguments.length < 4 ? new orig(a, b || 0, c || 1) : new orig(a, b, c, d || 0, e || 0, f || 0, g || 0)))) : orig());
object.LCID = Response.LCID;
return object;
};
date.prototype = orig.prototype;
return date;
})(Date);


var x = new Date();
document.writeln(x);
document.writeln(x.LCID);
document.writeln(x.getFullYear());
document.writeln(typeof x);
document.writeln(Object.prototype.toString.call(x));
document.writeln(x instanceof Date);
document.writeln("<br/>");

Response.LCID = 456;

var y = new Date();
document.writeln(y);
document.writeln(y.LCID);
document.writeln(y.getFullYear());
document.writeln(typeof y);
document.writeln(Object.prototype.toString.call(y));
document.writeln(y instanceof Date);
document.writeln("<br/>");

document.writeln(Date());
document.writeln(new Date());
document.writeln(new Date(2012));
document.writeln(new Date(2012, 7));
document.writeln(new Date(2012, 7, 14));
document.writeln(new Date(2012, 7, 14, 9));
document.writeln(new Date(2012, 7, 14, 9, 45));
document.writeln(new Date(2012, 7, 14, 9, 45, 27));
document.writeln(new Date(2012, 7, 14, 9, 45, 27, 687));

This is also available as an updated fiddle:

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[1]:

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Ok this does it using the [`__defineGetter__`][1] method, however I am not sure how it will behave on all browser but that the only way I could find to achieve what you want (well I think this time it is what you want)

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working example.

var Response = {
LCID: 321
};

var d = Date.prototype;

d._LCID = Number(0);
d.getLCID = function() {
if (d._LCID != Response.LCID) d._LCID = Response.LCID;
return d._LCID ;
};
d.__defineGetter__("LCID", function() {
return this.getLCID();
});

document.write((new Date()).LCID);
Response.LCID = 456;
document.write((new Date()).LCID);​



For alternative to ```__defineGetter__``` please see [this post][2]
[1]:

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[2]:

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