reflection - Instantiating a JavaScript object by calling prototype.constructor.apply

Question

Let me start with a specific example of what I'm trying to do.

I have an array of year, month, day, hour, minute, second and millisecond components in the form [ 2008, 10, 8, 00, 16, 34, 254 ]. I'd like to instantiate a Date object using the following standard constructor:

new Date(year, month, date [, hour, minute, second, millisecond ])

How can I pass my array to this constructor to get a new Date instance? [ Update: My question actually extends beyond this specific example. I'd like a general solution for built-in JavaScript classes like Date, Array, RegExp, etc. whose constructors are beyond my reach. ]

I'm trying to do something like the following:

var comps = [ 2008, 10, 8, 00, 16, 34, 254 ];
var d = Date.prototype.constructor.apply(this, comps);

I probably need a "new" in there somewhere. The above just returns the current time as if I had called "(new Date()).toString()". I also acknowledge that I may be completely in the wrong direction with the above :)

Note: No eval() and no accessing the array items one by one, please. I'm pretty sure I should be able to use the array as is.

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Update: Further Experiments

Since no one has been able to come up with a working answer yet, I've done more playing around. Here's a new discovery.

I can do this with my own class:

function Foo(a, b) {
    this.a = a;
    this.b = b;

    this.toString = function () {
        return this.a + this.b;
    };
}

var foo = new Foo(1, 2);
Foo.prototype.constructor.apply(foo, [4, 8]);
document.write(foo); // Returns 12 -- yay!

But it doesn't work with the intrinsic Date class:

var d = new Date();
Date.prototype.constructor.call(d, 1000);
document.write(d); // Still returns current time :(

Neither does it work with Number:

var n = new Number(42);
Number.prototype.constructor.call(n, 666);
document.write(n); // Returns 42

Maybe this just isn't possible with intrinsic objects? I'm testing with Firefox BTW.

Answer 1

I've done more investigation of my own and came up with the conclusion that this is an impossible feat, due to how the Date class is implemented.

I've inspected the SpiderMonkey source code to see how Date was implemented. I think it all boils down to the following few lines:

static JSBool
Date(JSContext *cx, JSObject *obj, uintN argc, jsval *argv, jsval *rval)
{
    jsdouble *date;
    JSString *str;
    jsdouble d;

    /* Date called as function. */
    if (!(cx->fp->flags & JSFRAME_CONSTRUCTING)) {
        int64 us, ms, us2ms;
        jsdouble msec_time;

        /* NSPR 2.0 docs say 'We do not support PRMJ_NowMS and PRMJ_NowS',
         * so compute ms from PRMJ_Now.
         */
        us = PRMJ_Now();
        JSLL_UI2L(us2ms, PRMJ_USEC_PER_MSEC);
        JSLL_DIV(ms, us, us2ms);
        JSLL_L2D(msec_time, ms);

        return date_format(cx, msec_time, FORMATSPEC_FULL, rval);
    }

    /* Date called as constructor. */
    // ... (from here on it checks the arg count to decide how to create the date)

When Date is used as a function (either as Date() or Date.prototype.constructor(), which are exactly the same thing), it defaults to returning the current time as a string in the locale format. This is regardless of any arguments that are passed in:

alert(Date()); // Returns "Thu Oct 09 2008 23:15:54 ..."
alert(typeof Date()); // Returns "string"

alert(Date(42)); // Same thing, "Thu Oct 09 2008 23:15:54 ..."
alert(Date(2008, 10, 10)); // Ditto
alert(Date(null)); // Just doesn't care

I don't think there's anything that can be done at the JS level to circumvent this. And this is probably the end of my pursuit in this topic.

I've also noticed something interesting:

    /* Set the value of the Date.prototype date to NaN */
    proto_date = date_constructor(cx, proto);
    if (!proto_date)
        return NULL;
    *proto_date = *cx->runtime->jsNaN;

Date.prototype is a Date instance with the internal value of NaN and therefore,

alert(Date.prototype); // Always returns "Invalid Date"
                       // on Firefox, Opera, Safari, Chrome
                       // but not Internet Explorer

IE doesn't disappoint us. It does things a bit differently and probably sets the internal value to -1 so that Date.prototype always returns a date slightly before epoch.

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Update

I've finally dug into ECMA-262 itself and it turns out, what I'm trying to achieve (with the Date object) is -- by definition -- not possible:

15.9.2 The Date Constructor Called as a Function

When Date is called as a function rather than as a constructor, it returns a string representing the current time (UTC).

NOTE The function call Date(…) is not equivalent to the object creation expression new Date(…) with the same arguments.

15.9.2.1 Date ( [ year [, month [, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] ] ] )

All of the arguments are optional; any arguments supplied are accepted but are completely ignored. A string is created and returned as if by the expression (new Date()).toString().