construct an object given a constructor and an array of parameters

arbitrary_constructor.js

function createObject (ctor, args) {
    function F () {
        ctor.apply(this, args);
    }
    F.prototype = ctor.prototype;
    F.prototype.constructor = ctor; // IE might need this
    return new F();
}

// quick test:
function Foo(a,b,c) {
    this.a = a; this.b = b; this.c = c;
}
createObject(Foo, [1,2,3]) instanceof Foo; // true
createObject(Foo, [1,2,3]).a // 1

/* another way to do it: create a generic constructor that takes an array of parameters */

function makeCtor (ctor) {
    function F (args) {
        ctor.apply(this, args);
    }
    F.prototype = ctor.prototype;
    F.prototype.constructor = ctor; // IE might need this
    return F;
}

var ctor = makeCtor(Foo);
new ctor([1, 2, 3]) instanceof Foo; // true
new ctor(['a', 'b', 'c']).a // 'a'

Something like this could be needed in a generic routine that constructs objects from meta data (json-schema for instance). You may expect that this would work:

var foo = new Foo.apply([1, 2, 3]);

but it doesn't.

The really cool part (imho) is that obj instanceof F is true, but no code could test for that since F is scoped locally to the createObject and makeCtor functions.