dynamic object construction in javascript?

There’s no simple, straightforward way to do this with a constructor function. This is because special things happen when you use the new keyword to call a constructor function, and so if you’re not going to do that, you have to emulate all of those special things. They are:

1. Creating a new object instance (you’re doing that).
2. Setting that object’s internal prototype to constructor function’s prototype property.
3. Setting that object’s constructor property.
4. Calling the constructor function with that object instance as the this value (you’re doing that).
5. Handling the special return value from the constructor function.

I think that’s about it, but worth double-checking in the spec.

So if you can avoid it and just use the constructor function directly, I’d do that. 🙂 If you can’t, though, you can still do it, it’s just awkward and involves workarounds. (See also this related answer here on StackOverflow, although I cover all of the ground here [and then some] as well.)

Your biggest issue is #2 above: Setting the internal prototype of the object. For a long time, there was no standard way to do this. Some browsers supported a __proto__ property that did it, so you can use that if it’s there. The good news is that ECMAScript 5 introduces a way to do this explicitly: Object.create. So cutting-edge browsers like Chrome will have that. But if you’re dealing with a browser that has neither Object.create nor __proto__, it gets a bit ugly:

1) Define a custom constructor function.

2) Set its prototype property to the prototype property of the real constructor function

3) Use it to create a blank object instance.

That handles the prototype for you. Then you continue with:

4) Replace the constructor property on that instance with the real constructor function.

5) Call the real constructor function via apply.

6) If the return value of the real constructor function is an object, use it instead of the one you created; otherwise, use the one you created.

Something like this (live example):

function applyConstruct(ctor, params) {
    var obj, newobj;

    // Use a fake constructor function with the target constructor's
    // `prototype` property to create the object with the right prototype
    function fakeCtor() {
    }
    fakeCtor.prototype = ctor.prototype;
    obj = new fakeCtor();

    // Set the object's `constructor`
    obj.constructor = ctor;

    // Call the constructor function
    newobj = ctor.apply(obj, params);

    // Use the returned object if there is one.
    // Note that we handle the funky edge case of the `Function` constructor,
    // thanks to Mike's comment below. Double-checked the spec, that should be
    // the lot.
    if (newobj !== null
        && (typeof newobj === "object" || typeof newobj === "function")
       ) {
        obj = newobj;
    }

    // Done
    return obj;
}

You could take it a step further and only use the fake constructor if necessary, looking to see if Object.create or __proto__ are supported first, like this (live example):

function applyConstruct(ctor, params) {
    var obj, newobj;

    // Create the object with the desired prototype
    if (typeof Object.create === "function") {
        // ECMAScript 5 
        obj = Object.create(ctor.prototype);
    }
    else if ({}.__proto__) {
        // Non-standard __proto__, supported by some browsers
        obj = {};
        obj.__proto__ = ctor.prototype;
        if (obj.__proto__ !== ctor.prototype) {
            // Setting it didn't work
            obj = makeObjectWithFakeCtor();
        }
    }
    else {
        // Fallback
        obj = makeObjectWithFakeCtor();
    }

    // Set the object's constructor
    obj.constructor = ctor;

    // Apply the constructor function
    newobj = ctor.apply(obj, params);

    // If a constructor function returns an object, that
    // becomes the return value of `new`, so we handle
    // that here.
    if (typeof newobj === "object") {
        obj = newobj;
    }

    // Done!
    return obj;

    // Subroutine for building objects with specific prototypes
    function makeObjectWithFakeCtor() {
        function fakeCtor() {
        }
        fakeCtor.prototype = ctor.prototype;
        return new fakeCtor();
    }
}

On Chrome 6, the above uses Object.create; on Firefox 3.6 and Opera, it uses __proto__. On IE8, it uses the fake constructor function.

The above is fairly off-the-cuff, but it mostly handles the issues I’m aware of in this area.