Use of .apply() with 'new' operator. Is this possible?

Question

In JavaScript, I want to create an object instance (via the new operator), but pass an arbitrary number of arguments to the constructor. Is this possible?

What I want to do is something like this (but the code below does not work):

function Something(){
    // init stuff
}
function createSomething(){
    return new Something.apply(null, arguments);
}
var s = createSomething(a,b,c); // 's' is an instance of Something

---

The Answer

From the responses here, it became clear that there's no in-built way to call .apply() with the new operator. However, people suggested a number of really interesting solutions to the problem.

My preferred solution was this one from Matthew Crumley (I've modified it to pass the arguments property):

var createSomething = (function() {
function F(args) {
    return Something.apply(this, args);
}
F.prototype = Something.prototype;

return function() {
    return new F(arguments);
}
})();

Answer 1

You could do it like this:

var Ctor = function (a, b) {
    if (arguments.length === 0) {
        return;
    }

    this.a = a;
    this.b = b;
};

var c = new Ctor;
    c.constructor.apply(c, [3, 4]);

print(c.a); // 3
print(c.b); // 4

The downside is that the constructor function will be executed twice.

Answer 2

if you're interested in an eval-based solution

function createSomething() {
 var q = [];
 for(var i = 0; i < arguments.length; i++)
  q.push("arguments[" + i + "]");
 return eval("new Something(" + q.join(",") + ")");
}

Answer 3

You could move the init stuff out into a separate method of Something's prototype:

function Something() {
    // Do nothing
}

Something.prototype.init = function() {
    // Do init stuff
};

function createSomthing() {
    var s = new Something();
    s.init.apply(s, arguments);
    return s;
}

var s = createSomething(a,b,c); // 's' is an instance of Something

Answer 4

Here's a generalized solution that can call any constructor with an array of arguments:

function construct(constructor, args) {
    function F() {
        return constructor.apply(this, args);
    }
    F.prototype = constructor.prototype;
    return new F();
}

An object created by calling construct(Class, [1, 2, 3]) would be identical to an object created with new Class(1, 2, 3).

You could also make a more specific version so you don't have to pass the constructor every time. This is also slightly more efficient, since it doesn't need to create a new instance of the inner function every time you call it.

var createSomething = (function() {
    function F(args) {
        return Something.apply(this, args);
    }
    F.prototype = Something.prototype;

    return function(args) {
        return new F(args);
    }
})();

The reason for creating and calling the outer anonymous function like that is to keep function F from polluting the global namespace. It's sometimes called the module pattern.

Answer 5

You can't call a constructor with a variable number of arguments like you want with the new operator.

What you can do is change the constructor slightly. Instead of:

function Something() {
    // deal with the "arguments" array
}
var obj = new Something.apply(null, [0, 0]);  // doesn't work!

Do this instead:

function Something(args) {
    // shorter, but will substitute a default if args.x is 0, false, "" etc.
    this.x = args.x || SOME_DEFAULT_VALUE;

    // longer, but will only put in a default if args.x is not supplied
    this.x = (args.x !== undefined) ? args.x : SOME_DEFAULT_VALUE;
}
var obj = new Something({x: 0, y: 0});

Or if you must use an array:

function Something(args) {
    var x = args[0];
    var y = args[1];
}
var obj = new Something([0, 0]);