Use of .apply() with 'new' operator. Is th

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 built-in 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 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 createSomething() {
    var s = new Something();
    s.init.apply(s, arguments);
    return s;
}

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

Answer 2

@Matthew I think it's better to fix the constructor property also.

// Invoke new operator with arbitrary arguments
// Holy Grail pattern
function invoke(constructor, args) {
    var f;
    function F() {
        // constructor returns **this**
        return constructor.apply(this, args);
    }
    F.prototype = constructor.prototype;
    f = new F();
    f.constructor = constructor;
    return f;
}

Answer 3

An improved version of @Matthew's answer. This form has the slight performance benefits obtained by storing the temp class in a closure, as well as the flexibility of having one function able to be used to create any class

var applyCtor = function(){
    var tempCtor = function() {};
    return function(ctor, args){
        tempCtor.prototype = ctor.prototype;
        var instance = new tempCtor();
        ctor.prototype.constructor.apply(instance,args);
        return instance;
    }
}();

This would be used by calling applyCtor(class, [arg1, arg2, argn]);

Answer 4

function createSomething() {
    var args = Array.prototype.concat.apply([null], arguments);
    return new (Function.prototype.bind.apply(Something, args));
}

If your target browser doesn't support ECMAScript 5 Function.prototype.bind, the code won't work. It is not very likely though, see compatibilty table.

Answer 5

since ES6 this is possible through the Spread operator, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator#Apply_for_new

This answer was already, sort of given in comment https://stackoverflow.com/a/42027742/7049810, but seems to have been missed by most

Answer 6

This answer is a little late, but figured anyone who sees this might be able to use it. There is a way to return a new object using apply. Though it requires one little change to your object declaration.

function testNew() {
    if (!( this instanceof arguments.callee ))
        return arguments.callee.apply( new arguments.callee(), arguments );
    this.arg = Array.prototype.slice.call( arguments );
    return this;
}

testNew.prototype.addThem = function() {
    var newVal = 0,
        i = 0;
    for ( ; i < this.arg.length; i++ ) {
        newVal += this.arg[i];
    }
    return newVal;
}

testNew( 4, 8 ) === { arg : [ 4, 8 ] };
testNew( 1, 2, 3, 4, 5 ).addThem() === 15;

For the first if statement to work in testNew you have to return this; at the bottom of the function. So as an example with your code:

function Something() {
    // init stuff
    return this;
}
function createSomething() {
    return Something.apply( new Something(), arguments );
}
var s = createSomething( a, b, c );

Update: I've changed my first example to sum any number of arguments, instead of just two.