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?

Answer 1

Solution without ES6 or polyfills:

var obj = _new(Demo).apply(["X", "Y", "Z"]);


function _new(constr)
{
    function createNamedFunction(name)
    {
        return (new Function("return function " + name + "() { };"))();
    }

    var func = createNamedFunction(constr.name);
    func.prototype = constr.prototype;
    var self = new func();

    return { apply: function(args) {
        constr.apply(self, args);
        return self;
    } };
}

function Demo()
{
    for(var index in arguments)
    {
        this['arg' + (parseInt(index) + 1)] = arguments[index];
    }
}
Demo.prototype.tagged = true;


console.log(obj);
console.log(obj.tagged);

output

Demo {arg1: "X", arg2: "Y", arg3: "Z"}


... or "shorter" way:

var func = new Function("return function " + Demo.name + "() { };")();
func.prototype = Demo.prototype;
var obj = new func();

Demo.apply(obj, ["X", "Y", "Z"]);

edit:
I think this might be a good solution:

this.forConstructor = function(constr)
{
    return { apply: function(args)
    {
        let name = constr.name.replace('-', '_');

        let func = (new Function('args', name + '_', " return function " + name + "() { " + name + "_.apply(this, args); }"))(args, constr);
        func.constructor = constr;
        func.prototype = constr.prototype;

        return new func(args);
    }};
}

Answer 2

Actually the simplest method is:

function Something (a, b) {
  this.a = a;
  this.b = b;
}
function createSomething(){
    return Something;
}
s = new (createSomething())(1, 2); 
// s == Something {a: 1, b: 2}

Answer 3

Suppose you've got an Items constructor which slurps up all the arguments you throw at it:

function Items () {
    this.elems = [].slice.call(arguments);
}

Items.prototype.sum = function () {
    return this.elems.reduce(function (sum, x) { return sum + x }, 0);
};

You can create an instance with Object.create() and then .apply() with that instance:

var items = Object.create(Items.prototype);
Items.apply(items, [ 1, 2, 3, 4 ]);

console.log(items.sum());

Which when run prints 10 since 1 + 2 + 3 + 4 == 10:

$ node t.js
10

Answer 4

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 5

See also how CoffeeScript does it.

s = new Something([a,b,c]...)

becomes:

var s;
s = (function(func, args, ctor) {
  ctor.prototype = func.prototype;
  var child = new ctor, result = func.apply(child, args);
  return Object(result) === result ? result : child;
})(Something, [a, b, c], function(){});

Answer 6

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]);