Creating New Javascript Object With Variable Arguments

Question

I have class Foo, and want to make a function FooPrime which is equivalent to call new Foo() with the arguments passed to FooPrime;

In other words, if I do:

FooPrime(1, 2, 3);

it should be the same as calling to:

new Foo(1, 2, 3)

Normally I'd use call or apply for this sort of thing, and I can get almost the same effect using apply:

var FooPrime = function() {
    return Foo.apply({}, arguments);
}

However:

Foo.apply({}, [1, 2, 3]);

is not exactly the same as calling:

new Foo(1, 2, 3)

so my question is, is there any way to make a function which does exactly the same thing as new, with any number of arguments?

new (Function.prototype.bind.apply(Foo, [null].concat(argsArray)))(); (borrowed from babel) — zerkms
– zerkms, Commented Mar 14, 2016 at 2:19

Oriol · Accepted Answer · 2016-03-14 14:48:58Z

3

In ECMAScript 5 you can polyfill [[Construct]] with something like this:

function isObject(o) {
  return Object(o) === o;
}
function construct(F, args) {
  var obj = isObject(F.prototype) ? Object.create(Foo.prototype) : {},
      res = Function.prototype.apply.call(F, obj, args);
  return isObject(res) ? res : obj;
}
var FooPrime = function() {
  return construct(Foo, arguments);
}

ECMAScript 6 provides the simpler and more reliable Reflect.construct

var FooPrime = function() {
  return Reflect.construct(Foo, arguments);
}

Or you can also use the spread operator, which has more browser support than Reflect.

var FooPrime = function() {
  return new Foo(...arguments);
}

edited Mar 14, 2016 at 14:48

answered Mar 14, 2016 at 2:28

Oriol

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2 Comments

machineghost Over a year ago

That's perfect, except unfortunately it doesn't work with es6 classes, ie. class Foo (it gives TypeError: Class constructors cannot be invoked without 'new'). By any chance is there an equivalent for an es6 class (but without the use of Reflect, since Chrome doesn't currently support it)?

Oriol Over a year ago

@machineghost You can use the spread operator: new Foo(...arguments)

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