JavaScript: How to create a new instance of a class without using the new keyword?

A simpler, cleaner way with no "factories"

function Person(name) {
  if (!(this instanceof Person)) return new Person(name);
  this.name = name;
}

var p1 = new Person('Fred');
var p2 = Person('Barney');

p1 instanceof Person  //=> true
p2 instanceof Person  //=> true

Doesn't this work?

function factory(class_, ...arg) {
    return new class_(...arg);
}

I don't understand why you can't use new.

If you really don't want to use the new keyword, and you don't mind only supporting Firefox, you can set the prototype yourself. There's not really any point to this though, since you can just use Dave Hinton's answer.

// This is essentially what the new keyword does
function factory(clazz) {
    var obj = {};
    obj.__proto__ = clazz.prototype;
    var result = clazz.call(obj);
    return (typeof result !== 'undefined') ? result : obj;
};