newApply

newApply.js

// newApply: Invoke a consturctor function correctly, passing an arbitrary
// number of arguments

function newApply(fn, argsArray) { // (c) Christian C. Salvadó - MIT License
  var F = newApply, obj, ret, t, isPrimitive;
  if (this instanceof F) return; // this function is used as a dummy constructor

  F.prototype = fn.prototype; // set up the prototype and constructor properties
  F.prototype.constructor = fn;

  obj = new F; // create a new instance using this function as a constructor
  ret = fn.apply(obj, argsArray); // apply original constructor function
  t = typeof ret; // get the type of the returned value
  // check if the returned value is a primitive
  isPrimitive = (t == "object") ? ret === null : t != "function";
  return isPrimitive ? obj : ret;
}


// Exampe usage:

function Foo(a,b,c) {
  this.a = a;
  this.b = b;
  this.c = c;
}

var args = ['a','b','c'];
var obj = newApply(Foo, args);

console.log(
  obj instanceof Foo, // true
  Foo.prototype.isPrototypeOf(obj), // true
  obj.a === 'a', // true
  obj.b === 'b', // true
  obj.c === 'c'  // true
);