Constructor that works without new keyword.

flexible-constructor.js

function MyConstructor(foo){
  if (!(this instanceof MyConstructor)){
    return new MyConstructor(foo);
  }
  this.foo = foo;
  return this;
}

var instance1 = MyConstructor('foo1');
var instance2 = new MyConstructor('foo2');

console.log(instance1.foo);
console.log(instance2.foo);