Why you shouldn't miss new when invoking a constructor function in javascript

hero.js

//taken from Object Oriented Javascript by Stoyan Stefanov

var Hero() {
  this.occupation = "Ninja!"
  this.say = function() {
    return "I'm a " + this.occupation;
  }
}

/*  correct invocation of constructor function
> var h = new Hero()
> h.say()
> "I'm a Ninja!"
> this.say()
> undefined
*/

/*  wrong invocation. The global object is 'clobbered'
> var h2 = Hero()
> undefined
> this.say()
> "I'm a Ninja!"
*/