dynamic languages - How does JavaScript .prototype work?

Question

I'm not that into dynamic programming languages but I've written my fair share of JavaScript code. I never really got my head around this prototype-based programming, does any one know how this works?

var obj = new Object();
obj.prototype.test = function() { alert('Hello?'); };
var obj2 = new obj();
obj2.test();

I remember a lot discussion I had with people a while back (I'm not exactly sure what I'm doing) but as I understand it, there's no concept of a class. It's just an object, and instances of those objects are clones of the original, right?

But what is the exact purpose of this ".prototype" property in JavaScript? How does it relate to instantiating objects?

Update: correct way

var obj = new Object(); // not a functional object
obj.prototype.test = function() { alert('Hello?'); }; // this is wrong!

function MyObject() {} // a first class functional object
MyObject.prototype.test = function() { alert('OK'); } // OK

Also these slides really helped a lot.

Answer 1

In a language implementing classical inheritance like Java, C# or C++ you start by creating a class--a blueprint for your objects--and then you can create new objects from that class or you can extend the class, defining a new class that augments the original class.

In JavaScript you first create an object (there is no concept of class), then you can augment your own object or create new objects from it. It's not difficult, but a little foreign and hard to metabolize for somebody used to the classical way.

Example:

//Define a functional object to hold persons in JavaScript
var Person = function(name) {
  this.name = name;
};

//Add dynamically to the already defined object a new getter
Person.prototype.getName = function() {
  return this.name;
};

//Create a new object of type Person
var john = new Person("John");

//Try the getter
alert(john.getName());

//If now I modify person, also John gets the updates
Person.prototype.sayMyName = function() {
  alert('Hello, my name is ' + this.getName());
};

//Call the new method on john
john.sayMyName();