Extend an object with an extension in JS

Question

Can someone explain to me what is happening here?

function extend( obj, extension ){
  for ( var key in extension ){
    obj[key] = extension[key];
  }
}

Context: Used in the Observer Pattern https://addyosmani.com/resources/essentialjsdesignpatterns/book/#observerpatternjavascript

For example:

// Extend the controlling checkbox with the Subject class
extend( controlCheckbox, new Subject() );

With

function Subject(){
  this.observers = new ObserverList();
}

Subject.prototype.addObserver = function( observer ){
  this.observers.add( observer );
};

Subject.prototype.removeObserver = function( observer ){
  this.observers.removeAt( this.observers.indexOf( observer, 0 ) );
};

Subject.prototype.notify = function( context ){
  var observerCount = this.observers.count();
  for(var i=0; i < observerCount; i++){
    this.observers.get(i).update( context );
  }
};

What I think is going on: The for loop goes through the Subject's properties that I added before and then adds it to the Observer-object.

What I don't understand: How come it only adds those properties that I added before (i.e. observers, addObserver, removeObserver, notify) but not ALL properties of the extension object?

For example when I print out the extension object I can see that the constructor- or the __proto__ property is NOT added.

Answer 1

The function extend is making obj inherit the properties and methods of extension.

So, in this case, controlCheckbox, is inheriting the properties and methods of Subject.

You will get all the properties and methods from Subject but __proto__ is not a property or method - it is the actual object that is used in the lookup chain to resolve methods, etc. There is a good post here which discusses __proto__

UPDATE Indeed, proto is a property - but it is a special kind of property. From here:

Contains a reference to the internal prototype of the specified object.

An object can have only a single prototype (not including inherited prototypes in the prototype chain)...

So in the chain, you will have a __proto__ for controlCheckbox and one of the properties of its __proto__ should be the __proto__ from Subjects containing all its properties and methods.

UPDATE 2 When using for (propName in obj) loops, the proto property is not added because it is not enumerable. Only enumerable inherited properties are returned. You can search on Enumerable and propertyIsEnumerable() to get more information.