Question

I’m having some trouble figuring out how the prototype chain is set up given the following code.

var Model = {
    prototype: {
        init: function(){},
        log: function(){ console.log('instance method log was called') }
    },
    log: function(){ console.log('class method log was called') },
    create: function() {
        var object = Object.create(this);
        object.parent = this;
        object.prototype = object.fn = Object.create(this.prototype);
        return object;
    },
    init: function() {
        var instance = Object.create(this.prototype);
        instance.parent = this;
        instance.init.apply(instance, arguments);
        return instance;
    }
}

var User = Model.create();
User.log(); // 'class method log was called'

// create a new method after creation
Model.warn = function() { console.warn('warn was called') }

User.warn() // 'warn was called'

var user = User.init();
user.log(); // 'instance method log was called'

Specifically this line confuses me in the create method:

object.prototype = object.fn = Object.create(this.prototype);

I understand how the create method is creating a new object who’s prototype points back to Model, but the penultimate line seems to overwrite that prototype with a new object (Model.prototype). However, it seems the original prototype is still intact, as I can add methods to Model even after creating a new object, and the new object can still access it.

can someone shed some light on what is actually happening?

Edit – I should point out this code comes from Javascript Web Applications by O’reilly

Answer 1

Ok. so just for delete noise code, you are asking about this code:

var Model = {
    prototype: {},
    create: function() {
        var object = Object.create(this);
        object.prototype = Object.create(this.prototype);
        return object;
    }
};

The first line of Model.create() function is easy, it creates a new object who extends (prototypes) Model.

But you are counfused because the next line overwrites the object “prototype” property. It’s not the object’s prototype, the object prototype is still Model and it’s stored on a hidden property called [[Prototype]], the property what the code is modifying has nothing to do with the object’s [[Prototype]] it only has the same name. Let’s change the name to understand it and it will be the same:

var Model = {
    blablabla: {},
    create: function() {
        var object = Object.create(this);
        object.blablabla = Object.create(this.blablabla);
        return object;
    }
};

It extends Model.blablabla so when you change object.blablabla it does not affect Model.blablabla.

var SubModel = Model.create();
SubModel.blablabla.newMethod = function() { };
console.log(Model.blablabla.newMethod); // undefined

Many times choose the right name for a field is more important than what it looks like.