javascript,debugging,prototype,google-chrome-devtools,prototypal-inheritance , Why does the __proto__ object evaluate to “Object” in the debugger?


Why does the __proto__ object evaluate to “Object” in the debugger?

Question:

Tag: javascript,debugging,prototype,google-chrome-devtools,prototypal-inheritance

I'm reading Kyle Simpson's "YDKJS: this & Object Prototypes", and looking at his example of behavior delegation. Here's the code below:

Foo = {
    init: function(who) {
        this.me = who;
    },
    identify: function() {
        return "I am " + this.me;
    }
};

Bar = Object.create( Foo );

Bar.speak = function() {
    alert( "Hello, " + this.identify() + "." );
};

var b1 = Object.create( Bar );
b1.init( "b1" );
var b2 = Object.create( Bar );
b2.init( "b2" );

b1.speak();
b2.speak();

He uses this diagram to describe the relationships between these objects:

Now my question:

When tinkering with this code in Chrome Developer tools, I see that the __proto__ object evaluates to Object. Why is that? Shouldn't the __proto__ object depict the actual relationship in the diagram, where b1 and b2 delegate to Bar which in-turn delegates to Foo? Why are these relationships not explicitly named in the __proto__ object? After-all, this implementation uses the Object.create() method, and places an argument in the proto parameter (the first parameter). Wouldn't it be expected that the console would return that named argument as the value of the __proto__ object?

enter image description here


Answer:

This isn't quite so simple. The __proto__ property is a dynamic link to the .constructor's prototype.

However, .constructor isn't overwritten in these cases (though it can be in certain hand-made libraries).

When you're looking for a reference to Foo to show up in that chain, it's a very "classical" viewpoint, and less to do with prototypal inheritance, which inherits from a concrete instance, rather than a class, and in the case of JS, references (when not scalar), rather than copies.

The tl;dr reason is simply: Object.create's constructor is Object, {}'s constructor is Object and Object.prototype's constructor is Object.


The Long Way

First thing to know:
names in these lookups aren't tied to variables, but rather they're tied to functions, typically.
Once upon a time, the field where this information could be found was hidden, and now it's found it's way to the .name property of a function (function Foo () { } Foo.name; //Foo) in many browsers now (much like __proto__ used to be invisible.

Second bit:
names which you see in console references, and in type-checks, aren't based on the proto, but on a property called .constructor.

// Equivalent assignments
var obj1 = { };
var obj2 = new Object();
var obj3 = Object.create(Object.prototype);

obj1.constructor; // Object
obj2.constructor; // Object
obj3.constructor; // Object

If we were to change the dynamic a little, and introduce the more "classical" approach to instance-creation, which is paseé, but will see a resurgence in ES6 with the class sugar...

function Foo () {
  this.isFoo = true;
}

var foo = new Foo();
foo.isFoo; // true
foo.constructor; // Foo
foo.__proto__ === Foo.prototype; // true
foo.__proto__.constructor; // Object

That last little bit there is telling, for your immediate question; Foo.prototype is nothing but a plain ol' Object instance.
In the days of JS where everybody was looking for the best possible way of making JS feel like Java / C#, you'd see something like:

function Foo () { }

function Bar() { }

Bar.prototype = new Foo();

var foo = new Foo();
var bar = new Bar();
bar instanceof Bar; // true
bar instanceof Foo; // true

On one hand, this sort of works, as we've been able to reuse method's of Foo on instances of Bar.
Grand.
The immediate downside is that all instances of Bar share the same instance of Foo, including all of its instance-specific properties.

While this method is far from what I'd suggest as a pattern for reuse, it does show off your dilemma well.

 // using the last set of assignments
 bar instanceof Foo;
 // equivalent to:
 bar.constructor === Foo; // false
 bar.__proto__.constructor === Foo; // true
 // not equivalent to
 bar.__proto__ == Foo;

More modern forms of "classical" inheritance (calling a super constructor, copying prototypes of other constructors onto your new constructor, etc) do a better job of allowing you to compose and reuse methods...

But that comes at the cost of being able to rely on the __proto__ chain for looking up where those loaned methods come from.

Historically, the "bad for reuse, good for type-checking" model let you search
val.__proto__.__proto__.__proto__.__proto__. (...) .__proto__.constructor
until it matched your instanceof, or until you hit Object at the end of the line.

Newer forms copy the values directly onto val.__proto__ (/ val.constructor.prototype, which we've seen is the same object, at creation of val), which means your proto chain runs out very quickly.


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