Why Is Foo.hasownproperty('__proto__') Equal To False?

var foo = { bar : 5 } Why is foo.hasOwnProperty('__proto__') equal to false? It can't be from any object in the prototype chain higher up, because it is specific to this very ob

Solution 1:

In fact, __proto__ is inherited from Object.prototype:

foo.hasOwnProperty('__proto__')              // falseObject.prototype.hasOwnProperty('__proto__') // true

And according to MDN article,

There is nothing special about the __proto__ property. It is simply an accessor property -- a property consisting of a getter function and a setter function -- on Object.prototype.

As you say, intuitively it can seem that, since __proto__ is intrinsically related to each object, it should be an own property.

But it isn't like this. Instead, Object.prototype.__proto__ has a getter function which returns differently when called on different objects.

You can obtain something similar if you run

Object.defineProperty(
    Object.prototype,
    'self',
    {get: function(){returnthis}}
)

Now you can call .self on different objects and you will get different results.

Also note this behavior isn't exclusive of __proto__. For example, the id property of an HTML element isn't an own property neither:

var el = document.createElement('div');
el.id = 'foo';
el.hasOwnProperty('id');                // falseElement.prototype.hasOwnProperty('id'); // true