[hajile]

`__proto__` doesn't necessarily equal `.prototype`.

    var foo = Object.create(null)
    //now foo.prototype and foo.__proto__ are both undefined
    foo.prototype = {abc:123}
    //foo.__proto__ is still undefined. Need to use Object.setPrototypeOf()

In older JS code, I've seen people trying to abuse prototypes. One result in this kind of thing is often retaining references to those hidden `__proto__` leading to memory leaks.

Also, `__proto__` is deprecated. If you're writing JS, you should be using `.getPrototypeOf()` instead.

> Could you please expand this part? "Primitive" has specific meaning in JavaScript.

    var fn = function () {}
    fn.bar = "abc"

    Object.keys(fn) //=> ["bar"]

    //likewise
    (1).__proto__ === Number.prototype //=> true

JS is torn on the idea of whether something is primitive or an object. You see this (for example) in Typescript with the primitive number being different from the Number type which represents a number object. To get at the primitive, you must actually call `.valueOf()` which returns the primitive in question. Meanwhile, you can attach your own properties to the function object -- a fact exploited by many, many libraries including modern ones like React. You can also add your own `.valueOf()` to allow your code to better interact with JS operators, but I believe that to pretty much always be a bad practice.

[sergeykish]

Yes, I know about null [[Prototype]] and who is owner of prototype and __proto__. I like how it is not real property

    var foo = {
      __proto__: null
    }
    foo.__proto__
    undefined

As you can see I do not store it and do not modify it, it is assertion, kind of instanceof.

> Also, `__proto__` is deprecated

Object.getPrototypeOf would be too verbose in this example. I could have defined

    Object.defineProperty(Object.prototype, "proto", {
      get() { Object.getPrototypeOf(this) }
    })

but why bother? We both know that I meant [[Prototype]].

> Primitive

> fn.bar = "abc"

is syntactic sugar for

fn["bar"] = "abc"

do not follow.

> (1).__proto__ === Number.prototype

number is primitive

    typeof 1
    "number"
    1 instanceof Number
    false

number is wrapped in Number when we access property

    Number.prototype.foo = function () { return this }
    1..foo()
    //Number {1}
    typeof 1..foo()
    //"object"
    1..foo() instanceof Number 
    //true

Function is not primitive

    typeof Math.max
    "function"
    Math.max instanceof Function
    true
    Math.max instanceof Object 
    true

it is an object, we can attach properties to object.

As I remember valueOf is complicated by hints, exposed in JavaScript [1]. I've played with removal

    delete Array.prototype.toString
    delete Object.prototype.toString
    Uncaught TypeError: Cannot convert object to primitive value

Unfortunately it converts to string and than converts string to number.

    delete Array.prototype.valueOf 
    delete Object.prototype.valueOf 
    +[]
    //0

[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...