I have not claimed JS is the weirdest. But I have not claimed "Normally C++/PHP/Ruby/Python devs don't really encounter these notorious problems, for many years now" either.

Eigenclass (singleton_class) explained in another thread. I have not encountered Pythons for/else [1] yet.

Right, typeof null exposed by Microsoft IE 2 (?). Web is many times bigger now yet even such a small mistake is not fixed.

I have issue + of being concatenator, I prefer string interpolation, separate operators. Implicit type conversion often does not make sense spoils a lot

    [] * 2
    //0
    foo = {}
    bar = {}
    foo[bar] = 1  // just throw please
    Object.keys(foo)
    //["[object Object]"]

> they could also silently fail as well.

But they don't. If only these rules were defined as library. I am sure it would be ditched long ago. Actually this may be argument in favor of operator overloading in JavaScript, the way to fix it.

> Foo being an instance of function is MUCH more honest

    class Foo
    end

    Foo.send(:initialize)
    TypeError (already initialized class)
    # wrong one
 
    Foo.instance_method(:initialize).call
    NoMethodError (undefined method `call' for #<UnboundMethod: Foo(BasicObject)#initialize()>)
    # does not allow unbound

    Foo.new

new constructs an object and calls initialize. Same in JavaScript

    function Foo () {
      console.log(this)
    }
    new Foo
    // Foo {}
    Foo()
    // Window

It kind of make sense — new creates an object of constructor.prototype and calls constructor. I can't see how it is MUCH more honest than if new creates an object of prototype and calls prototype.constructor. By that logic Object.create is not honest

    Object.create(Object.prototype) // expects [[Prototype]] not constructor
    Object.create(null)

And even if it was

    foo = {}
    bar = Object.create(foo)
    bar.__proto__ === foo 
    //true
    bar.__proto__.__proto__ === Object.prototype
    //true
    bar.__proto__.__proto__.__proto__ === null 
    //true

    class Foo {}
    class Bar extends Foo {}
    bar = new Bar 
    bar.__proto__ === Bar.prototype 
    bar.__proto__.__proto__ === Foo.prototype
    bar.__proto__.__proto__.__proto__ === Object.prototype
    bar.__proto__.__proto__.__proto__.__proto__  === null

I don't need constructor except in new, otherwise I use it only to access prototype. Absence of languages adopting this approach confirms its usability issues.

> This is even more true because you are looking at the primitive rather than the function object which contains the primitive.

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

[1] https://book.pythontips.com/en/latest/for_-_else.html

`__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.

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...