Learn about the instanceof, the Function, And the Object

instanceof.md

The three property

*.__proto__: the constructor function of an object.
*.prototype: the value of prototype is used to initialize __proto__.
*.constructor: the constructor function to create new object.

> const o = new Object();
> o.__proto__ === Object.prototype
true

Deep dive into the parent

> Object.__proto__
[Function]  // let's call it func
> const func = Object.__proto__
> Object.prototype
{}  // which is an object that has no __proto__, let's call it obj
> const obj = Object.prototype
> Object.constructor
[Function: Function]    // yes, it can own new child
> Object.constructor === Function

> Function.__proto__
[Function]
> Function.__proto__ === func
true
> Function.prototype
[Function]
> Function.prototype === func
true
> Function.constructor
[Function: Function]
> Function.constructor === Function
true

> func.__proto__
{}
> func.__proto__ === obj
true
> func.prototype
undefined
> func.constructor
[Function: Function]
> func.constructor === Function
true

> obj.__proto__
null
> obj.prototype
undefined
> obj.constructor
[Function: Object]
> obj.constructor === O
true

> const a = new Object()
> a instanceof Object
true
> a.__proto__
{}  // obj
> a.prototype
undefined
> a.constructor
[Function: Object]  // Object

Remove the progress

const func = Object.__proto__
const obj = Object.prototype
const a = new Object()

Conclusion

pointer	value	typeof	__proto__	prototype	constructor
Object	[Function: Object]	function	func	obj	Function
Function	[Function: Function]	function	func	func	Function
func	[Function]	function	obj		Function
obj	{}	object			Object
a	{}	object	obj		Object

a, func, obj cannot be used as right expression of "instanceof"
All names in the table refers to an unique object.

Pesudo code

class obj {
    constructor() [[native code]]
}
class func extends obj {
    constructor() [[native code]]
}
const Object = obj.constructor;
const Function = func.constructor;

What is class in JavaScript?

> typeof (class A {})
'function'

The class declaration creates a new class with a given name using prototype-based inheritance.

Object instanceof Function is true

Object is a function;  
Object is an instance of Function (
    Object is a Function object;  
    Object is an instance of the Function constructor;
);
Object inherits from Function.prototype aka. func;

Object instanceof Object is true

func inherits from Object.prototype aka. obj;  
obj is an object;  
Object inherits from obj;  

Function instance of Object is true

func is an object;
Function inherits from func;
func inherits from obj.

Function instanceof Function is true

Function is an instance of itself (naturally, since it’s a function, and thus an instance of Function).

The prototype chain looks like this

Object   ---> Function.prototype ---> Object.prototype ---> null
Function ---> Function.prototype ---> Object.prototype ---> null

Back to the definition, from the MDN

The instanceof operator tests whether an object has in its prototype chain the prototype property of a constructor.
in another word,
The instanceof operator tests whether the prototype property of an constructor of something exists in a prototype chain of an object.

Future reading

Inheritance and the prototype chain, MDN