Function.prototype.__proto__ 是什么意思？

Question

I am a beginner in Javascript and am learning about Object Oriented and prototyping in it.

As far as I know, Object is a function and is created by Function because Object.__proto__ === Function.prototype but looking through various diagrams online, I am quite confused by fact that How Function.prototype.__proto__ === Object.prototype .

What does Function.prototype.__proto__ mean?

Isn't it something that is developed by language owners as Function is the first thing from which everything arrives.

Then what does it mean? Am I lacking some important fact? I looked through other StackOverflow answers but can't find anything related to it.

Answer 1

myFunc.prototype is the __proto__ of any object constructed by calling new myFunc() .

Thinking in terms of classical (Java- or C++-style) OO, you could say that myFunc is a constructor and (therefore) myFunc.prototype is the class. In that sense, myFunc.prototype.__proto__ is the superclass; that is, the prototype of the prototype of all objects created with new myFunc .

One useful thing you can do to myFunc.prototype.__proto__ is assign to it to create a superclass relationship, eg

myFunc.prototype.__proto__ = mySuperclassConstructor.prototype

This idiom sheds light on why Function.prototype.__proto__ === Object.prototype holds (the core of your question): it simply means that Function is a subclass of Object — Or in other words, JavaScript runtimes do something equivalent to the above code snippet in their prelude so as to make Function a subclass of Object (as they should, per ECMA-262 §§ 19.2.2 and 19.2.3)

Careful though, while __proto__ happens to work on all modern (2019) JavaScript implementations (node.js and browsers), its use is both non-standard and slow . Consider using “real” ES6 classes instead.

Answer 2

TL;DR

__proto__ is a property of an object which enables you to look up in the prototype chain. While prototype is a property of a Function which enables you to add shareable functionalities to a constructor function.

Long Answer

Understand this from an example, let's say you create a constructor function.

function A() {} and then create an instance of it, var a = new A() .

Then, add a function as following:

A.prototype.getA = function () { return 'A'; } A.prototype.getA = function () { return 'A'; } .

Now, if you try to access a.getA() , you'll get the result, ie getA will be executed.

But how does it know about the function getA even though getA has been added after the instance a was created. It's because of using __proto__ , you can traverse up in the chain (you must have heard about prototype chaining).

Technically, __proto__ is a property of an object, while prototype is a property of function . But how could functions have property? Because everything in JavaScript is converted implicitly to an object. Ever wondered how could you something like this: 'test'.toUpperCase() ? Isn't string literals are 'not object' and they are primitives?

Read this for reference: http://jayendra.co.in/objects-in-javascript/

Now to answer your question:

What does Function.prototype.__proto__ mean?

You are trying to access the prototype property of Function constructor function. Remember, prototype itself is an object , so you can access properties like constructor and __proto__ .

Function.prototype.__proto__ === Object.prototype

To enable chaining, when you access __proto__ property, you are looking up!

Any function can access all the properties of an object. How?

A property of Object , let's say toString . You can do, A.toString() // A created at the start . But we never created a function toString for A constructor function!