Question

I know this will work:

function Foo() {};
Foo.prototype.talk = function () {
    alert('hello~\n');
};

var a = new Foo;
a.talk(); // 'hello~\n'

But if I want to call

Foo.talk() // this will not work
Foo.prototype.talk() // this works correctly

I find some methods to make Foo.talk work,

1. Foo.__proto__ = Foo.prototype
2. Foo.talk = Foo.prototype.talk

Are there other ways to do this? I don’t know whether it is right to do so. Do you use class methods or static methods in your JavaScript code?

Answer 1

First off, remember that JavaScript is primarily a prototypal language, rather than a class-based language¹. Foo isn’t a class, it’s a function, which is an object. You can instantiate an object from that function using the new keyword which will allow you to create something similar to a class in a standard OOP language.

I’d suggest ignoring __proto__ most of the time because it has poor cross browser support, and instead focus on learning about how prototype works.

If you have an instance of an object created from a function² and you access one of its members (methods, attributes, properties, constants etc) in any way, the access will flow down the prototype hierarchy until it either (a) finds the member, or (b) doesn’t find another prototype.

The hierarchy starts on the object that was called, and then searches its prototype object. If the prototype object has a prototype, it repeats, if no prototype exists, undefined is returned.

For example:

foo = {bar: 'baz'};
console.log(foo.bar); // logs "baz"

foo = {};
console.log(foo.bar); // logs undefined

function Foo(){}
Foo.prototype = {bar: 'baz'};
f = new Foo();
console.log(f.bar);
// logs "baz" because the object f doesn't have an attribute "bar"
// so it checks the prototype
f.bar = 'buzz';
console.log( f.bar ); // logs "buzz" because f has an attribute "bar" set

---

It looks to me like you’ve at least somewhat understood these “basic” parts already, but I need to make them explicit just to be sure.

In JavaScript, everything is an object³.

everything is an object.

function Foo(){} doesn’t just define a new function, it defines a new function object that can be accessed using Foo.

This is why you can access Foo‘s prototype with Foo.prototype.

What you can also do is set more functions on Foo:

Foo.talk = function () {
  alert('hello world!');
};

This new function can be accessed using:

Foo.talk();

I hope by now you’re noticing a similarity between functions on a function object and a static method.

Think of f = new Foo(); as creating a class instance, Foo.prototype.bar = function(){...} as defining a shared method for the class, and Foo.baz = function(){...} as defining a public static method for the class.

---

ECMAScript 2015 introduced a variety of syntactic sugar for these sorts of declarations to make them simpler to implement while also being easier to read. The previous example can therefore be written as:

class Foo {
  bar() {...}

  static baz() {...}
}

which allows bar to be called as:

const f = new Foo()
f.bar()

and baz to be called as:

Foo.baz()

---

_{1: class was a “Future Reserved Word” in the ECMAScript 5 specification, but ES6 introduces the ability to define classes using the class keyword.}

_{2: essentially a class instance created by a constructor, but there are many nuanced differences that I don’t want to mislead you}

_{3: primitive values—which include undefined, null, booleans, numbers, and strings—aren’t technically objects because they’re low-level language implementations. Booleans, numbers, and strings still interact with the prototype chain as though they were objects, so for the purposes of this answer, it’s easier to consider them “objects” even though they’re not quite.}