Class vs. static method in JavaScript

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

Is there some 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

ES6 supports now class & static keywords like a charm :

class Foo {
    constructor() {}

    talk() {
        console.log("i am not static");
    }

    static saying() {
        console.log(this.speech);
    }

    static get speech() {
        return "i am static method";
    }

}

Answer 2

In your case, if you want to Foo.talk():

function Foo() {};
// But use Foo.talk would be inefficient
Foo.talk = function () {
    alert('hello~\n');
};

Foo.talk(); // 'hello~\n'

But it's an inefficient way to implement, using prototype is better.

---

Another way, My way is defined as static class:

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

Foo.talk(); // 'hello~\n'

Above static class doesn't need to use prototype because it will be only constructed once as static usage.

https://github.com/yidas/js-design-patterns/tree/master/class

Answer 3

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 it's prototype object. If the prototype object has a prototype, it repeats, if no prototype exists, undefined is returned.

For example:

foo = {bar: 'baz'};
alert(foo.bar); //alerts "baz"

foo = {};
alert(foo.bar); //alerts undefined

function Foo(){}
Foo.prototype = {bar: 'baz'};
f = new Foo();
alert(f.bar);
//alerts "baz" because the object f doesn't have an attribute "bar"
//so it checks the prototype
f.bar = 'buzz';
alert( f.bar ); //alerts "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 therefor 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.}

Answer 4

If you have to write static methods in ES5 I found a great tutorial for that:

//Constructor
var Person = function (name, age){
//private properties
var priv = {};

//Public properties
this.name = name;
this.age = age;

//Public methods
this.sayHi = function(){
    alert('hello');
}
}


// A static method; this method only 
// exists on the class and doesn't exist  
// on child objects
Person.sayName = function() {
   alert("I am a Person object ;)");  
};

see @https://abdulapopoola.com/2013/03/30/static-and-instance-methods-in-javascript/

Answer 5

Just additional notes. Using class ES6, When we create static methods..the Javacsript engine set the descriptor attribute a lil bit different from the old-school "static" method

function Car() {

}

Car.brand = function() {
  console.log('Honda');
}

console.log(
  Object.getOwnPropertyDescriptors(Car)
);

it sets internal attribute (descriptor property) for brand() to

..
brand: [object Object] {
    configurable: true,
    enumerable: true,
    value: ..
    writable: true

}
..

compared to

class Car2 {
   static brand() {
     console.log('Honda');
   }
}

console.log(
  Object.getOwnPropertyDescriptors(Car2)
);

that sets internal attribute for brand() to

..
brand: [object Object] {
    configurable: true,
    enumerable: false,
    value:..
    writable: true
  }

..

see that enumerable is set to false for static method in ES6.

it means you cant use the for-in loop to check the object

for (let prop in Car) {
  console.log(prop); // brand
}

for (let prop in Car2) {
  console.log(prop); // nothing here
}

static method in ES6 is treated like other's class private property (name, length, constructor) except that static method is still writable thus the descriptor writable is set to true { writable: true }. it also means that we can override it

Car2.brand = function() {
   console.log('Toyota');
};

console.log(
  Car2.brand() // is now changed to toyota
);

Answer 6

When i faced such a situation, i have done something like this:

Logger = {
    info: function (message, tag) {
        var fullMessage = '';        
        fullMessage = this._getFormatedMessage(message, tag);
        if (loggerEnabled) {
            console.log(fullMessage);
        }
    },
    warning: function (message, tag) {
        var fullMessage = '';
        fullMessage = this._getFormatedMessage(message, tag);
        if (loggerEnabled) {
            console.warn(fullMessage);`enter code here`
        }
    },
    _getFormatedMessage: function () {}
};

so now i can call the info method as Logger.info("my Msg", "Tag");