Static properties and methods

We can also assign a method to the class function, not to its "prototype". Such methods are called static.

An example:

class User {
  static staticMethod() {
    alert(this === User);
  }
}

User.staticMethod(); // true

That actually does the same as assigning it as a property:

class User() { }

User.staticMethod = function() {
  alert(this === User);
};

The value of this inside User.staticMethod() is the class constructor User itself (the “object before dot” rule).

Usually, static methods are used to implement functions that belong to the class, but not to any particular object of it.

For instance, we have Article objects and need a function to compare them. The natural choice would be Article.compare, like this:

class Article {
  constructor(title, date) {
    this.title = title;
    this.date = date;
  }

  static compare(articleA, articleB) {
    return articleA.date - articleB.date;
  }
}

// usage
let articles = [
  new Article("HTML", new Date(2019, 1, 1)),
  new Article("CSS", new Date(2019, 0, 1)),
  new Article("JavaScript", new Date(2019, 11, 1))
];

articles.sort(Article.compare);

alert( articles[0].title ); // CSS

Here Article.compare stands “over” the articles, as a means to compare them. It’s not a method of an article, but rather of the whole class.

Another example would be a so-called “factory” method. Imagine, we need few ways to create an article:

  1. Create by given parameters (title, date etc).
  2. Create an empty article with today’s date.

The first way can be implemented by the constructor. And for the second one we can make a static method of the class.

Like Article.createTodays() here:

class Article {
  constructor(title, date) {
    this.title = title;
    this.date = date;
  }

  static createTodays() {
    // remember, this = Article
    return new this("Today's digest", new Date());
  }
}

let article = Article.createTodays();

alert( article.title ); // Todays digest

Now every time we need to create a today’s digest, we can call Article.createTodays(). Once again, that’s not a method of an article, but a method of the whole class.

Static methods are also used in database-related classes to search/save/remove entries from the database, like this:

// assuming Article is a special class for managing articles
// static method to remove the article:
Article.remove({id: 12345});

Static properties

A recent addition
This is a recent addition to the language. Examples work in the recent Chrome.

Static properties are also possible, just like regular class properties:

class Article {
  static publisher = "Ilya Kantor";
}

alert( Article.publisher ); // Ilya Kantor

That is the same as a direct assignment to Article:

Article.publisher = "Ilya Kantor";

Statics and inheritance

Statics are inherited, we can access Parent.method as Child.method.

For instance, Animal.compare in the code below is inherited and accessible as Rabbit.compare:

class Animal {

  constructor(name, speed) {
    this.speed = speed;
    this.name = name;
  }

  run(speed = 0) {
    this.speed += speed;
    alert(`${this.name} runs with speed ${this.speed}.`);
  }

  static compare(animalA, animalB) {
    return animalA.speed - animalB.speed;
  }

}

// Inherit from Animal
class Rabbit extends Animal {
  hide() {
    alert(`${this.name} hides!`);
  }
}

let rabbits = [
  new Rabbit("White Rabbit", 10),
  new Rabbit("Black Rabbit", 5)
];

rabbits.sort(Rabbit.compare);

rabbits[0].run(); // Black Rabbit runs with speed 5.

Now we can call Rabbit.compare assuming that the inherited Animal.compare will be called.

How does it work? Again, using prototypes. As you might have already guessed, extends gives Rabbit the [[Prototype]] reference to Animal.

So, Rabbit function now inherits from Animal function. And Animal function normally has [[Prototype]] referencing Function.prototype, because it doesn’t extend anything.

Here, let’s check that:

class Animal {}
class Rabbit extends Animal {}

// for static properties and methods
alert(Rabbit.__proto__ === Animal); // true

// the next step up leads to Function.prototype
alert(Animal.__proto__ === Function.prototype); // true

// the "normal" prototype chain for object methods
alert(Rabbit.prototype.__proto__ === Animal.prototype);

This way Rabbit has access to all static methods of Animal.

Summary

Static methods are used for the functionality that doesn’t relate to a concrete class instance, doesn’t require an instance to exist, but rather belongs to the class as a whole, like Article.compare – a generic method to compare two articles.

Static properties are used when we’d like to store class-level data, also not bound to an instance.

The syntax is:

class MyClass {
  static property = ...;

  static method() {
    ...
  }
}

That’s technically the same as assigning to the class itself:

MyClass.property = ...
MyClass.method = ...

Static properties are inherited.

Technically, for class B extends A the prototype of the class B itself points to A: B.[[Prototype]] = A. So if a field is not found in B, the search continues in A.

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