Static properties and methods

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

In a class, they are prepended by static keyword, like this:

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

User.staticMethod(); // true

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

class User() { }

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

User.staticMethod(); // true

The value of this in User.staticMethod() call 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. A natural solution would be to add Article.compare method, 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 “above” 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.
  3. …or else somehow.

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 ); // Today's 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, they look like regular class properties, but prepended by static:

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";

Inheritance of static properties and methods

Static properties and methods are inherited.

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

class Animal {
  static planet = "Earth";

  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.

alert(Rabbit.planet); // Earth

Now when we call Rabbit.compare, 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 extends Animal creates two [[Prototype]] references:

  1. Rabbit function prototypally inherits from Animal function.
  2. Rabbit.prototype prototypally inherits from Animal.prototype.

As a result, inheritance works both for regular and static methods.

Here, let’s check that by code:

class Animal {}
class Rabbit extends Animal {}

// for statics
alert(Rabbit.__proto__ === Animal); // true

// for regular methods
alert(Rabbit.prototype.__proto__ === Animal.prototype); // true

Summary

Static methods are used for the functionality that belongs to the class “as a whole”. It doesn’t relate to a concrete class instance.

For example, a method for comparison Article.compare(article1, article2) or a factory method Article.createTodays().

They are labeled by the word static in class declaration.

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() {
    ...
  }
}

Technically, static declaration is the same as assigning to the class itself:

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

Static properties and methods are inherited.

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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