en.javascript.info/1-js/09-classes/03-static-properties-methods/article.md

# 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:

```js run
class User {
*!*
  static staticMethod() {
*/!*
    alert(this === User);
  }
}

User.staticMethod(); // true
```

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

```js
function 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:

```js run
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("Mind", new Date(2019, 1, 1)),
  new Article("Body", new Date(2019, 0, 1)),
  new Article("JavaScript", new Date(2019, 11, 1))
];

*!*
articles.sort(Article.compare);
*/!*

alert( articles[0].title ); // Body
```

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.
3. ...

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:

```js run
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:

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

## Static properties

[recent browser=Chrome]

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

```js run
class Article {
  static publisher = "Ilya Kantor";
}

alert( Article.publisher ); // Ilya Kantor
```

That is the same as a direct assignment to `Article`:

```js
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`:

```js run
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 also gives `Rabbit` the `[[Prototype]]` reference to `Animal`.


![](animal-rabbit-static.png)

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:

```js run
class Animal {}
class Rabbit extends Animal {}

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

// and the next step is Function.prototype
alert(Animal.__proto__ === Function.prototype); // true

// that's in addition to 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:

```js
class MyClass {
  static property = ...;

  static method() {
    ...
  }
}
```

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

```js
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`.