Classes

The “class” construct allows to define prototype-based classes with a clean, nice-looking syntax.

The “class” syntax

The class syntax is versatile, we’ll start with a simple example first.

Here’s a prototype-based class User:

function User(name) {
  this.name = name;
}

User.prototype.sayHi = function() {
  alert(this.name);
}

let user = new User("John");
user.sayHi();

…And that’s the same using class syntax:

class User {

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

  sayHi() {
    alert(this.name);
  }

}

let user = new User("John");
user.sayHi();

It’s easy to see that the two examples are alike. Just please note that methods in a class do not have a comma between them. Novice developers sometimes forget it and put a comma between class methods, and things don’t work. That’s not a literal object, but a class syntax.

So, what exactly does class do? We may think that it defines a new language-level entity, but that would be wrong.

The class User {...} here actually does two things:

  1. Declares a variable User that references the function named "constructor".
  2. Puts into User.prototype methods listed in the definition. Here it includes sayHi and the constructor.

Here’s the code to dig into the class and see that:

class User {
  constructor(name) { this.name = name; }
  sayHi() { alert(this.name); }
}

// proof: User is the "constructor" function
alert(User === User.prototype.constructor); // true

// proof: there are two methods in its "prototype"
alert(Object.getOwnPropertyNames(User.prototype)); // constructor, sayHi

Here’s the illustration of what class User creates:

So class is a special syntax to define a constructor together with its prototype methods.

…But not only that. There are minor tweaks here and there:

Constructors require new
Unlike a regular function, a class constructor can’t be called without new:
class User {
  constructor() {}
}

alert(typeof User); // function
User(); // Error: Class constructor User cannot be invoked without 'new'
Different string output
If we output it like alert(User), some engines show "class User...", while others show "function User...".

Please don’t be confused: the string representation may vary, but that’s still a function, there is no separate “class” entity in JavaScript language.

Class methods are non-enumerable
A class definition sets enumerable flag to false for all methods in the "prototype". That’s good, because if we for..in over an object, we usually don’t want its class methods.
Classes have a default constructor() {}
If there’s no constructor in the class construct, then an empty function is generated, same as if we had written constructor() {}.
Classes always use strict
All code inside the class construct is automatically in strict mode.

Getters/setters

Classes may also include getters/setters. Here’s an example with user.name implemented using them:

class User {

  constructor(name) {
    // invokes the setter
    this.name = name;
  }

  get name() {
    return this._name;
  }

  set name(value) {
    if (value.length < 4) {
      alert("Name is too short.");
      return;
    }
    this._name = value;
  }

}

let user = new User("John");
alert(user.name); // John

user = new User(""); // Name too short.

Internally, getters and setters are also created on the User prototype, like this:

Object.defineProperties(User.prototype, {
  name: {
    get() {
      return this._name
    },
    set(name) {
      // ...
    }
  }
});

Only methods

Unlike object literals, no property:value assignments are allowed inside class. There may be only methods and getters/setters. There is some work going on in the specification to lift that limitation, but it’s not yet there.

If we really need to put a non-function value into the prototype, then we can alter prototype manually, like this:

class User { }

User.prototype.test = 5;

alert( new User().test ); // 5

So, technically that’s possible, but we should know why we’re doing it. Such properties will be shared among all objects of the class.

An “in-class” alternative is to use a getter:

class User {
  get test() {
    return 5;
  }
}

alert( new User().test ); // 5

From the external code, the usage is the same. But the getter variant is a bit slower.

Class Expression

Just like functions, classes can be defined inside another expression, passed around, returned etc.

Here’s a class-returning function (“class factory”):

function makeClass(phrase) {
  // declare a class and return it
  return class {
    sayHi() {
      alert(phrase);
    };
  };
}

let User = makeClass("Hello");

new User().sayHi(); // Hello

That’s quite normal if we recall that class is just a special form of a function-with-prototype definition.

And, like Named Function Expressions, such classes also may have a name, that is visible inside that class only:

// "Named Class Expression" (alas, no such term, but that's what's going on)
let User = class MyClass {
  sayHi() {
    alert(MyClass); // MyClass is visible only inside the class
  }
};

new User().sayHi(); // works, shows MyClass definition

alert(MyClass); // error, MyClass not visible outside of the class

Static methods

We can also assign methods 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 function property:

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:

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(2016, 1, 1)),
  new Article("Body", new Date(2016, 0, 1)),
  new Article("JavaScript", new Date(2016, 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.

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});

Summary

The basic class syntax looks like this:

class MyClass {
  constructor(...) {
    // ...
  }
  method1(...) {}
  method2(...) {}
  get something(...) {}
  set something(...) {}
  static staticMethod(..) {}
  // ...
}

The value of MyClass is a function provided as constructor. If there’s no constructor, then an empty function.

In any case, methods listed in the class declaration become members of its prototype, with the exception of static methods that are written into the function itself and callable as MyClass.staticMethod(). Static methods are used when we need a function bound to a class, but not to any object of that class.

In the next chapter we’ll learn more about classes, including inheritance.

Tasks

importance: 5

Rewrite the Clock class from prototypes to the modern “class” syntax.

P.S. The clock ticks in the console, open it to see.

Open the sandbox for the task.

Tutorial map

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