Classes

JavaScript classes, introduced in ECMAScript 2015, are primarily syntactical sugar over JavaScript's existing prototype-based inheritance. The class syntax does not introduce a new object-oriented inheritance model to JavaScript.

Defining classes

Classes are in fact "special functions", and just as you can define function expressions and function declarations, the class syntax has two components: class expressions and class declarations.

Class declarations

One way to define a class is using a class declaration. To declare a class, you use the class keyword with the name of the class ("Rectangle" here).

class Rectangle {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }
}

Hoisting

An important difference between function declarations and class declarations is that function declarations are hoisted and class declarations are not. You first need to declare your class and then access it, otherwise code like the following will throw a ReferenceError:

var p = new Rectangle(); // ReferenceError

class Rectangle {}

Class expressions

A class expression is another way to define a class. Class expressions can be named or unnamed. The name given to a named class expression is local to the class's body. (it can be retrieved through the class's (not an instance's) .name property, though)

// unnamed
var Rectangle = class {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }
};
console.log(Rectangle.name);
// output: "Rectangle"

// named
var Rectangle = class Rectangle2 {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }
};
console.log(Rectangle.name);
// output: "Rectangle2"

Note: Class expressions also suffer from the same hoisting issues mentioned for Class declarations.

Class body and method definitions

The body of a class is the part that is in curly brackets {}. This is where you define class members, such as methods or constructor.

Strict mode

The bodies of class declarations and class expressions are executed in strict mode i.e. constructor, static and prototype methods, getter and setter functions are executed in strict mode.

Constructor

The constructor method is a special method for creating and initializing an object created with a class. There can only be one special method with the name "constructor" in a class. A SyntaxError will be thrown if the class contains more than one occurrence of a constructor method.

A constructor can use the super keyword to call the constructor of the super class.

Prototype methods

See also method definitions.

class Rectangle {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }
  // Getter
  get area() {
    return this.calcArea();
  }
  // Method
  calcArea() {
    return this.height * this.width;
  }
}

const square = new Rectangle(10, 10);

console.log(square.area); // 100

Static methods

The static keyword defines a static method for a class. Static methods are called without instantiating their class and cannot be called through a class instance. Static methods are often used to create utility functions for an application.

class Point {
  constructor(x, y) {
    this.x = x;
    this.y = y;
  }

  static distance(a, b) {
    const dx = a.x - b.x;
    const dy = a.y - b.y;

    return Math.hypot(dx, dy);
  }
}

const p1 = new Point(5, 5);
const p2 = new Point(10, 10);

console.log(Point.distance(p1, p2)); // 7.0710678118654755

Boxing with prototype and static methods

When a static or prototype method is called without a value for this, the this value will be undefined inside the method. This behavior will be the same even if the "use strict" directive isn't present, because code within the class syntax is always executed in strict mode.

class Animal { 
  speak() {
    return this;
  }
  static eat() {
    return this;
  }
}

let obj = new Animal();
obj.speak(); // Animal {}
let speak = obj.speak;
speak(); // undefined

Animal.eat() // class Animal
let eat = Animal.eat;
eat(); // undefined

If the above is written using traditional function–based syntax, then autoboxing in method calls will happen in non–strict mode based on the initial this value. If the inital value is undefined, this will be set to the global object.

Autoboxing will not happen in strict mode, the this value remains as passed.

function Animal() { }

Animal.prototype.speak = function() {
  return this;
}

Animal.eat = function() {
  return this;
}

let obj = new Animal();
let speak = obj.speak;
speak(); // global object

let eat = Animal.eat;
eat(); // global object

Instance properties

Instance properties must be defined inside of class methods:

class Rectangle {
  constructor(height, width) {    
    this.height = height;
    this.width = width;
  }
}

Static class-side properties and prototype data properties must be defined outside of the ClassBody declaration:

Rectangle.staticWidth = 20;
Rectangle.prototype.prototypeWidth = 25;

Sub classing with extends

The extends keyword is used in class declarations or class expressions to create a class as a child of another class.

class Animal { 
  constructor(name) {
    this.name = name;
  }
  
  speak() {
    console.log(this.name + ' makes a noise.');
  }
}

class Dog extends Animal {
  speak() {
    console.log(this.name + ' barks.');
  }
}

var d = new Dog('Mitzie');
d.speak(); // Mitzie barks.

If there is a constructor present in subclass, it needs to first call super() before using "this".

One may also extend traditional function-based "classes":

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

Animal.prototype.speak = function () {
  console.log(this.name + ' makes a noise.');
}

class Dog extends Animal {
  speak() {
    console.log(this.name + ' barks.');
  }
}

var d = new Dog('Mitzie');
d.speak(); // Mitzie barks.

Note that classes cannot extend regular (non-constructible) objects. If you want to inherit from a regular object, you can instead use Object.setPrototypeOf():

var Animal = {
  speak() {
    console.log(this.name + ' makes a noise.');
  }
};

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

// If you do not do this you will get a TypeError when you invoke speak
Object.setPrototypeOf(Dog.prototype, Animal);

var d = new Dog('Mitzie');
d.speak(); // Mitzie makes a noise.

Species

You might want to return Array objects in your derived array class MyArray. The species pattern lets you override default constructors.

For example, when using methods such as map() that returns the default constructor, you want these methods to return a parent Array object, instead of the MyArray object. The Symbol.species symbol lets you do this:

class MyArray extends Array {
  // Overwrite species to the parent Array constructor
  static get [Symbol.species]() { return Array; }
}

var a = new MyArray(1,2,3);
var mapped = a.map(x => x * x);

console.log(mapped instanceof MyArray); // false
console.log(mapped instanceof Array);   // true

Super class calls with super

The super keyword is used to call corresponding methods of super class.

class Cat { 
  constructor(name) {
    this.name = name;
  }
  
  speak() {
    console.log(this.name + ' makes a noise.');
  }
}

class Lion extends Cat {
  speak() {
    super.speak();
    console.log(this.name + ' roars.');
  }
}

var l = new Lion('Fuzzy');
l.speak(); 
// Fuzzy makes a noise.
// Fuzzy roars.

Mix-ins

Abstract subclasses or mix-ins are templates for classes. An ECMAScript class can only have a single superclass, so multiple inheritance from tooling classes, for example, is not possible. The functionality must be provided by the superclass.

A function with a superclass as input and a subclass extending that superclass as output can be used to implement mix-ins in ECMAScript:

var calculatorMixin = Base => class extends Base {
  calc() { }
};

var randomizerMixin = Base => class extends Base {
  randomize() { }
};

A class that uses these mix-ins can then be written like this:

class Foo { }
class Bar extends calculatorMixin(randomizerMixin(Foo)) { }

Specifications

Browser compatibility

Running in Scratchpad

A class can't be redefined. If you're playing with code in Scratchpad (Firefox menu Tools > Web Developer > Scratchpad) and you 'Run' a definition of a class with the same name twice, you'll get a confusing SyntaxError: redeclaration of let <class-name>.

To re-run a definition, use Scratchpad's menu Execute > Reload and Run.
Please vote for bug #1428672.

See also

• Functions
• class declaration
• class expression
• super
• Blog post: "ES6 In Depth: Classes"
• Fields and public/private class properties proposal (stage 3)