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
classkeyword 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:const 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
let Rectangle = class {
constructor(height, width) {
this.height = height;
this.width = width;
}
};
console.log(Rectangle.name);
// output: "Rectangle"
// named
let Rectangle = class Rectangle2 {
constructor(height, width) {
this.height = height;
this.width = width;
}
};
console.log(Rectangle.name);
// output: "Rectangle2"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 body of a class is executed in strict mode, i.e., code written here is subject to stricter syntax for increased performance, some otherwise silent errors will be thrown, and certain keywords are reserved for future versions of ECMAScript.
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); // 100Static 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)); // 7.0710678118654755Boxing 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 body's syntactic boundary 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 initial 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 objectInstance 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;Field declarations
Public field declarations
With the JavaScript field declaration syntax, the above example can be written as:
class Rectangle {
height = 0;
width;
constructor(height, width) {
this.height = height;
this.width = width;
}
}
By declaring fields up-front, class definitions become more self-documenting, and the fields are always present.
As seen above, the fields can be declared with or without a default value.
Private field declarations
Using private fields, the definition can be refined as below.
class Rectangle {
#height = 0;
#width;
constructor(height, width) {
this.#height = height;
this.#width = width;
}
}
It's an error to reference private fields from outside of the class; they can only be read or written within the class body. By defining things which are not visible outside of the class, you ensure that your classes' users can't depend on internals, which may change version to version.
Private fields cannot be created later through assigning to them, the way that normal properties can.
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 {
constructor(name) {
super(name); // call the super class constructor and pass in the name parameter
}
speak() {
console.log(this.name + ' barks.');
}
}
let d = new Dog('Mitzie');
d.speak(); // Mitzie barks.
If there is a constructor present in the 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.');
}
}
let 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():const 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);
let 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; }
}
let a = new MyArray(1,2,3);
let 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. This is one advantage over prototype-based inheritance.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.`);
}
}
let 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:
let calculatorMixin = Base => class extends Base {
calc() { }
};
let 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)) { }
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