Classes

JavaScript classes are introduced in ECMAScript 6 and are syntactical sugar over JavaScript's existing prototype-based inheritance. The class syntax is not introducing a new object-oriented inheritance model to JavaScript. JS classes provide a much simpler and clearer syntax to create objects and dealing with inheritance.

Defining classes

Classes are in fact functions, and just like you can define function expressions and function declarations, the class syntax has the two opponents:

• class expressions and
• class declarations.

Class declarations

One way to define a class is a class declaration. For that, you need a class keyword and a name of the class ("Polygon" here).

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

Hoisting

A 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 Polygon(); // ReferenceError

class Polygon {}

Class expressions

A class expression is another way to define a class and it can be named or unnamed. If named, the name of the class is local the class body only.

// unnamed
var Polygon = class {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }
};

// named
var Polygon = class Polygon {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }
};

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 constructors.

Strict mode

The body of class declarations and class expressions is 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 a parent class.

Prototype methods

See also method definitions.

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Static methods

The static keyword defines a static method for a class. Static methods are called without instantiating their class nor are they callable when the class is instantiated. Static methods are often used to create utility functions for an application.

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Sub classing with extends

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

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Sub classing built-in objects

TBD

Super class calls with super

The super keyword is used to call functions on an object's parent.

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ES5 inheritance syntax and ES6 classes syntax compared

TBD

Examples

TBD

Specifications

Browser compatibility

See also

• Functions
• class declaration
• class expression
• super