path: root/files/ar/web/javascript/reference/classes/index.html

---
title: Classes
slug: Web/JavaScript/Reference/Classes
tags:
  - Classes
  - Constructors
  - ECMAScript 2015
  - Inheritance
  - Intermediate
  - JavaScript
  - NeedsTranslation
  - TopicStub
translation_of: Web/JavaScript/Reference/Classes
---
<div>{{JsSidebar("Classes")}}</div>

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

<h2 id="Defining_classes">Defining classes</h2>

<p>Classes are in fact "special <a href="/en-US/docs/Web/JavaScript/Reference/Functions">functions</a>", and just as you can define <a href="/en-US/docs/Web/JavaScript/Reference/Operators/function">function expressions</a> and <a href="/en-US/docs/Web/JavaScript/Reference/Statements/function">function declarations</a>, the class syntax has two components: <a href="/en-US/docs/Web/JavaScript/Reference/Operators/class">class expressions</a> and <a href="/en-US/docs/Web/JavaScript/Reference/Statements/class">class declarations</a>.</p>

<h3 id="Class_declarations">Class declarations</h3>

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

<pre class="brush: js">class Rectangle {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }
}</pre>

<h4 id="Hoisting">Hoisting</h4>

<p>An important difference between <strong>function declarations</strong> and <strong>class declarations</strong> is that function declarations are {{Glossary("Hoisting", "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 {{jsxref("ReferenceError")}}:</p>

<pre class="brush: js example-bad">var p = new Rectangle(); // ReferenceError

class Rectangle {}
</pre>

<h3 id="Class_expressions">Class expressions</h3>

<p>A <strong>class expression</strong> 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) <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/name">.name</a> property, though)</p>

<pre class="brush: js">// 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"
</pre>

<p><strong>Note:</strong> Class <strong>expressions</strong> also suffer from the same hoisting issues mentioned for Class <strong>declarations</strong>.</p>

<h2 id="Class_body_and_method_definitions">Class body and method definitions</h2>

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

<h3 id="Strict_mode">Strict mode</h3>

<p>The bodies of <em>class declarations</em> and <em>class expressions</em> are executed in <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode">strict mode</a> i.e. constructor, static and prototype methods, getter and setter functions are executed in strict mode.</p>

<h3 id="Constructor">Constructor</h3>

<p>The <code><a href="/en-US/docs/Web/JavaScript/Reference/Classes/constructor">constructor</a></code> method is a special method for creating and initializing an object created with a <code>class</code>. There can only be one special method with the name "constructor" in a class. A {{jsxref("SyntaxError")}} will be thrown if the class contains more than one occurrence of a <code>constructor</code> method.</p>

<p>A constructor can use the <code>super</code> keyword to call the constructor of the super class.</p>

<h3 id="Prototype_methods">Prototype methods</h3>

<p>See also <a href="/en-US/docs/Web/JavaScript/Reference/Functions/Method_definitions">method definitions</a>.</p>

<pre class="brush: js">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</pre>

<h3 id="Static_methods">Static methods</h3>

<p>The <code><a href="/en-US/docs/Web/JavaScript/Reference/Classes/static">static</a></code> keyword defines a static method for a class. Static methods are called without <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Introduction_to_Object-Oriented_JavaScript#The_object_(class_instance)" title='An example of class instance is "var john = new Person();"'>instantiating </a>their class and <strong>cannot </strong>be called through a class instance. Static methods are often used to create utility functions for an application.</p>

<pre class="brush: js">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</pre>

<h3 id="Boxing_with_prototype_and_static_methods">Boxing with prototype and static methods</h3>

<p>When a static or prototype method is called without a value for <em>this</em>, the <em>this</em> value will be <font face="consolas, Liberation Mono, courier, monospace"><span style="background-color: rgba(220, 220, 220, 0.498039);">undefined</span></font> inside the method. This behavior will be the same even if the <code>"use strict"</code> directive isn't present, because code within the <code>class</code> syntax is always executed in strict mode.</p>

<pre class="brush: js">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</pre>

<p>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 <em>this</em> value. If the inital value is <code>undefined</code>, <em>this</em> will be set to the global object.</p>

<p>Autoboxing will not happen in strict mode, the <em>this</em> value remains as passed.</p>

<pre class="brush: js">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
</pre>

<h3 id="Instance_properties">Instance properties</h3>

<p>Instance properties must be defined inside of class methods:</p>

<pre class="brush: js">class Rectangle {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }
}</pre>

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

<pre class="brush: js">Rectangle.staticWidth = 20;
Rectangle.prototype.prototypeWidth = 25;
</pre>

<p> </p>

<h2 id="Sub_classing_with_extends">Sub classing with <code>extends</code></h2>

<p>The <code><a href="/en-US/docs/Web/JavaScript/Reference/Classes/extends">extends</a></code> keyword is used in <em>class declarations</em> or <em>class expressions</em> to create a class as a child of another class.</p>

<pre class="brush: js">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.
</pre>

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

<p>One may also extend traditional function-based "classes":</p>

<pre class="brush: js">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.
</pre>

<p>Note that classes cannot extend regular (non-constructible) objects. If you want to inherit from a regular object, you can instead use {{jsxref("Object.setPrototypeOf()")}}:</p>

<pre class="brush: js">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.
</pre>

<h2 id="Species">Species</h2>

<p>You might want to return {{jsxref("Array")}} objects in your derived array class <code>MyArray</code>. The species pattern lets you override default constructors.</p>

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

<pre class="brush: js">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 =&gt; x * x);

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

<h2 id="Super_class_calls_with_super">Super class calls with <code>super</code></h2>

<p>The <code><a href="/en-US/docs/Web/JavaScript/Reference/Operators/super">super</a></code> keyword is used to call corresponding methods of super class.</p>

<pre class="brush: js">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.

</pre>

<h2 id="Mix-ins">Mix-ins</h2>

<p>Abstract subclasses or <em>mix-ins</em> 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.</p>

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

<pre class="brush: js">var calculatorMixin = Base =&gt; class extends Base {
  calc() { }
};

var randomizerMixin = Base =&gt; class extends Base {
  randomize() { }
};
</pre>

<p>A class that uses these mix-ins can then be written like this:</p>

<pre class="brush: js">class Foo { }
class Bar extends calculatorMixin(randomizerMixin(Foo)) { }</pre>

<h2 id="Specifications">Specifications</h2>

<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Status</th>
   <th scope="col">Comment</th>
  </tr>
  <tr>
   <td>{{SpecName('ES2015', '#sec-class-definitions', 'Class definitions')}}</td>
   <td>{{Spec2('ES2015')}}</td>
   <td>Initial definition.</td>
  </tr>
  <tr>
   <td>{{SpecName('ES2016', '#sec-class-definitions', 'Class definitions')}}</td>
   <td>{{Spec2('ES2016')}}</td>
   <td> </td>
  </tr>
  <tr>
   <td>{{SpecName('ES2017', '#sec-class-definitions', 'Class definitions')}}</td>
   <td>{{Spec2('ES2017')}}</td>
   <td> </td>
  </tr>
  <tr>
   <td>{{SpecName('ESDraft', '#sec-class-definitions', 'Class definitions')}}</td>
   <td>{{Spec2('ESDraft')}}</td>
   <td> </td>
  </tr>
 </tbody>
</table>

<h2 id="Browser_compatibility">Browser compatibility</h2>



<p>{{Compat("javascript.classes")}}</p>

<h2 id="Running_in_Scratchpad">Running in Scratchpad</h2>

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

<p>To re-run a definition, use Scratchpad's menu Execute &gt; Reload and Run.<br>
 Please vote for bug <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=1428672">#1428672</a>.</p>

<h2 id="See_also">See also</h2>

<ul>
 <li><a href="/en-US/docs/Web/JavaScript/Reference/Functions">Functions</a></li>
 <li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/class"><code>class</code> declaration</a></li>
 <li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/class"><code>class</code> expression</a></li>
 <li>{{jsxref("Operators/super", "super")}}</li>
 <li><a href="https://hacks.mozilla.org/2015/07/es6-in-depth-classes/">Blog post: "ES6 In Depth: Classes"</a></li>
 <li><a href="https://github.com/tc39/proposal-class-fields">Fields and public/private class properties proposal (stage 3)</a></li>
</ul>