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The class declaration creates a new class with a given name using prototype-based inheritance.

You can also define a class using a class expression. But unlike the class expression, the class declaration doesn't allow an existing class to be declared again and will throw a type error if attempted. 

Syntax

class name [extends] {
  // class body
}

Description

Just like with class expressions, the class body of a class declaration is executed in strict mode. The constructor property is optional.

Class declarations are not hoisted (unlike function declarations).

Examples

A simple class declaration

In the following example, we first define a class named Polygon, then extend it to create a class named Square. Note that super(), used in the constructor, can only be used in constructors and must be called before the this keyword can be used.

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

class Square extends Polygon {
  constructor(length) {
    super(length, length);
    this.name = 'Square';
  }
}

Attempting to declare a class twice

Re-declaring a class using the class declaration throws a type error.

class Foo {};
class Foo {}; // Uncaught TypeError: Identifier 'Foo' has already been declared

The same error is thrown when a class has been defined before using the class expression.

var Foo = class {};
class Foo {}; // Uncaught TypeError: Identifier 'Foo' has already been declared

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Class definitions' in that specification.
Standard Initial definition.
ECMAScript 2016 (ECMA-262)
The definition of 'Class definitions' in that specification.
Standard  
ECMAScript 2017 (ECMA-262)
The definition of 'Class definitions' in that specification.
Standard  
ECMAScript Latest Draft (ECMA-262)
The definition of 'Class definitions' in that specification.
Living Standard  

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support42 Yes45 No ?10.1
Array subclassing43 ? ? No ? ?
Allowed in sloppy mode49 ? ? No ? ?
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic support4242 Yes45 No ?10.1
Array subclassing4343 ? ? No ? ?
Allowed in sloppy mode4949 ? ? No ? ?

See also

ডকুমেন্ট ট্যাগ এবং অবদানকারী

 সর্বশেষ হালনাগাদ করেছেন: indiamos,