class expression

The class expression is one way to define a class in ECMAScript 2015. Similar to function expressions, class expressions can be named or unnamed. If named, the name of the class is local to the class body only. JavaScript classes use prototype-based inheritance.

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Syntax

var MyClass = class [className] [extends] {
  // class body
};

Description

A class expression has a similar syntax to a class statement (declaration). However, with class expressions, you are able to omit the class name ("binding identifier"), which you can't with class statements. Additionally, class expressions allow you to redefine/re-declare classes and don't throw any type errors like class declaration. The constructor property is optional. And, typeof the classes generated using this keyword will always be "function".

Just like with class statements, the class body of class expressions is executed in strict mode.

'use strict';
var Foo = class {}; // constructor property is optional
var Foo = class {}; // Re-declaration is allowed

typeof Foo; //returns "function"
typeof class {}; //returns "function"

Foo instanceof Object; // true
Foo instanceof Function; // true
class Foo {}; // Throws TypeError, doesn't allow re-declaration

Examples

A simple class expression

This is just a simple anonymous class expression which you can refer to using the variable "Foo".

var Foo = class {
  constructor() {}
  bar() {
    return 'Hello World!';
  }
};

var instance = new Foo();
instance.bar(); // "Hello World!"
Foo.name; // "Foo"

Named class expressions

If you want to refer to the current class inside the class body, you can create a named class expression. This name is only visible in the scope of the class expression itself.

var Foo = class NamedFoo {
  constructor() {}
  whoIsThere() {
    return NamedFoo.name;
  }
}
var bar = new Foo();
bar.whoIsThere(); // "NamedFoo"
NamedFoo.name; // ReferenceError: NamedFoo is not defined
Foo.name; // "NamedFoo"

Specifications

Browser compatibility

See also

• function expression
• class statement
• Classes