class

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since March 2017.

The class declaration creates a binding of a new class to a given name.

You can also define classes using the class expression.

Try it

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

console.log(new Polygon(4, 3).area);
// Expected output: 12

Syntax

js
class name {
  // class body
}
class name extends otherName {
  // class body
}

Description

The class body of a class declaration is executed in strict mode. The class declaration is very similar to let:

class declarations are scoped to blocks as well as functions.
class declarations can only be accessed after the place of declaration is reached (see temporal dead zone). For this reason, class declarations are commonly regarded as non-hoisted (unlike function declarations).
class declarations do not create properties on globalThis when declared at the top level of a script (unlike function declarations).
class declarations cannot be redeclared by any other declaration in the same scope.

Outside the class body, class declarations can be re-assigned like let, but you should avoid doing so. Within the class body, the binding is constant like const.

js
class Foo {
  static {
    Foo = 1; // TypeError: Assignment to constant variable.
  }
}

class Foo2 {
  bar = (Foo2 = 1); // TypeError: Assignment to constant variable.
}

class Foo3 {}
Foo3 = 1;
console.log(Foo3); // 1

Examples

A class declaration

In the following example, we first define a class named Rectangle, then extend it to create a class named FilledRectangle.

Note that super(), used in the constructor, can only be used in constructors, and must be called before the this keyword can be used.

js
class Rectangle {
  constructor(height, width) {
    this.name = "Rectangle";
    this.height = height;
    this.width = width;
  }
}

class FilledRectangle extends Rectangle {
  constructor(height, width, color) {
    super(height, width);
    this.name = "Filled rectangle";
    this.color = color;
  }
}