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, which allows redeclarations and omitting class names. Attempting to place class declaration in the same scope, under the same name, will throw a SyntaxError.

Try it


class name [extends otherName] {
  // class body


The class body of a class declaration is executed in strict mode. The constructor method is optional.

Class declarations behave like let and const and are not hoisted (unlike function declarations).


A simple 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.

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

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

Attempting to declare a class twice

Re-declaring a class using the class declaration throws a SyntaxError.

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

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

let Foo = class {};
class Foo {} // Uncaught SyntaxError: Identifier 'Foo' has already been declared

If you're experimenting in a REPL, such as the Firefox web console (Tools > Web Developer > Web Console), and you run two class declarations with the same name in two separate inputs, you may get the same re-declaration error. See further discussion of this issue in Firefox bug 1580891. The Chrome console allows class re-declarations between different REPL inputs.


ECMAScript Language Specification
# sec-class-definitions

Browser compatibility

BCD tables only load in the browser

See also