class

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).

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) {
    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;
  }
}

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.

• function declaration
• class expression
• Classes