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.


class name [extends] {
  // class body


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

Class declarations are not hoisted (unlike function declarations).


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) { = 'Polygon';
    this.height = height;
    this.width = width;

class Square extends Polygon {
  constructor(length) {
    super(length, length); = 'Square';

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