ladybird/Libraries/LibJS/Tests/classes/class-advanced-extends.js

test("extending function", () => {
  class A extends function () {
    this.foo = 10;
  } {}

  expect(new A().foo).toBe(10);
});

test("extending null", () => {
  class A extends null {}

  expect(Object.getPrototypeOf(A.prototype)).toBeNull();

  expect(() => {
    new A();
  }).toThrowWithMessage(ReferenceError, "|this| has not been initialized");
});

test("extending String", () => {
  class MyString extends String {}

  const ms = new MyString("abc");
  expect(ms).toBeInstanceOf(MyString);
  expect(ms).toBeInstanceOf(String);
  expect(ms.charAt(1)).toBe("b");

  class MyString2 extends MyString {
    charAt(i) {
      return `#${super.charAt(i)}`;
    }
  }

  const ms2 = new MyString2("abc");
  expect(ms2.charAt(1)).toBe("#b");
});