ladybird/Libraries/LibJS/Tests/operators/in-operator-basic.js

test("in operator with objects", () => {
  const o = { foo: "bar", bar: undefined };
  expect("" in o).toBeFalse();
  expect("foo" in o).toBeTrue();
  expect("bar" in o).toBeTrue();
  expect("baz" in o).toBeFalse();
  expect("toString" in o).toBeTrue();
});

test("in operator with arrays", () => {
  const a = ["hello", "friends"];
  expect(0 in a).toBeTrue();
  expect(1 in a).toBeTrue();
  expect(2 in a).toBeFalse();
  expect("0" in a).toBeTrue();
  expect("hello" in a).toBeFalse();
  expect("friends" in a).toBeFalse();
  expect("length" in a).toBeTrue();
});

test("in operator with string object", () => {
  const s = new String("foo");
  expect("length" in s).toBeTrue();
});

test("error when used with primitives", () => {
  ["foo", 123, null, undefined].forEach(value => {
    expect(() => {
      "prop" in value;
    }).toThrowWithMessage(TypeError, "'in' operator must be used on an object");
  });
});