describe("[[IsExtensible]] trap normal behavior", () => {
  test("forwarding when not defined in handler", () => {
    expect(Object.isExtensible(new Proxy({}, { isExtensible: null }))).toBeTrue();
    expect(Object.isExtensible(new Proxy({}, { isExtensible: undefined }))).toBeTrue();
    expect(Object.isExtensible(new Proxy({}, {}))).toBeTrue();
  });

  test("correct arguments supplied to trap", () => {
    let o = {};
    let p = new Proxy(o, {
      isExtensible(target) {
        expect(target).toBe(o);
        return true;
      },
    });

    expect(Object.isExtensible(p)).toBeTrue();
  });
});

describe("[[Call]] invariants", () => {
  test("return value must match the target's extensibility", () => {
    let o = {};
    Object.preventExtensions(o);

    let p = new Proxy(o, {
      isExtensible() {
        return true;
      },
    });

    expect(() => {
      Object.isExtensible(p);
    }).toThrowWithMessage(
      TypeError,
      "Proxy handler's isExtensible trap violates invariant: return value must match the target's extensibility"
    );
  });
});