ladybird/Libraries/LibJS/Tests/builtins/Proxy/Proxy.handler-preventExtensions.js

describe("[[PreventExtension]] trap normal behavior", () => {
  test("forwarding when not defined in handler", () => {
    let p = new Proxy({}, { preventExtensions: null });
    expect(Object.preventExtensions(p)).toBe(p);
    p = new Proxy({}, { preventExtensions: undefined });
    expect(Object.preventExtensions(p)).toBe(p);
    p = new Proxy({}, {});
    expect(Object.preventExtensions(p)).toBe(p);
  });

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

    Object.preventExtensions(o);
    Object.preventExtensions(p);
  });
});

describe("[[PreventExtensions]] invariants", () => {
  test("cannot return false", () => {
    let p = new Proxy(
      {},
      {
        preventExtensions() {
          return false;
        },
      }
    );

    expect(() => {
      Object.preventExtensions(p);
    }).toThrowWithMessage(TypeError, "Object's [[PreventExtensions]] method returned false");
  });

  test("cannot return true if the target is extensible", () => {
    let o = {};
    let p = new Proxy(o, {
      preventExtensions() {
        return true;
      },
    });

    expect(() => {
      Object.preventExtensions(p);
    }).toThrowWithMessage(
      TypeError,
      "Proxy handler's preventExtensions trap violates invariant: cannot return true if the target object is extensible"
    );

    Object.preventExtensions(o);
    expect(Object.preventExtensions(p)).toBe(p);
  });
});