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

describe("[[Set]] trap normal behavior", () => {
  test("forwarding when not defined in handler", () => {
    expect((new Proxy({}, { set: undefined }).foo = 1)).toBe(1);
    expect((new Proxy({}, { set: null }).foo = 1)).toBe(1);
    expect((new Proxy({}, {}).foo = 1)).toBe(1);
  });

  test("correct arguments supplied to trap", () => {
    let o = {};
    let p = new Proxy(o, {
      set(target, prop, value, receiver) {
        expect(target).toBe(o);
        expect(prop).toBe("foo");
        expect(value).toBe(10);
        expect(receiver).toBe(p);
        return true;
      },
    });

    p.foo = 10;
  });

  test("conditional return value", () => {
    let p = new Proxy(
      {},
      {
        set(target, prop, value) {
          if (target[prop] === value) {
            target[prop] *= 2;
          } else {
            target[prop] = value;
          }
        },
      }
    );

    p.foo = 10;
    expect(p.foo).toBe(10);
    p.foo = 10;
    expect(p.foo).toBe(20);
    p.foo = 10;
    expect(p.foo).toBe(10);
  });
});

describe("[[Set]] invariants", () => {
  test("cannot return true for a non-configurable, non-writable property", () => {
    let o = {};
    Object.defineProperty(o, "foo", { value: 10 });

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

    expect(() => {
      p.foo = 12;
    }).toThrowWithMessage(
      TypeError,
      "Proxy handler's set trap violates invariant: cannot return true for a property on the target which is a non-configurable, non-writable own data property"
    );
  });

  test("cannot return true for a non-configurable accessor property with no setter", () => {
    let o = {};
    Object.defineProperty(o, "foo", { get() {} });

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

    expect(() => {
      p.foo = 12;
    }).toThrowWithMessage(
      TypeError,
      "Proxy handler's set trap violates invariant: cannot return true for a property on the target which is a non-configurable own accessor property with an undefined set attribute"
    );
  });
});