jest not implemented window.alert()

How I solved this problem is actually define the window.alert method at the top of the test file as a jest spy. This should work for any window method (in my case I was actually testing window.open).

Be sure to call mockClear() in your test, since this is a global object and it's calls will persist across tests.

window.alert = jest.fn();

test("login api resolves true", () => {
  window.alert.mockClear();
  /* ... */
})

The default test environment for Jest is a browser-like environment provided by jsdom.

jsdom implements most of what an actual browser would provide (including the global window object), but it doesn't implement everything.

Specifically for this case, jsdom does not implement window.alert, and instead throws an Error when it is called as can be seen in the source code here.

---

As long as you know why your code is launching the alert and know that your test is working properly aside from the Error then you can suppress the Error by providing an empty implementation for window.alert:

test("login api resolves true", () => {
  const jsdomAlert = window.alert;  // remember the jsdom alert
  window.alert = () => {};  // provide an empty implementation for window.alert
  return expect(AuthManager.login("test", "test")).resolves.toMatchObject(
    expect.objectContaining({
      accessToken: expect.any(String),
      email: expect.any(String),
      expiresIn: expect.any(Number),
      refreshToken: expect.any(String),
      userFullName: expect.any(String),
      userId: expect.any(Number)
    })
  );  // SUCCESS
  window.alert = jsdomAlert;  // restore the jsdom alert
});