Test functionality with timeouts in react-testing-library and jest

Question

I'm trying to follow TDD, and I have a span that should appear on screen after 5 seconds. I haven't implemented the span at all, so the test should fail, but currently it passes the test expect(messageSpan).toBeInTheDocument.

Here are my two tests:

it("doesn't show cta message at first", () => {
    render(<FAB />);
    const messageSpan = screen.queryByText(
      "Considering a career in nursing? Join our team!"
    );
    expect(messageSpan).toBeNull(); // passes, as it should
  });

  it("should show the cta message after 5 secs", () => {
    render(<FAB />);
    setTimeout(() => {
      const messageSpan = screen.getByText( // also tried queryByText instead of get
        "Considering a career in nursing? Join our team!"
      );
      expect(messageSpan).toBeInTheDocument(); // also passes, even though messageSpan should throw an error.
    }, 5000);
  });

Here's my FAB component, where you can see there's no message at all:

export default function FAB() {
  return (
// using styled-components; there's no content in any of these.
    <StyledFABContainer>
      <StyledFABButton>
        <BriefcaseIcon />
      </StyledFABButton>
    </StyledFABContainer>
  );
}

To complicate things, I don't plan to have a set function I call for the setTimeout. I will simply set state after a set time of 5 secs. So don't think I can use the suggestions in the timer mocks section of jest docs: https://jestjs.io/docs/timer-mocks

My two questions are:

a) Why would this pass and not throw an error/null? b) How can I properly test setTimeout functionality in RTL?

UPDATE: Have tried using various combinations of useFakeTimers, act, waitFor etc., but no luck. Here's my current test as it's written out, and throwing two errors - one saying I need to use act when changing state (which I am, but still) and one saying my messageSpan is null:

it("cta message to display after 5 secs", async () => {
    jest.useFakeTimers();
    const el = document.createElement("div");
    act(() => {
      ReactDOM.render(<FAB />, el);
    });
    jest.advanceTimersByTime(5000);
    const messageSpan = screen.queryByText(
      "Considering a career in nursing? Join our team!"
    );
    expect(messageSpan).toBeInTheDocument();
  });

Answer 1

You can not use setTimeout like this in your tests. An obvious reason would be that you do not want to wait 5 seconds in your test to then continue. Imagine a component that would change after 10min. You cant wait that long but should use jests mocked timers API instead. You can progress the nodejs timer so your component changes, then immediately make your assertion. Other remarks about the tests you wrote:

• If you are awaiting changes you should use await waitFor(() => ...) from testing library. It checks the expectations to pass every 50ms by default for a total of 5sec. If all expectations pass it continues. You should make asynchronous assertions there.
• "expect(messageSpan).toBeNull()" should be "expect(messageSpan).not.toBeInTheDocument()". Its good to go with the accessibility way testing library provides.