Jest unit test: setTimeout not firing in async test

Question

I'm trying to understand how asynchronous testing works in Jest.

What I'm trying to do is similar to an example from the Jest documentation. This works fine ..

function doAsync(c) {
  c(true)
}

test('doAsync calls both callbacks', () => {

  expect.assertions(2);

  function callback1(data) {
    expect(data).toBeTruthy();
  }

  function callback2(data) {
    expect(data).toBeTruthy();
  }

  doAsync(callback1);
  doAsync(callback2);
});

But I want to delay the callback invocations so I tried this ....

 function doAsync(c) {
    setTimeout(() => {
      console.log('timeout fired')
      c(true)
    }, 1000)
  }

but the test fails with the message Expected two assertions to be called but received zero assertion calls..

The log message 'timeout fired' doesn't appear in the console.

Please can someone explain why it fails?

Answer 1

You need to use jest's timer mocks https://jestjs.io/docs/en/timer-mocks

First you tell jest to use mock timers, then you run the timers within your test.

It would look something like:

function doAsync(c) {
  setTimeout(() => {
      c(true)
    }, 1000)
}

jest.useFakeTimers()

test('doAsync calls both callbacks', () => {

  expect.assertions(2);

  function callback1(data) {
    expect(data).toBeTruthy();
  }

  function callback2(data) {
    expect(data).toBeTruthy();
  }

  doAsync(callback1);
  doAsync(callback2);

  jest.runAllTimers(); // or jest.advanceTimersByTime(1000)
});

Answer 2

Use of jest.runAllTimers(); can lead to below error:

Ran 100000 timers, and there are still more! Assuming we've hit an infinite recursion and bailing out...

After going through the JEST timer mocks documentation, it occurred to me that the setTimeout goes into infinite recursion as mentioned in the docs. It is recommended to use jest.runOnlyPendingTimers() and this solves the infinite recursion error.

Fast forward and exhaust only currently pending timers (but not any new timers that get created during that process)

jest.runOnlyPendingTimers();