Jest: Timer and Promise Don't Work Well. (Settimeout and Async Function)

Jest: Timer and Promise don't work well. (setTimeout and async function)

Yes, you're on the right track.

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What happens

await simpleTimer(callback) will wait for the Promise returned by simpleTimer() to resolve so callback() gets called the first time and setTimeout() also gets called. jest.useFakeTimers() replaced setTimeout() with a mock so the mock records that it was called with [ () => { simpleTimer(callback) }, 1000 ].

jest.advanceTimersByTime(8000) runs () => { simpleTimer(callback) } (since 1000 < 8000) which calls setTimer(callback) which calls callback() the second time and returns the Promise created by await. setTimeout() does not run a second time since the rest of setTimer(callback) is queued in the PromiseJobs queue and has not had a chance to run.

expect(callback).toHaveBeenCalledTimes(9) fails reporting that callback() was only called twice.

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Additional Information

This is a good question. It draws attention to some unique characteristics of JavaScript and how it works under the hood.

Message Queue

JavaScript uses a message queue. Each message is run to completion before the runtime returns to the queue to retrieve the next message. Functions like setTimeout() add messages to the queue.

Job Queues

ES6 introduces Job Queues and one of the required job queues is PromiseJobs which handles "Jobs that are responses to the settlement of a Promise". Any jobs in this queue run after the current message completes and before the next message begins.
then() queues a job in PromiseJobs when the Promise it is called on resolves.

async / await

async / await is just syntactic sugar over promises and generators. async always returns a Promise and await essentially wraps the rest of the function in a then callback attached to the Promise it is given.

Timer Mocks

Timer Mocks work by replacing functions like setTimeout() with mocks when jest.useFakeTimers() is called. These mocks record the arguments they were called with. Then when jest.advanceTimersByTime() is called a loop runs that synchronously calls any callbacks that would have been scheduled in the elapsed time, including any that get added while running the callbacks.

In other words, setTimeout() normally queues messages that must wait until the current message completes before they can run. Timer Mocks allow the callbacks to be run synchronously within the current message.

Here is an example that demonstrates the above information:

jest.useFakeTimers();

test('execution order', async () => {
  const order = [];
  order.push('1');
  setTimeout(() => { order.push('6'); }, 0);
  const promise = new Promise(resolve => {
    order.push('2');
    resolve();
  }).then(() => {
    order.push('4');
  });
  order.push('3');
  await promise;
  order.push('5');
  jest.advanceTimersByTime(0);
  expect(order).toEqual([ '1', '2', '3', '4', '5', '6' ]);
});

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How to get Timer Mocks and Promises to play nice

Timer Mocks will execute the callbacks synchronously, but those callbacks may cause jobs to be queued in PromiseJobs.

Fortunately it is actually quite easy to let all pending jobs in PromiseJobs run within an async test, all you need to do is call await Promise.resolve(). This will essentially queue the remainder of the test at the end of the PromiseJobs queue and let everything already in the queue run first.

With that in mind, here is a working version of the test:

jest.useFakeTimers() 

it('simpleTimer', async () => {
  async function simpleTimer(callback) {
    await callback();
    setTimeout(() => {
      simpleTimer(callback);
    }, 1000);
  }

  const callback = jest.fn();
  await simpleTimer(callback);
  for(let i = 0; i < 8; i++) {
    jest.advanceTimersByTime(1000);
    await Promise.resolve(); // allow any pending jobs in the PromiseJobs queue to run
  }
  expect(callback).toHaveBeenCalledTimes(9);  // SUCCESS
});

setTimeout with Promise wrapper not working as expected with Jest async/await

You just need to pass mockCallback as the first argument to setTimeout:

const sleep = ms => new Promise(res => setTimeout(res, ms));

it('should call callback after specified delay', async () => {
  const mockCallback = jest.fn();

  setTimeout(mockCallback, 1000);  // <= pass mockCallback as first argument

  expect(mockCallback).not.toHaveBeenCalled();  // Success!

  await sleep(1000);

  expect(mockCallback).toHaveBeenCalled();  // Success!
});

How to properly use Promises and Timers with Jest

So with a follow up comment from @Bergi, I relaized the done wasn't actually necessary either. I just needed to re-order some things. I then ran into a similar issue when testing chains of promises that further highlighted this so I added some cases for that.

jest.useFakeTimers();

describe('timers test', () => {
  it('Using a plain timer works as expected', () => {
    const mock = jest.fn();
    setTimeout(mock, 5000);

    jest.runAllTimers();
    expect(mock).toHaveBeenCalled();
  });

  it('Using a timer to mock a promise resolution', async () => {
    const mock = jest.fn(() => {
      return new Promise((resolve) => setTimeout(resolve, 500));
    });

    const handler = jest.fn();

    const actual = mock().then(handler);
    jest.runAllTimers();
    await actual;

    expect(handler).toHaveBeenCalled();
  });

  it('Using a timer to mock a promise rejection', async () => {
    const mock = jest.fn(() => {
      return new Promise((resolve, reject) => setTimeout(reject, 500));
    });

    const handler = jest.fn();

    const actual = mock().catch(handler);
    jest.runAllTimers();
    await actual;

    expect(handler).toHaveBeenCalled();
  });

  it('Using a timer to mock a promise resolve -> delay -> resolve chain', async () => {
    const mockA = jest.fn(() => {
      return Promise.resolve();
    });

    const mockB = jest.fn(() => {
      return new Promise((resolve, reject) => {
        setTimeout(resolve, 500);
      });
    });

    const handler = jest.fn();

    const actual = mockA()
      .then(() => {
        const mockProm = mockB();
        jest.runAllTimers();
        return mockProm;
      })
      .then(handler);

    jest.runAllTimers();
    await actual;

    expect(mockA).toHaveBeenCalled();
    expect(mockB).toHaveBeenCalled();
    expect(handler).toHaveBeenCalled();
  });

  it('Using a timer to mock a promise resolve -> delay -> reject chain', async () => {
    const mockA = jest.fn(() => {
      return Promise.resolve();
    });

    const mockB = jest.fn(() => {
      return new Promise((resolve, reject) => {
        setTimeout(reject, 500);
      });
    });

    const handler = jest.fn();

    const actual = mockA()
      .then(() => {
        const mockProm = mockB();
        jest.runAllTimers();
        return mockProm;
      })
      .catch(handler);

    await actual;

    expect(mockA).toHaveBeenCalled();
    expect(mockB).toHaveBeenCalled();
    expect(handler).toHaveBeenCalled();
  });
});

---

@Bergi' comment led me to the solution. I ended up making use of the done function, and removing the await. This seems to work at least in this minimal test case.

jest.useFakeTimers();

describe('timers test', () => {
  it('plain timer works as expected', () => {
    const mock = jest.fn();
    setTimeout(mock, 5000);

    jest.runAllTimers();
    expect(mock).toHaveBeenCalled();
  });

  it('Using a timer to mock a promise resolution results in a jest timeout error', async (done) => {
    const mock = jest.fn().mockImplementation(() => {
      return new Promise((resolve) => setTimeout(resolve, 500));
    });

    // make the handler invoke done to replace the await    
    const handler = jest.fn(done);

    mock().then(handler);
    jest.runAllTimers();

    expect(handler).toHaveBeenCalled();
  });

  it('Using a timer to mock a promise rejection results in a jest timeout error', async (done) => {
    const mock = jest.fn().mockImplementation(() => {
      return new Promise((resolve, reject) => setTimeout(reject, 500));
    });

    // make the handler invoke done to replace the await
    const handler = jest.fn(done);

    mock().catch(handler);
    jest.runAllTimers();

    expect(handler).toHaveBeenCalled();
  });
});

Jest fake timers with promises

The current best alternative is to use the async versions of fake-timers. So you would do

await clock.tickAsync(1000); // doesn't wait 1000ms but is async

Instead of calling clock.tick. Please see the answer below for more details.

At the moment, it's not supported

You're not doing anything wrong - it doesn't work at the moment - sorry. The following things have to happen before this will work from our end:

• Jest needs to merge the ongoing work to merge lolex as their fake timer implementation here https://github.com/facebook/jest/pull/5171
• Lolex needs to support pumping through promises - we've discussed this with the V8 team in a recent Node.js collaborator summit. That would expose a hook we'll use to allow doing something like advanceTimeByTime(100) and have that work with promises.

The problem in a gist is that the .then(spy) only gets called later.

As we are volunteers - there is no concrete timeline for these things. I hope SimenB does the merge in the coming 2-3 months and I'll follow up with the hook with the V8 team next month.

What you can do now

You can always write an async test:

// note this is an async function now
it('resolves in a given amount of time', async () => {
  // this is in a promise.reoslve.then to not 'lock' on the await
  Promise.resolve().then(() => jest.advanceTimersByTime(100));
  await timeout(100);
});

You can add expectations after the timeout if there is anything else you want to wait for.

Jest unit test - How do I call through async function in a setTimeout repeating function

So it sounds like I have to...resolve some pending promise here?

Yes, exactly.

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The short answer is that a Promise callback gets queued in PromiseJobs by the then chained to the Promise returned by doAsyncStuff, and the way the test is written, that callback never has a chance to run until the test is already over.

To fix it, give the Promise callbacks a chance to run during your test:

updater.js

export const doAsyncStuff = async () => { };

code.js

import { doAsyncStuff } from './updater';

export function repeatMe() {
  setTimeout(() => {
    doAsyncStuff().then((response) => {
      if (response) {
        console.log("I get here!");
        repeatMe();
      }
    })
  }, 5000);
}

code.test.js

import * as updater from './updater';
import { repeatMe } from './code';

test('repeatMe', async () => {
  jest.useFakeTimers();

  let doAsyncStuff = jest.spyOn(updater, 'doAsyncStuff');
  doAsyncStuff.mockResolvedValue(true);

  repeatMe();

  jest.advanceTimersByTime(5000);
  expect(doAsyncStuff).toHaveBeenCalledTimes(1);  // Success!

  await Promise.resolve();  // let callbacks in PromiseJobs run

  jest.advanceTimersByTime(5000);
  expect(doAsyncStuff).toHaveBeenCalledTimes(2);  // Success!

  await Promise.resolve();  // let callbacks in PromiseJobs run

  jest.advanceTimersByTime(5000);
  expect(doAsyncStuff).toHaveBeenCalledTimes(3);  // Success!

  // ... and so on ...
});

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The complete details of exactly what happens and why can be found in my answer here

How to mock an async function's delay time using Jest

Mock funcA to return a deferred promise, to be resolved later. I know sinon provides a promise helper to cover deferred so jest may include a similar construct. Otherwise here is one of the simple implementations from that answer:

class Deferred {
  constructor() {
    this.promise = new Promise((resolve, reject) => {
      this.reject = reject
      this.resolve = resolve
    })
  }
}

Then the mock is something like:

    const deferred = new Deferred()
    const spyOnFuncA = jest.spyOn(module2, 'funcA').mockImplementation(() => deferred.promise)
    mainFunc() // avoid uncaught exceptions with `.catch`
      .catch(err => expect(err).toBe(null))
    expect(spyOnFuncA).toBeCalled()
    expect(spyOnFuncB).not.toBeCalled()
    await deferred.resolve('whatever')
    expect(spyOnFuncB).toBeCalled()

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