jest.fn().waitUntilCalledNTimes() Feature

🚀 Feature Proposal

Add some kind to feature to jest to wait until a mock function has been called n times.

Motivation

We have an API that recursively sends periodic notifications to another service, we need to check that this works, we currently have a setTimeout wrapped in a promise to make the test wait for a period thats deffinetly longer than needed.

Example

test('some test', async () => {
  const someMock = jest.fn()

  await someMock.waitUntilCalledNTimes(n) // on the nth call, continue.

  expect(someOtherMock).hasBeenCalled()
})

Pitch

Why does this feature belong in the Jest core platform?

It would be an addition to the core jest mocking ability, allowing for dynamic tests ensuring that a function has to of been called n times before moving on to check for its asssertions

Im happy to give this ago myself, if its something thats wanted

[rickhanlonii]

Yeah that makes sense @benjaminkay93

I think this makes more sense in userland, as you said, it’s pretty niche

Here’s an example of how you could implement, let me know what you think

import { someApiEndpoint } form '../api-manager'
import helper from '../helpers'

const createWaitableMock = () => {
  let resolve;
  let times;
  let calledCount = 0;
  const mock = jest.fn();
  mock.mockImplementation(() => {
    calledCount +=1;
    if (resolve && calledCount >= times) {
      resolve(); 
    }
  });

  
  mock.waitToHaveBeenCalled = t => {
    times = t;
    return new Promise(r => {
      resolve = r;
    })
  }

  return mock;
}

test('some api endpoint test', async () => {
  helper.sendPayload = createWaitableMock();

  const req = {body: [ 'some', 'array' ]}
  const response = someApiEndpoint(req)

  expect(response).toBe('Replay Started!')

  await helper.sendPayload.waitToHaveBeenCalled(2) // explicit number of calls to be made then continue with assertions

  expect(helper.sendPayload).toHaveBeenCalledTimes(2) // I know here redundant but in actual code we have many calls out to things that could be assert to of happened twice
})

[SimenB]

await waitForExpect(() => {
  expect(someMock).toHaveBeenCalledTimes(n);
});

https://www.npmjs.com/package/wait-for-expect

Is that enough?