Jest Timers and Promises in polling.

faketimers-polling-workaround.js

jest.useFakeTimers();

const pollingPromise = poll();
const cancelAdvance = advanceTimersContinuously();
const status = await pollingPromise;
cancelAdvance();

expect(status).toEqual('COMPLETE');

function advanceTimersContinuously() {
  let isCancelled = false;

  async function advance() {
    let mockSetTimeoutFnCallCount = 0;

    while (!isCancelled) {
      // 1. Make sure the setTimeout mock being queued to be called.
      await Promise.resolve();
      const isMockSetTimeoutFnBeingCalled =
        mockSetTimeoutFnCallCount !== setTimeout.mock.calls.length;

      if (isMockSetTimeoutFnBeingCalled) {
        mockSetTimeoutFnCallCount = setTimeout.mock.calls.length;
        
        // 2. Fast forward and exhaust only currently pending timers.
        jest.runOnlyPendingTimers();
        
        // 3. Flush the "PromiseJobs".
        await Promise.resolve();
      }
    }
  }

  advance();

  return () => {
    isCancelled = true;
  };
}