Unit Testing & Mocking Asynchronous Code

1class FakeTimerHarness {
2  constructor() {
3    this.currentTime = 0;
4    this.queue = [];
5  }
6
7  setTimeout(fn, delay) {
8    const id = this.queue.length + 1;
9    this.queue.push({ id, runAt: this.currentTime + delay, fn });
10    return id;
11  }
12
13  clearTimeout(id) {
14    this.queue = this.queue.filter((item) => item.id !== id);
15  }
16
17  advanceTime(ms) {
18    this.currentTime += ms;
19    this.queue.sort((a, b) => a.runAt - b.runAt);
20    while (this.queue.length && this.queue[0].runAt <= this.currentTime) {
21      const task = this.queue.shift();
22      task.fn();
23    }
24  }
25}
26
27function createMockFn() {
28  const calls = [];
29  const fn = (...args) => {
30    calls.push(args);
31    return 'mocked-result';
32  };
33  fn.calls = calls;
34  return fn;
35}
36
37const clock = new FakeTimerHarness();
38const mockApi = createMockFn();
39const logs = [];
40
41function debouncedFetch(api, delay) {
42  let timerId = null;
43  return (query) => {
44    if (timerId) {
45      clock.clearTimeout(timerId);
46      logs.push('cancelled-timer');
47    }
48    timerId = clock.setTimeout(() => {
49      logs.push('executed:' + query);
50      api(query);
51      timerId = null;
52    }, delay);
53  };
54}
55
56const trigger = debouncedFetch(mockApi, 100);
57
58trigger('apple');
59clock.advanceTime(50);
60trigger('banana');
61clock.advanceTime(100);
62
63const callCount = mockApi.calls.length;
64const executedArg = mockApi.calls[0] ? mockApi.calls[0][0] : 'none';
65
66logs.push('calls:' + callCount);
67logs.push('arg:' + executedArg);
68
69console.log(logs.join(' '));