Sleep

Write delay(ms, value), the classic sleep helper. It returns a promise that fulfills with value once ms milliseconds have passed. Without a value, it fulfills with undefined. It never rejects.

Even delay(0) waits for a timer, like setTimeout(fn, 0): the promise doesn't fulfill until that timer fires.

async function countdown() {
  console.log(3);
  await delay(1000);
  console.log(2);
  const word = await delay(1000, "go");
  console.log(word); // "go", two seconds after 3
}

What the tests check

1. 1returns a promise

   useFakeTimers();
   expect(delay(100)).toBeInstanceOf(Promise);
   

2. 2doesn't fulfill before ms have passed

   const clock = useFakeTimers();
   let done = false;
   delay(100).then(() => {
     done = true;
   });
   await clock.tickAsync(99);
   expect(done).toBe(false);
   

3. 3fulfills once ms have passed

   const clock = useFakeTimers();
   let done = false;
   delay(100).then(() => {
     done = true;
   });
   await clock.tickAsync(100);
   expect(done).toBe(true);
   

4. 4fulfills with the value it was given

   const clock = useFakeTimers();
   let result;
   delay(50, "ready").then((value) => {
     result = value;
   });
   await clock.tickAsync(50);
   expect(result).toBe("ready");
   

5. 5fulfills with undefined when there's no value

   const clock = useFakeTimers();
   let result = "not yet";
   delay(10).then((value) => {
     result = value;
   });
   await clock.tickAsync(10);
   expect(result).toBeUndefined();
   

6. 6several delays finish in order of their times

   const clock = useFakeTimers();
   const order = [];
   delay(300, "c").then((value) => order.push(value));
   delay(100, "a").then((value) => order.push(value));
   delay(200, "b").then((value) => order.push(value));
   await clock.tickAsync(300);
   expect(order).toEqual(["a", "b", "c"]);
   

7. 7delay(0) still waits for a timer

   const clock = useFakeTimers();
   let done = false;
   delay(0).then(() => {
     done = true;
   });
   await flush();
   expect(done).toBe(false);
   await clock.tickAsync(0);
   expect(done).toBe(true);
   

Learn more

• PromisesLesson
• Timers: setTimeout & setIntervalLesson