我如何用玩笑测试承诺延迟? [英] How do I test promise delays with jest?

问题描述

这是我用来延迟进程的代码(用于退避)

Here's my code which I use to delay process (for backoff)

export function promiseDelay(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

我想测试它，但我不能.我尝试使用 fakeTimers 但我的测试永远不会结束.

I would want to test it but I'm not able to. I tried working with fakeTimers but my test never ends.

test('promiseDelay delays for 1s', async (done) => {
    jest.useFakeTimers();
    Promise.resolve().then(() => jest.advanceTimersByTime(100));
    await promiseDelay(100);
  });

推荐答案

promiseDelay 返回一个在 ms 之后解析的 Promise 所以在 then 中调用一个 spycode> 并测试 spy 是否在不同的时间间隔后被调用:

promiseDelay returns a Promise that resolves after ms so call a spy in then and test to see if the spy has been called after different intervals:

describe('promiseDelay', () => {

  beforeEach(() => { jest.useFakeTimers(); });
  afterEach(() => { jest.useRealTimers(); });

  test('should not resolve until timeout has elapsed', async () => {
    const spy = jest.fn();
    promiseDelay(100).then(spy);  // <= resolve after 100ms

    jest.advanceTimersByTime(20);  // <= advance less than 100ms
    await Promise.resolve();  // let any pending callbacks in PromiseJobs run
    expect(spy).not.toHaveBeenCalled();  // SUCCESS

    jest.advanceTimersByTime(80);  // <= advance the rest of the time
    await Promise.resolve();  // let any pending callbacks in PromiseJobs run
    expect(spy).toHaveBeenCalled();  // SUCCESS
  });

});

注意测试代码是同步的，Timer Mocks make setTimeout 同步但 then 在 PromiseJobs 中排队回调，因此在测试 spy 是否已被调用之前，需要允许任何排队的回调运行.

Note that test code is synchronous and Timer Mocks make setTimeout synchronous but then queues a callback in PromiseJobs so any queued callbacks need to be allowed to run before testing if the spy has been called.

这可以通过使用 async 测试函数并在已解析的 Promise 上调用 await 来完成，这有效地将测试的其余部分排在PromiseJobs 的结尾允许任何挂起的回调在测试继续之前运行.

This can be done by using an async test function and calling await on a resolved Promise which effectively queues the rest of the test at the end of PromiseJobs allowing any pending callbacks to run before the test continues.

在我的回答此处中提供了有关承诺和假计时器如何交互的其他信息.

Additional information about how promises and fake timers interact is available in my answer here.

这篇关于我如何用玩笑测试承诺延迟?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！