如何“刷新”笑话模拟（不使用等待）

Question

I come from the AngularJS world but now using React and Jest (with Jest Mock).

I'd like to do this....

test('should update internal tracking variable', async () => {
        api.post = jest.fn().mock;

        expect(obj.postCallFinished).toBe(false)
        obj.begin() //this calls api.post internally with await
        expect(obj.postCallFinished).toBe(false)

        api.post.flush()
        expect(obj.postCallFinished).toBe(true)   

})

I don't want to use await in this instance on the obj.begin call. I need more fine grain control and want to check internal tracking variables so that I can slowly step through all the callbacks of my app (without breaking encapsulation on the functions). I need to do state based testing and it's important that I can slowly step through each blocking call in turn.

Please can someone help me figure out how I get control over the promise and slowly resolve the mock imperatively?

Answer 1

It sounds like begin is an async function that calls await on a series of async functions.

You can spy on the functions called within begin and retrieve the Promise that each returns by using mockfn.mock.results . Then you can await each of those Promises individually within the test to check the state at each step.

Here is a simple example to get you started:

class MyClass {
  constructor() {
    this.state = 0;
  }

  async first() {
    await Promise.resolve();  // <= do something asynchronous
    this.state = 1;
  }

  async second() {
    await Promise.resolve();  // <= do something asynchronous
    this.state = 2;
  }

  async begin() {
    await this.first();
    await this.second();
    await Promise.resolve();  // <= do something asynchronous
    this.state = 3;
  }
}

test('walk through begin', async () => {
  const firstSpy = jest.spyOn(MyClass.prototype, 'first');
  const secondSpy = jest.spyOn(MyClass.prototype, 'second');

  const instance = new MyClass();
  const promise = instance.begin();
  expect(instance.state).toBe(0);  // Success!
  await firstSpy.mock.results[0].value;  // <= wait for the Promise returned by first
  expect(instance.state).toBe(1);  // Success!
  await secondSpy.mock.results[0].value;  // <= wait for the Promise returned by second
  expect(instance.state).toBe(2);  // Success!
  await promise;  // <= wait for the Promise returned by begin
  expect(instance.state).toBe(3);  // Success!
})