Async callback timeout when unit testing nested async function

Question

I want to implement a function which hits some apis asynchronously and does some calculations. However I would like to use promises (also because I thought its easy to write unit test for them)

This is the function I want to write.

const CalcFactory = (someApi1, someApi2, someApi3) => 
 async function calculation(params){
  return new Promise((res, rej) =>{
    const data1 = someApi1.getData() // getData is async
    someApi2.getData((err, data) =>{
       if(err)
         rej(err);
       else
         res();
      });
    });
 }

The question now is how to test and mock this? My first test is if all the api methods are beeing called but I already struggle with that.

const Calculator = CalcFactory(api1Mock, api2Mock, api3Mock);

if('should hit all external apis', () => {
return Calculator(somedada).then(()=>{
   expect(api1Mock.getData).toHaveBeenCalledWith(someData);
   expect(api2Mock.getData).toHaveBeenCalledWith(somedata);
   });

});

the mocks look like this:

const api1Mock = {
   someData: jest.fn(),
}

const api2Mock = {
someData: jest.fn().mockResolvedValue({
      data:{},
   })
}

However I always get a timeout when I run the test. I am also not really sure if this is a good approach to write the function but I really would like to use a promise.

Estus Flask · Accepted Answer · 2020-06-11 18:38:46Z

1

There's getData in one place and someData in another. Considering the promise is not rejected, it's not a problem. Calculator(somedada) promise isn't resolved because someApi2.getData is incorrectly mocked and results in pending promise.

It's an antipattern to return a promise from async function without using await. This means that either it doesn't benefit from being async, or promise constructor contains too much besides promisfication of non-promise API.

In case the use of async is justified, it can be:

async function calculation(params){
  ...
  const data1 = someApi1.getData()
  const data2 = await new Promise((res, rej) =>{
      someApi2.getData((err, data) => {
       if(err)
         rej(err);
       else
         res();
      });
  });
  ...
}

In Node, error-first callbacks can be promisified with util.promisify.

And mocked someApi2.getData should use a callback like expected:

const api2Mock = {
  getData: jest.fn().mockImplementation(cb => cb(null, 'data'))
}

If someApi2.getData is used more than once, it makes sense to promisify it instead of using promise constructor every time.

answered Jun 11, 2020 at 18:38

Estus Flask

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1 Comment

zlZimon Over a year ago

thank, I promisified someApi2.getData and just await the promise which works fine.

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