Jest: Timer and Promise don't work well. (setTimeout and async function)

Question

Any ideas on this code

jest.useFakeTimers() 

it(\'simpleTimer\', async () => {
  async function simpleTimer(callback) {
    await callback()    // LINE-A         


        

Answer 1

There is a use case I just couldn't find a solution:

function action(){
  return new Promise(function(resolve, reject){
    let poll
    (function run(){
      callAPI().then(function(resp){
        if (resp.completed) {
          resolve(response)
          return
        }
        poll = setTimeout(run, 100)
      })
    })()
  })
}

And the test looks like:

jest.useFakeTimers()
const promise = action()
// jest.advanceTimersByTime(1000) // this won't work because the timer is not created
await expect(promise).resolves.toEqual(({completed:true})
// jest.advanceTimersByTime(1000) // this won't work either because the promise will never resolve

Basically the action won't resolve unless the timer advances. Feels like a circular dependency here: promise need timer to advance to resolve, fake timer need promise to resolve to advance.