How to test promise before it runs finally with JEST

December 19, 2021

I have a function like this:

let settings = {
  isRun: false
}
function testPromise(promise) {
  settings.isRun = true;
  return promise.finally(() => settings.isRun = false;)
}

How can test isRun = true when run testPromise before finally expression run. I tried with

it('should isRun true when testPromise running', async () => {
   expect(settings.isRun).toBeFalsy();
   await testPromise(Promise.resolve('run')).then(() => {
     expect(settings.isRun).toBeTruthy();
   }
   expect(settings.isRun).toBeFalsy();

but it does not work

>Solution :

I think you can execute but not wait for the promise to observe the intermittent value change you want to see.

it('should isRun true when testPromise running', () => {
    expect(settings.isRun).toBeFalsy();
    const promise = testPromise(Promise.resolve('run'));
    // At this point, promise is not resolved yet, so the callback in finally has not executed yet.
    expect(settings.isRun).toBeTruthy();
    // If you want to test it changed back to fasle
    promise.then(() => {
        expect(settings.isRun).toBeFalsy();
    });
}