如何在诺言中处理这种情况？

Question

I have an array where each element will go through 2 nested async tasks if some conditions hold. Otherwise, will go through 1 normal sync task and when all done return the array with the new updates.

Here is a simplified version of my code:

    someFunction(array) {
    array.forEach((element, index) => {
      if (some checks hold) {
        asyncTask1(element).then(updatedElement => {
          asyncTask2(updatedElement).then(elementFinal => {
            array[index] = elementFinal;
          });
        });
        return;
      }
      array[index] = normalSyncTask(element);
    });
  }

I need this function to return the updated array only when all elements are processed:

someFunction(array).then(updatedArray => { // Do something with the updated array });

This was my attempt:

someFunction(array) {
    var promises = [];
    array.forEach((element, index) => {
      if (some checks hold) {
        var promise = asyncTask1(element);
        promise.then(updatedElement => {
          asyncTask2(updatedElement).then(elementFinal => {
            array[index] = elementFinal;
          });
        });
        promises.push(promise);
        return;
      }
      array[index] = normalSyncTask(element);
    });
    return Promise.all(promises).then(() => Promise.resolve(array));
  }

Excuse my stupid attempt but I cannot really get a good grasp of how promises exactly work no matter how many tutorials I read.

Answer 1

This should work:

someFunction(array){
    const promises = array.map((element, index) => {
        if(some checks hold){
            return asyncTask1(element).then(asyncTask2);
        }
        return normalSyncTask(element);
    });

    return Promise.all(promises).then((values) => {
        console.log('Everything is OK', values);
    });
}

You generate an array of promises using Array.map and wait for all of them to be resolved using Promise.all .

The values that are not a promise will be ignored by Promise.all and be returned with the promises results in the values array returned by the function.

Answer 2

Your was very close with your attempt

function someFunction(array) {
    var promises = [];
    array.forEach((element, index) => {
        if (some checks hold) {
            // push Promise from last async function
            promises.push(asyncTask1(element).then(updatedElement => {
                return asyncTask2(updatedElement).then(elementFinal => {
                    array[index] = elementFinal;
                });
            }));
            return;
        }
        array[index] = normalSyncTask(element);
    });
    return Promise.all(promises).then(() => array);
}