在最后一个承诺解析为递归承诺时捕获

Question

I am calling a function which essentially returns a promise that resolves with either a list or nothing. I then call the same function for each item in the list. Eventually, all will resolve with nothing. I would like to run some code only when all resolve, but cannot figure out how to do that.

To simplify my problem I created this example. I have control only over the recursive function. Saving all the promises to an array and passing it to Promises.all() in sample below:

 function randomLowerInt(int) { return parseInt(Math.random() * int); } function smallerArray(size) { const arr = []; const smallerSize = randomLowerInt(size); for (let i = 0; i < smallerSize; i++) { arr.push(i); } return arr; } function createPromise(arrLength) { const secnds = parseInt(Math.random() * 20) * 1000; const timeCreated = new Date().getTime(); const p = new Promise(res => { setTimeout(() => { const timeResolved = new Date().getTime(); res({ timeCreated, timeResolved, arrLength }); }, secnds); }); return p; } function recursive(arr) { arr.forEach(() => { createPromise(arr.length).then(({ timeCreated, timeResolved, arrLength }) => { // console.log({ // timeCreated, // timeResolved // }); const smallerArr = smallerArray(arrLength); recursive(smallerArr); }); }); } recursive([1, 2, 3, 4, 5]); const promises = []; function recursive2(arr) { arr.forEach(() => { const p = createPromise(arr.length).then(({ timeCreated, timeResolved, arrLength }) => { const smallerArr = smallerArray(arrLength); recursive2(smallerArr); return ({ timeCreated, timeResolved }); }); promises.push(p); }); } recursive2([1, 2, 3, 4, 5]); console.log('Waiting...'); Promise.all(promises).then(vals => console.log(vals)); 

doesn't work because Promise.all() will get called before the array is fully populated.

Answer 1

If the expected result is only the timeCreated and timeResolved properties in a single object you can declare an array as a variable, .push() the values to the array return Promise.all() from .map() instead of .forEach() and log the result at chained .then() .

The pattern can be substituted for use of .reduce() , async/await with for..of loop or other patterns.

let res = []
function recursive(arr) {
  return arr.map((promise) => {
    return createPromise(arr.length).then(({ timeCreated, timeResolved, arrLength }) => {
      console.log({ timeCreated, timeResolved });
      res.push({ timeCreated, timeResolved });
      const smallerArr = smallerArray(arrLength);
      return Promise.all(recursive(smallerArr));
    });
  });
}

Promise.all(recursive([1, 2, 3, 4, 5])).then(() => console.log(res));