[Promise] Execute Asynchronous Functions in Parallel (Promise.all() 구현하기)

https://leetcode.com/problems/execute-asynchronous-functions-in-parallel/description/

Execute Asynchronous Functions in Parallel - LeetCode

Given an array of asynchronous functions functions, return a new promise promise. Each function in the array accepts no arguments and returns a promise.

promise resolves:

• When all the promises returned from functions were resolved successfully. The resolved value of promise should be an array of all the resolved values of promises in the same order as they were in the functions.

promise rejects:

• When any of the promises returned from functions were rejected. promise should also reject with the reason of the first rejection.

Please solve it without using the built-in Promise.all function.

 

Example 1:

Input: functions = [
  () => new Promise(resolve => setTimeout(() => resolve(5), 200))
]
Output: {"t": 200, "resolved": [5]}
Explanation: 
promiseAll(functions).then(console.log); // [5]

The single function was resolved at 200ms with a value of 5.

Example 2:

Input: functions = [
    () => new Promise(resolve => setTimeout(() => resolve(1), 200)), 
    () => new Promise((resolve, reject) => setTimeout(() => reject("Error"), 100))
]
Output: {"t": 100, "rejected": "Error"}
Explanation: Since one of the promises rejected, the returned promise also rejected with the same error at the same time.

Example 3:

Input: functions = [
    () => new Promise(resolve => setTimeout(() => resolve(4), 50)), 
    () => new Promise(resolve => setTimeout(() => resolve(10), 150)), 
    () => new Promise(resolve => setTimeout(() => resolve(16), 100))
]
Output: {"t": 150, "resolved": [4, 10, 16]}
Explanation: All the promises resolved with a value. The returned promise resolved when the last promise resolved.

 

Constraints:

• functions is an array of functions that returns promises
• 1 <= functions.length <= 10

 

solution

/**
 * @param {Array<Function>} functions
 * @return {Promise<any>}
 */
var promiseAll = async function(functions) {
  return new Promise((resolve, reject) => {
    const answer = [];
    let count = 0;
    for(let i = 0; i < functions.length; i++) {
      functions[i]().then(a => {
        count++;
        answer[i] = a;
        if(count === functions.length) {
          resolve(answer);
        }
      }).catch((error) => {
        reject(error);
      });
    }
  })
};

/**
* const promise = promiseAll([() => new Promise(res => res(42))])
* promise.then(console.log); // [42]
*/

promise가 끝나는 순서에 상관없이 순서대로 resolve안에 값을 넣어야하므로 answer의 index번호와 functions의 index번호 순에 맞게 resolve 값을 집어넣는다.