2636-promise-pool.js

/**
 * 2636. Promise Pool
 * https://leetcode.com/problems/promise-pool/
 * Difficulty: Medium
 *
 * Given an array of asynchronous functions functions and a pool limit n, return an asynchronous
 * function promisePool. It should return a promise that resolves when all the input functions
 * resolve.
 *
 * Pool limit is defined as the maximum number promises that can be pending at once. promisePool
 * should begin execution of as many functions as possible and continue executing new functions
 * when old promises resolve. promisePool should execute functions[i] then functions[i + 1] then
 * functions[i + 2], etc. When the last promise resolves, promisePool should also resolve.
 *
 * For example, if n = 1, promisePool will execute one function at a time in series. However,
 * if n = 2, it first executes two functions. When either of the two functions resolve, a 3rd
 * function should be executed (if available), and so on until there are no functions left to
 * execute.
 *
 * You can assume all functions never reject. It is acceptable for promisePool to return a promise
 * that resolves any value.
 */

/**
 * @param {Function[]} functions
 * @param {number} n
 * @return {Promise<any>}
 */
var promisePool = async function(functions, n) {
  const next = () => functions[n++]?.().then(next);
  return Promise.all(functions.slice(0, n).map(f => f().then(next)));
};