Add solution #2721

Author: JoshCrozier

README.md

@@ -407,6 +407,7 @@
 2677|[Chunk Array](./2677-chunk-array.js)|Easy|
 2695|[Array Wrapper](./2695-array-wrapper.js)|Easy|
 2703|[Return Length of Arguments Passed](./2703-return-length-of-arguments-passed.js)|Easy|
+2721|[Execute Asynchronous Functions in Parallel](./2721-execute-asynchronous-functions-in-parallel.js)|Medium|
 3042|[Count Prefix and Suffix Pairs I](./3042-count-prefix-and-suffix-pairs-i.js)|Easy|
 3110|[Score of a String](./3110-score-of-a-string.js)|Easy|
 3392|[Count Subarrays of Length Three With a Condition](./3392-count-subarrays-of-length-three-with-a-condition.js)|Easy|

solutions/2721-execute-asynchronous-functions-in-parallel.js

@@ -0,0 +1,41 @@
+/**
+ * 2721. Execute Asynchronous Functions in Parallel
+ * https://leetcode.com/problems/execute-asynchronous-functions-in-parallel/
+ * Difficulty: Medium
+ *
+ * Given an array of asynchronous functions functions, return a new promise promise.
+ * Each function in the array accepts no arguments and returns a promise. All the
+ * promises should be executed in parallel.
+ *
+ * promise resolves:
+ * - When all the promises returned from functions were resolved successfully in
+ * parallel. 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. The promise
+ * should resolve when all the asynchronous functions in the array have completed
+ * execution in parallel.
+ *
+ * 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.
+ */
+
+/**
+ * @param {Array<Function>} functions
+ * @return {Promise<any>}
+ */
+var promiseAll = function(functions) {
+  return new Promise((resolve, reject) => {
+    const promises = new Array(functions.length);
+    let count = 0;
+    functions.forEach((fn, i) => {
+      fn().then(result => {
+        promises[i] = result;
+        if (++count === promises.length) {
+          resolve(promises);
+        }
+      }).catch(reject);
+    });
+  });
+};