problems(js): add “2636. Promise Pool”

Signed-off-by: Matej Focko <me@mfocko.xyz>

problems/promise-pool.js

@@ -0,0 +1,38 @@
+/**
+ * @param {Function[]} functions
+ * @param {number} n
+ * @return {Function}
+ */
+var promisePool = async function(functions, n) {
+    return new Promise((resolve, reject) => {
+        let in_progress = 0;
+        let next = 0;
+
+        function progress() {
+            if (next >= functions.length) {
+                if (in_progress == 0) {
+                    resolve();
+                }
+
+                return;
+            }
+
+            while (in_progress < n && next < functions.length) {
+                in_progress++;
+
+                functions[next++]().then(() => {
+                    in_progress--;
+                    progress();
+                });
+            }
+        }
+
+        progress();
+    });
+};
+
+/**
+ * const sleep = (t) => new Promise(res => setTimeout(res, t));
+ * promisePool([() => sleep(500), () => sleep(400)], 1)
+ *   .then(console.log) // After 900ms
+ */