--- /dev/null
+'use strict';
+const pTry = require('p-try');
+
+module.exports = concurrency => {
+ if (concurrency < 1) {
+ throw new TypeError('Expected `concurrency` to be a number from 1 and up');
+ }
+
+ const queue = [];
+ let activeCount = 0;
+
+ const next = () => {
+ activeCount--;
+
+ if (queue.length > 0) {
+ queue.shift()();
+ }
+ };
+
+ return fn => new Promise((resolve, reject) => {
+ const run = () => {
+ activeCount++;
+
+ pTry(fn).then(
+ val => {
+ resolve(val);
+ next();
+ },
+ err => {
+ reject(err);
+ next();
+ }
+ );
+ };
+
+ if (activeCount < concurrency) {
+ run();
+ } else {
+ queue.push(run);
+ }
+ });
+};