Add CountingSemaphore with async/await contract

Author: tshemsedinov

JavaScript/8-await-counting.js

@@ -0,0 +1,45 @@
+'use strict';
+
+const fs = require('fs');
+
+class CountingSemaphore {
+  constructor(concurrency) {
+    this.counter = concurrency;
+    this.queue = [];
+  }
+
+  enter() {
+    return new Promise(resolve => {
+      if (this.counter > 0) {
+        this.counter--;
+        resolve();
+        return;
+      }
+      this.queue.push(resolve);
+    });
+  }
+
+  leave() {
+    if (this.queue.length === 0) {
+      this.counter++;
+      return;
+    }
+    const resolve = this.queue.shift();
+    resolve();
+  }
+}
+
+// Usage
+
+const job = async task => {
+  console.log('try enter', task);
+  await semaphore.enter();
+  console.log('enter', task);
+  setTimeout(() => {
+    semaphore.leave();
+    console.log('leave', task);
+  }, 1000);
+};
+
+const semaphore = new CountingSemaphore(3);
+for (let i = 0; i < 100; i++) job(i);