Add counter functional object

Author: tshemsedinov

JavaScript/8-counter.js

@@ -0,0 +1,37 @@
+'use strict';
+
+function Counter() {}
+
+const counter = initial => {
+  const f = val => {
+    f.count += val;
+    Object.keys(f.events)
+      .map(key => parseInt(key))
+      .filter(n => n <= f.count)
+      .forEach(n => {
+        f.events[n].forEach(callback => callback(f.count));
+        delete f.events[n];
+      });
+    return f;
+  };
+  const fields = { count: 0, events: {} };
+  Object.setPrototypeOf(f, Counter.prototype);
+  Object.assign(f, fields);
+  return f(initial);
+};
+
+Counter.prototype.on = function(n, callback) {
+  const event = this.events[n];
+  if (event) event.push(callback);
+  else this.events[n] = [callback];
+  return this(0);
+};
+
+// Usage
+
+const c = counter(10);
+c.on(5, val => console.log('Counter > 5, value:', val));
+c.on(5, val => console.log('Counter > 5, value:', val));
+c.on(25, val => console.log('Counter > 25, value:', val));
+c(5);
+setTimeout(() => c(15), 100);