Add collector functor implementation

Author: tshemsedinov

JavaScript/8-collector.js

@@ -0,0 +1,51 @@
+'use strict';
+
+const util = require('util');
+
+function Collector() {
+}
+
+util.inherits(Collector, Function);
+
+const collect = (expected) => {
+  const collector = (key, value) => {
+    if (collector.finished) return collector;
+    collector.count++;
+    collector.data[key] = value;
+    if (value instanceof Error) {
+      collector.callback(value, collector.data);
+      return collector;
+    }
+    if (collector.expected === collector.count) {
+      collector.callback(null, collector.data);
+    }
+    return collector;
+  };
+
+  const fields = {
+    count: 0,
+    expected,
+    data: {},
+    callback: null,
+    finished: false
+  };
+
+  Object.setPrototypeOf(collector, Collector.prototype);
+  return Object.assign(collector, fields);
+};
+
+Collector.prototype.done = function(callback) {
+  this.callback = callback;
+  return this;
+};
+
+// Usage
+
+const dc = collect(3).done((err, data) => {
+  console.log('Done callback ');
+  console.dir({ err, data });
+});
+
+dc('key1', 'value1');
+dc('key2', 'value2');
+dc('key3', 'value3');