problems(js): add “2694. Event Emitter”

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/js/event-emitter.js

@@ -0,0 +1,39 @@
+class EventEmitter {
+  constructor() {
+    this.mapping = new Map();
+  }
+
+  subscribe(event, cb) {
+    if (!this.mapping.has(event)) {
+      this.mapping.set(event, new Array());
+    }
+
+    this.mapping.get(event).push(cb);
+
+    return {
+        unsubscribe: () => {
+          this.mapping.set(event, this.mapping.get(event).filter(s => s !== cb));
+        }
+    };
+  }
+
+  emit(event, args = []) {
+    if (!this.mapping.has(event)) {
+      return [];
+    }
+
+    return this.mapping.get(event).map(cb => cb(...args));
+  }
+}
+
+/**
+ * const emitter = new EventEmitter();
+ *
+ * // Subscribe to the onClick event with onClickCallback
+ * function onClickCallback() { return 99 }
+ * const sub = emitter.subscribe('onClick', onClickCallback);
+ *
+ * emitter.emit('onClick'); // [99]
+ * sub.unsubscribe(); // undefined
+ * emitter.emit('onClick'); // []
+ */