Add tests for demux

src/functions/demux.ts

@@ -1,5 +1,27 @@
 import { WritableOptions, Writable } from "stream";
 
+enum EventSubscription {
+    Last = 0,
+    First,
+    All,
+    Self,
+    Unhandled,
+}
+
+const eventsTarget = {
+    close: EventSubscription.Self,
+    data: EventSubscription.All,
+    drain: EventSubscription.Self,
+    end: EventSubscription.Self,
+    error: EventSubscription.Self,
+    finish: EventSubscription.Self,
+    pause: EventSubscription.Self,
+    pipe: EventSubscription.Unhandled,
+    readable: EventSubscription.Self,
+    resume: EventSubscription.Self,
+    unpipe: EventSubscription.Unhandled,
+};
+
 /**
  * Return a Duplex stream that is pushed data from multiple sources
  * @param streams Source streams to multiplex
@@ -76,4 +98,24 @@ class Demux extends Writable {
             });
         }
     }
+
+    public on(event: string, cb: any) {
+        switch (eventsTarget[event]) {
+            case EventSubscription.Self:
+                super.on(event, cb);
+                break;
+            case EventSubscription.All:
+                Object.keys(this.streamsByKey).forEach(key =>
+                    this.streamsByKey[key].stream.on(event, cb),
+                );
+                break;
+            case EventSubscription.Unhandled:
+                throw new Error(
+                    "Stream must be multiplexed before handling this event",
+                );
+            default:
+                super.on(event, cb);
+        }
+        return this;
+    }
 }