Protocol wire-tapping.

src/protocol.ts

@@ -4,6 +4,8 @@
 
 export const VERSION = "0";
 
+export const ALL: unique symbol = Symbol.for("*");
+
 // node with higher ID wins the election (must be unique and comparable)
 export type ID = string;
 
@@ -38,7 +40,7 @@ export interface Channel {
 }
 
 export type CreateChannel = (
-  id: ID,
+  id: ID | typeof ALL,
   options?: { version: string },
 ) => Channel;
 
@@ -49,25 +51,27 @@ export type CreateChannel = (
  * @returns a newly created Channel
  */
 export const multicast: CreateChannel = (
-  id: ID,
+  id,
   options: { version: string } = { version: VERSION },
 ) => {
   const bcast = new BroadcastChannel(`use-leader_${options.version}`);
+  const wiretap = id === ALL; // can listen to everyone's messages
 
   return {
     emit(params) {
       const message: Message = Object.assign(
-        { ver: options.version, snd: id },
+        { ver: options.version, snd: String(id) },
         params,
       );
+
       bcast.postMessage(message);
     },
 
     listen(fn) {
       const handler = ({ data: msg }: MessageEvent<Message>) => {
         if (typeof msg !== "object") return;
         // this message is for someone else, ignoring
-        if (msg.rcv && msg.rcv !== id) return;
+        if (!wiretap && msg.rcv && msg.rcv !== id) return;
 
         fn(msg);
       };

web/index.jsx

@@ -5,6 +5,7 @@ import { useState } from "preact/hooks";
 import { nanoid } from "nanoid";
 
 import { useLeader } from "../src/use-leader.ts";
+import { multicast, ALL } from "../src/protocol.ts";
 
 const IAmLeader = ({ leader = null }) => {
   const Label = () => {
@@ -100,23 +101,20 @@ const App = () => {
 };
 
 /** Logger */
-const bcast = new BroadcastChannel(`use-leader_0`);
-bcast.onmessage = (e) => {
-  const { snd, evt, rcv } = e.data;
 
+const channel = multicast(ALL);
+
+channel.listen(({ snd, evt, rcv }) => {
   const translate = {
     "🙋‍": "elect",
     "🙅‍": "disag",
     "🦸‍": "leadr",
     "💀": "xdead",
   };
 
-  if (rcv) {
-    console.log(`${evt} ${snd} -> ${rcv} :: ${translate[evt]} `);
-  } else {
-    console.log(`${evt} ${snd} :: ${translate[evt]}`);
-  }
-};
+  const transfer = [snd, rcv].filter(Boolean).join(" -> ");
+  console.log(`${evt} ${transfer} :: ${translate[evt]} `);
+});
 
 /** mono screen utils */
 const WIDTH = 40;