Some decorative messaging

pico.js

     State.websocket.onmessage = (e) => {
         const message = JSON.parse(e.data)
 
+        if(message.online) {
+            const difference = (l1, l2) => l1.filter(u => !l2.includes(u))
+            difference(message.online, State.online).forEach(username =>
+                State.posts.push({ts: message.ts, value: `${username} joined`}))
+            difference(State.online, message.online).forEach(username =>
+                State.posts.push({ts: message.ts, value: `${username} left`}))
+        }
+
         if(!doNotLog.has(message.kind)) {
             console.log(message)
         }