PubSub with Wildcards

A plain event emitter matches topics by string equality: subscribe to "user.login", get "user.login" events. Real message buses — MQTT, Redis keyspace notifications, analytics pipelines — go further with hierarchical topics and wildcards, so one subscription can catch a whole family of events. Subscribe to user.* and you hear every direct action; subscribe to user.** and you hear the entire subtree.

Implement pubsubWildcard(), returning { on, emit, off } where dotted topics support two wildcards: * for exactly one segment and ** for the rest.

Signature

function pubsubWildcard() {
  return {
    on,    // (pattern, handler) => unsubscribe
    emit,  // (topic, payload) => void
    off,   // (pattern, handler) => void
  };
}

Examples

const bus = pubsubWildcard();
bus.on('user.*', (payload, topic) => console.log(topic, payload));

bus.emit('user.login', { id: 1 });   // matches -> "user.login" {id:1}
bus.emit('user.logout', { id: 1 });  // matches -> "user.logout" {id:1}
bus.emit('user.login.ok', {});       // NO match — * is one segment only

bus.on('user.**', () => {});
bus.emit('user');               // matches (** allows zero remaining)
bus.emit('user.login.ok', {});  // matches (** allows any depth)

Notes

• Topics are dot-segmented — user.login.success is three levels. Match segment by segment.
• * = exactly one segment — user.* matches user.login but not user or user.login.ok. It may appear anywhere, including the middle (a.*.c).
• ** = the rest — matches zero or more remaining segments, and must be the final segment.
• Deliver to all matches — one emit can hit several subscribers (exact and wildcard); each handler is called as handler(payload, topic). Both on's return value and off can unsubscribe.