Observer

Observer notifies dependents when subject state changes without hard-wiring every listener. Event emitters, reactive stores, DOM events, and pub/sub buses are modern variants. Design must address subscription lifecycle, delivery order, error isolation, and memory leaks from forgotten listeners.

On interviews: compare push versus pull models. Discuss backpressure, debouncing, and when events should be domain facts versus technical signals.

Common pitfalls: synchronous observers blocking publishers; listeners never unsubscribed; implicit ordering assumptions across subscribers.

The trade-off is flexibility versus complexity—know when the simpler path is enough.

Checklist:

• Define unsubscribe or weak-reference policy.
• Isolate listener failures from publisher.
• Document event payload contracts and versioning.
• Prefer immutable event records for auditability.