Overview

The Observer pattern is a foundational design pattern in event-driven programming. Libraries like RxJS, Node's EventEmitter, and React's state management are all built on this concept.

Implement a createObservable() factory that returns an observable object with three methods:

• subscribe(id, callback) — registers a subscriber with a unique id and a callback function.
• next(value) — pushes a value to all active subscribers by calling their callbacks.
• unsubscribe(id) — removes a subscriber so it no longer receives future values.

Constraints

• Subscriber ids are unique strings.
• Unsubscribed observers must not receive future events.
• Calling subscribe with an existing id should replace the old callback.
• next should call subscribers in the order they were first subscribed.
• unsubscribe on a non-existent id is a no-op.
• Complexity target: $O(1)$ per operation (amortised).

Examples

const obs = createObservable();

const logA = [];
const logB = [];

obs.subscribe("a", (v) => logA.push(v));
obs.subscribe("b", (v) => logB.push(v));
obs.next(1);
obs.unsubscribe("a");
obs.next(2);

logA; // [1]
logB; // [1, 2]

// Re-subscribing replaces the callback.
const obs = createObservable();
const log = [];

obs.subscribe("x", (v) => log.push("old:" + v));
obs.next(1);
obs.subscribe("x", (v) => log.push("new:" + v));
obs.next(2);

log; // ["old:1", "new:2"]

Notes

• Think about the data structure: a Map is ideal for O(1) lookup while preserving insertion order.
• The callback is invoked synchronously inside next.
• This is not a full Rx-style Observable — no error/complete channels, just the core subscribe/next/unsubscribe contract.

Solution

function createObservable() {
  const subscribers = new Map();

  return {
    subscribe(id, callback) {
      subscribers.set(id, callback);
    },
    next(value) {
      for (const cb of subscribers.values()) {
        cb(value);
      }
    },
    unsubscribe(id) {
      subscribers.delete(id);
    },
  };
}

Resources

• Observer Pattern — Wikipedia
• RxJS Overview
• EventEmitter — Node.js