Implement a Publish-Subscribe Event Bus in JavaScript or TypeScript

Implement a publish-subscribe (pub/sub) utility in JavaScript or TypeScript for use inside one front-end application. Code should be able to subscribe a callback to a named topic, publish a payload to every current subscriber of a topic, and stop receiving messages when it no longer needs them. Write the implementation, explain your API choices, and describe how you would test it. ```hint Design the removal handle Think about what subscribing should give back so a caller can remove exactly its own subscription, even if the same function is subscribed twice. ``` ```hint Consider subscriptions changing mid-publish Consider what should happen if a subscriber unsubscribes itself, or adds another subscriber, while a publish is running. ``` ### Constraints and Clarifications - The utility runs inside one JavaScript runtime. There is no network transport, persistence, or cross-tab messaging. - Publishing to a topic with no subscribers is valid and does nothing. - The API surface is up to you. At a minimum, it must support subscribing, publishing, and unsubscribing. ### Clarifying Questions - Should delivery be synchronous, with subscribers called before `publish` returns, or asynchronous? - If one subscriber throws, should the remaining subscribers still receive the message, and how should the error be reported? - Should subscribing the same function twice create two subscriptions or be ignored? - Are extras needed, such as one-time subscriptions, wildcard topics, or replaying the last message to late subscribers? ### What a Strong Answer Covers - A clear API in which subscribe returns an unsubscribe function or token, and calling unsubscribe twice is safe. - Data structures chosen for constant-time subscribe and unsubscribe, with delivery in subscription order. - Stable behavior when subscribers are added or removed during a publish. - Error isolation, so one failing subscriber does not stop delivery to the others. - Cleanup of empty topics and a discussion of memory leaks from forgotten subscriptions. - Tests for ordering, unsubscribing, mid-publish changes, and error handling. ### Follow-up Questions 1. How would you add a `once` subscription, and what happens if it is unsubscribed before it fires? 2. How would you type the utility in TypeScript so each topic's payload type is checked at compile time? 3. What changes if delivery must be asynchronous but still preserve per-topic order? 4. How would a React component subscribe safely so it never receives messages after unmounting?