Futures, Promises, Tasks — Junior¶

At junior level, focus on this question:

What does a future/promise actually represent, and what are its three possible states?

A placeholder for a value that doesn't exist yet¶

const promise = fetch("https://api.example.com/data");
// `promise` is NOT the actual response data - it's a PLACEHOLDER
// representing "the response, whenever it arrives"

A future/promise has exactly three possible states: pending (the value isn't ready yet), resolved/fulfilled (the value is now available), or rejected (an error occurred instead of a value). Code that wants the eventual value attaches a callback (.then() in JavaScript, await in most languages) to be notified/resumed once the future transitions out of pending.

🎓 Takeaway: a future/promise is fundamentally just a container with these three states, plus a mechanism for other code to be notified when it transitions — everything else covered in this topic (eager/lazy execution, tasks) builds on top of this basic three-state container concept.

Test yourself¶

1. Why is a promise/future not the same thing as the actual value it will eventually contain?
2. Once a future transitions to "resolved," can it ever go back to "pending" or transition to "rejected"?
3. What mechanism lets code "wait" for a future to resolve without blocking a thread (recall the Async/Await concurrency-model page)?

Continue to middle.md.