## Question Compare JavaScript's `Promise.all`, `Promise.any`, and `Promise.allSettled`. Explain when each aggregate promise fulfills or rejects, what its result preserves, and how you would choose among them for three independent service calls. ### Constraints & Assumptions - Inputs may contain promises or plain values. - Completion order may differ from input order. - A rejected operation may continue running unless it supports external cancellation. - Discuss the empty-iterable case for each method. ### Clarifying Questions to Ask - Does “first” mean first in input order or first to settle? Settlement behavior and result ordering are separate. - Should individual errors be retained? That depends on the method and use case. - Is cancellation automatic after the aggregate settles? No. - Is one successful result enough for the caller? That is the key selection question. ```hint Compare four dimensions For each method, state its success condition, failure condition, returned shape, and ordering guarantee. ``` ### What a Strong Answer Covers - Fail-fast rejection and ordered fulfillment array for `Promise.all`. - First fulfillment and aggregate failure for `Promise.any`. - Wait-for-everything outcome records for `Promise.allSettled`. - Correct empty-input semantics and distinction from `Promise.race`. - Practical choices, error reporting, timeouts, and explicit cancellation. ### Follow-up Questions 1. How would you cancel remaining fetches after `Promise.any` fulfills? 2. How would you require at least two of three operations to succeed? 3. Why do result arrays preserve input order even when completion order differs?