promise.all( )

Interviewers ask for Promise. all when they want to see if you understand ordered fan-in: many async jobs, one result array that keeps input order. Naive move Push into an array as each promise resolves. Race the network: slower items finish later and scramble the order. Where it breaks Empty input must resolve to [] immediately. One rejection must reject the outer promise right away — do not wait for the rest. Non-thenables must still be accepted (wrap with Promise. resolve). The fix Create one outer promise. For each index, settle that input and write results[index] = value. Count successes; when the count hits length, resolve. On first reject, reject once and ignore later settlements. Remember: Index the results; never push. Reject-fast, order-preserving. Say this in the interview: I'd wrap each input in Promise.