Promise Merge
Problem statement
Implement a function that accepts two promises and returns a single Promise. This returned promise fulfills when both input promises fulfill, with a single value according to the following merge rules:
- Numbers: Should be added together.
- Strings: Should be concatenated.
- Arrays: Should be combined into a single array (flat).
- Plain Objects: Should be merged into a single object (shallow merge).
Rejection Cases
The returned promise should be rejected if:
- The types of the fulfilled results do not match (e.g., a number and an array). Reject with the string
'Unsupported data types'. - One of the input promises fails. Reject with the reason from the failing promise.
Example
await promiseMerge(Promise.resolve(1), Promise.resolve(2)); // 3
await promiseMerge(Promise.resolve('abc'), Promise.resolve('def')); // 'abcdef'
await promiseMerge(Promise.resolve({ a: 1 }), Promise.resolve({ b: 2 })); // { a: 1, b: 2 }How to approach Promise Merge
The strategy an interviewer expects you to reach for.
Promise merge waits on two promises, then combines values by type — add numbers, concat strings, reject on mismatched types. Naive move Chain p1.then(() => p2) serially, or coerce everything with + and hope. Where it breaks You need both results. Type mismatch should reject. If either input rejects, the merge should reject (fail-fast via Promise.all). The fix Promise.all([p1,p2]).then(([a,b]) => …) with explicit typeof branches for number/number and string/string; otherwise reject. Remember: all for coordination; typeof for the merge rule — no silent coercion. Say this in the interview: I'd Promise.all both inputs, then add numbers or concat strings, and reject when the resolved types do not match the allowed pairs.
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