How do you limit concurrency with Promise.all in JavaScript?
Batch items: for (let i = 0; i < items.length; i += size) { await Promise.all(items.slice(i, i + size).map(fn)); }. Process size items at a time to avoid overwhelming the server.
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More FAQs in Promise Combinators Best Practices in JavaScript
Choose the right combinator (all, allSettled, race, any), always handle errors, use allSettled for best-effort, clear timers in race, use AbortController for cancellation, limit concurrency for large sets, handle AggregateError, and check for empty arrays.
Use Promise.all when all operations must succeed (any failure is a total failure). Use Promise.allSettled when you want all results even if some fail (partial success).
Use try/catch. Check err instanceof AggregateError. Access err.errors (an array of all individual rejection reasons). This lets you see why each promise failed and handle accordingly.
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