ScriptdrillPromise.all with a concurrency limit
A JavaScript exercise from Scriptdrill. Solve it with a code editor in your browser that runs your function against example and hidden tests, and it is checked the moment you submit.
The task
Like `Promise.all(items.map(fn))`, but with at most `limit` calls of the async function `fn(item)` running at the same time. Resolve with the results in the order of `items`. Start the next item as soon as any running one finishes. If a call rejects, reject. Tests run on a fake clock: `setTimeout`, `clearTimeout` and `Date.now` are simulated, so waits are instant and exact.
What this exercise teaches
Workers share a counter, so whichever finishes first claims the next item at once, and storing by index keeps the results in input order. Working in fixed batches instead would wait for the slowest item of every batch.
A hint if you get stuck
Start `limit` workers. Each worker loops: take the next unclaimed index, `await fn(items[i])`, store the result at `results[i]`, repeat until no items are left. Then `await Promise.all(workers)`.
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