Map Async Limit

JavaScript

medium

25 mins

Implement a function mapAsyncLimit(arr, limit, asyncFn) that:

Runs asyncFn on each item of arr.
Only runs limit number of async operations concurrently.
Returns a Promise that resolves to an array of results in the same order as input.

Input:

arr: Array of items.
limit: Maximum number of concurrent async calls.
asyncFn: Asynchronous function returning a promise.

Output:

A promise that resolves to an array of results after processing all inputs using asyncFn.

Example Inputs & Outputs

// Example: delayFn = x => new Promise(resolve => setTimeout(() => resolve(x * 2), 100))
await mapAsyncLimit([1, 2, 3, 4], 2, delayFn);
// Output: [2, 4, 6, 8]

// Example: limit = 1 behaves like sequential map
await mapAsyncLimit([1, 2, 3], 1, delayFn);
// Output: [2, 4, 6]

Constraints & Edge Cases

arr.length can be 0 → should return an empty array.
limit should be >= 1.
asyncFn may fail → should propagate the error.
Results must maintain input order.
Should handle non-promise-returning functions gracefully (wrap in Promise.resolve() if needed).