Concurrently run an asynchronous function on a list of items given a concurrency limit.

concurrent.ts

/**
 * Concurrently run an asynchronous function on a list of items given a
 * concurrency limit. A concurrency limit equal to Infinity makes this function
 * behave like `Promise.all`.
 *
 * @params items - List of items to process
 * @params run - Asynchronous function to run for each item
 * @params concurrency - Maximum number of concurrent executions
 * @return A Promise that resolves to an array of results
 */
async function concurrent<Item = any, Result = any>(
  items: Iterable<Item>,
  run: (item: Item, index: number) => Promise<Result>,
  concurrency: number = Infinity
): Promise<Result[]> {
  const pool: (() => void)[] = []
  concurrency = Math.max(1, Math.floor(concurrency || 0))

  return Promise.all([...items].map((item, index) => (
    new Promise<Result>((resolve, reject) => {
      const start = () => run(item, index)
        .then((result) => { pool.pop()?.(), resolve(result) })
        .catch((error) => { pool.splice(0), reject(error) })

      index < concurrency ? start() : pool.unshift(start)
    })
  )))
}