Executes array of promise-returning functions with provided concurrency level

concurrentGen.ts

type Task<R> = () => Promise<R>

/** Execute array of promise-returning functions with provided concurrency level */
async function* concurrentGen<R>(tasks: Task<R>[], concurrency: number) {
  async function withIndex(task: Task<R>, index: number) {
    const result = await task()
    return { result, index }
  }

  const queues = tasks.splice(0, concurrency).map(withIndex)

  while (queues.some(Boolean)) {
    const { result, index } = await Promise.race(queues.filter(Boolean))
    const nextTask = tasks.shift()

    if (nextTask) queues[index] = withIndex(nextTask, index)
    else delete queues[index]

    yield result
  }
}