queue.js

function queue(concurrency = 1) {
  let running = 0
  const taskQueue = []

  const runTask = (task) => {
    running++
    task(() => {
      running--
      if (taskQueue.length > 0) {
        runTask(taskQueue.shift())
      }
    })
  }

  const enqueueTask = task => taskQueue.push(task)

  return {
    push: task =>
      running < concurrency ? runTask(task) : enqueueTask(task),
  }
}