React Hook for queueing and processing async tasks sequentially

useTaskQueue.ts

function useTaskQueue(params: {
  shouldProcess: boolean
}): {
  tasks: ReadonlyArray<Task>
  isProcessing: boolean
  addTask: (task: Task) => void
} {
  const [queue, setQueue] = React.useState<{
    isProcessing: boolean
    tasks: Array<Task>
  }>({isProcessing: false, tasks: []})

  React.useEffect(() => {
    if (!params.shouldProcess) return
    if (queue.tasks.length === 0) return
    if (queue.isProcessing) return

    const task = queue.tasks[0]
    setQueue((prev) => ({
      isProcessing: true,
      tasks: prev.tasks.slice(1),
    }))

    Promise.resolve(task()).finally(() => {
      setQueue((prev) => ({
        isProcessing: false,
        tasks: prev.tasks,
      }))
    })
  }, [queue, params.shouldProcess])

  return {
    tasks: queue.tasks,
    isProcessing: queue.isProcessing,
    addTask: React.useCallback((task) => {
      setQueue((prev) => ({
        isProcessing: prev.isProcessing,
        tasks: [...prev.tasks, task],
      }))
    }, []),
  }
}

type Task = () => Promise<void> | void

In this hook, there are two consecutive setQueue(...) calls inside the useEffect which must update the internal state in the correct order. Otherwise there's the possibility for inifinite re-renders which cause the above error. This only happens if the enqueued task() is very fast.

@sladkoff not sure this is correct? both setQueue() calls should be queued and, therefore, the task should be removed from the queue even if it runs really quickly

https://react.dev/learn/queueing-a-series-of-state-updates

Hi @majames, I just double-checked this in my codebase. I noticed that I can remove the flushSync(...) now and the original problem is not reproducible.

However, I was able to reproduce the problem on the code revision from a year ago... It could indeed be fixed by applying the flushSync(...) wrapper... It doesn't make a lot of sense to me. I'm assuming it has something to do with the context of my code and maybe some dependency constellation 🤷

You're probably right that this is not the correct way to do this.