type Task = (callback: (result: string) => void) => void

type TasksCallback = (results: string[]) => void

class TaskRunner {
  protected tasks: Task[] = []

  protected results: Set<string> = new Set()

  addTask(task: Task) {
    this.tasks.push(task)
  }
}

class ParallelTaskRunner extends TaskRunner {
  run(callback: TasksCallback) {
    const totalTasks = this.tasks.length

    this.tasks.forEach((task) => {
      task((result) => {
        this.results.add(result)

        if (this.results.size === totalTasks) {
          callback([...this.results])
        }
      })
    })
  }
}

class SerialTaskRunner extends TaskRunner {
  index = 0

  run(callback: TasksCallback) {
    const executeTask = () => {
      if (this.index >= this.tasks.length) {
        callback([...this.results])
        return
      }

      this.tasks[this.index]((result) => {
        this.results.add(result)
        this.index++

        executeTask()
      })
    }

    executeTask()
  }
}

• Node.js script to run multiple commands in parallel · Muffin Man¹

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Footnotes

1. concurrently ↩