A simple priority based task runner with a given cuncurrency for asynchronous tasks.

TaskRunner.js

// Task Runner
// Implement Asynchronous Task Runner
// tasks execute in an order of priority

// If the task runner have bandwidth it will pick immediately

// A "TaskRunner" Constructor takes one argument, "concurrency", and exposes one method "push" on its prototype.
// The "push" method takes two arguments. First one is task which is a "function" & second argument priority number

class TaskRunner {
  constructor(concurrency) {
    this.concurrency = concurrency;
    this.taskList = [];
  }

  // push
  push(task, priority) {
    if (this.taskList.length < this.concurrency) {
      this.execute(task);
    } else {
      // push it into taskList
      this.taskList.push({ id: new Date().getTime(), task, priority });
    }
  }

  // Task object id,  task, priority
  execute(task) {
    // start timeout here no response instead of done, invoke sleep or discard
    task(this.done);
  }

  logicForPriority(final, task) {
    if (task.priority > final.priority) {
      return task;
    }
  }

  done() {
    // next task is present
    if (this.taskList.length > 0) {
      // priority check
      // sort and pick first but FIFO will be lost
      let newTask = taskList.reduce(this.logicForPriority);
      this.execute(newTask.task);
      // decision to remove task from task list is upon execution not completion
      this.taskList = this.taskList.filter((t) => t.id !== newTask.id);
    }
  }
}

var r = new TaskRunner(3);
console.log(typeof r.push);
// r.push(exampleSimpleTask1, 1);
// executes immediately   3

// r.push(exampleSimpleTask2, 2);
// executes immediately    2

// r.push(exampleSimpleTask3, 1);
// executes immediately    1

// r.push(exampleSimpleTask4, 5);
// should wait until one of the running tasks completes
// r.push(exampleSimpleTask5, 4);
// should wait until one of the running tasks completes