NodeJS Design Patters (third edition) challenges

producer-consumer-promises.js

class TaskQueue {
  constructor(concurrency) {
    this.taskQueue = [];
    this.consumerQueue = [];

    for (let i = 0; i < concurrency; i++) {
      this.consumer();
    }
  }

  consumer() {
    return new Promise((resolve, reject) => {
      const task = this.getNextTask();
      resolve(task);
    })
      .then(task => task())
      .then(res => console.log(res))
      .then(() => {
        if (this.taskQueue.length !== 0) this.consumer();
      });
  }

  getNextTask() {
    return new Promise(resolve => {
      if (this.taskQueue.length !== 0) {
        return resolve(this.taskQueue.shift());
      }

      this.consumerQueue.push(resolve);
    });
  }

  runTask(task) {
    return new Promise((resolve, reject) => {
      const taskWrapper = () => {
        const taskPromise = task();
        taskPromise.then(resolve, reject);
        return taskPromise;
      };
      if (this.consumerQueue.length !== 0) {
        const consumer = this.consumerQueue.shift();
        consumer(taskWrapper);
      } else {
        this.taskQueue.push(taskWrapper);
      }
    });
  }
}