JavaScript Promise Queue

PromiseQueue.js

class PromiseQueue {
  constructor({concurrency = 1} = {}) {
    this.concurrency = concurrency;
    this.queue = [];
    this.pendingPromises = 0;

    this.push = this.push.bind(this);
    this._dequeue = this._dequeue.bind(this);
  }

  push(promiseGenerator) {
    return new Promise((resolve, reject) => {
      this.queue.push({promiseGenerator, resolve, reject});
      this._dequeue();
    });
  }

  _dequeue() {
    if (
      this.queue.length === 0 ||
      this.pendingPromises >= this.concurrency
    ) {
      return;
    }
    this.pendingPromises++;
    const {promiseGenerator, resolve, reject} = this.queue.shift();
    promiseGenerator()
      .then(resolve)
      .catch(reject)
      .finally(() => {
        this.pendingPromises--;
        this._dequeue();
      });
  }
}

// Example 

function somethingAsync() {
  return new Promise((resolve, reject) => {
    console.log('starting...');
    setTimeout(() => {
      console.log('...finishing');
      resolve('tada!');
    }, 1000);
  })
}

let queue = new PromiseQueue();

queue.push(somethingAsync);
queue.push(somethingAsync);

// starting...
// ...finishing
// starting...
// ...finishing

queue = new PromiseQueue({concurrency: 2});

queue.push(somethingAsync);
queue.push(somethingAsync);
queue.push(somethingAsync);

// starting...
// starting...
// ...finishing
// ...finishing
// starting...
// ...finishing

queue.push(somethingAsync)
  .then(result => console.log(result));

// starting...
// ...finishing
// tada!