A rate-limiting queue for promised jobs

queue.js

const _ = require('lodash')

/**
 * A rate-limiting queue for functions that return promises.
 *
 *     const q = new Queue({rate: 1000})
 *     Promise.all(_.range(10).map(n => q.push(() => n * n))).then(all => console.log(all))
 *     q.start()
 *     // ten seconds later => [ 0, 1, 4, 9, 16, 25, 36, 49, 64, 81 ]
 *
 *     const q = new Queue({rate: 125}) // try to not exceed 500 requests in 60 seconds
 *     Promise.all(bunchOfPeople.map(p => intercom.getUser(p))).then(intercomUsers => ...)
 *     q.start()
 *
 * The rate limiter is based on setInterval(). You can stop the queue with
 * queue.stop(). This will cancel the interval immediately and any pending jobs
 * will not run until you call queue.start() again.
 */
class Queue {
  constructor(options) {
    this.opts = _.defaults(options, {
      rate: 100
    })
    this.jobs = []
  }

  start() {
    if (this.timer) return
    this.timer = setInterval(() => this._next(), this.opts.rate)
  }

  stop() {
    if (!this.timer) return
    clearInterval(this.timer)
    this.timer = null
  }

  _next() {
    const job = this.jobs.shift()
    if (job) job()
  }

  push(job) {
    return new Promise((resolve, reject) => {
      this.jobs.push(() => Promise.resolve(job()).then(resolve).catch(reject))
    })
  }
}

module.exports = Queue