Rate-limited concurrent promises using an iterator

Deferred.js

class Deferred extends Promise {
  constructor() {
    let resolve, reject
    super((...args) => [resolve, reject] = args)
    Object.assign(this, {resolve, reject})
  }
}
Deferred.tasks = (list, limit = list.length) => {
  const promises = list.map(task => {
    function exec(iterator) {
      task()
      .then(this.resolve, this.reject)
      .then(() => {
        const {value} = iterator.next()
        if (value) value.exec(iterator)
      })
    }
    return Object.assign(new Deferred(), {exec})
  })
  const iterator = promises[Symbol.iterator]()
  Array.from(Array(limit), () => iterator.next().value.exec(iterator))
  return Promise.all(promises)
}