Min delay between values in rxjs

index.js

import Rx from 'rxjs/Rx'

function delayBetween(delay, first = false){
  let past = Date.now()

  return this.mergeMap((next, index) => {
    const present = Date.now()
    const futur = Math.max(past + (index === 0 && !first ? 0 : delay), present)
    past = futur
    return Rx.Observable.of(next).delay(futur - present)
  })
}

Rx.Observable.prototype.delayBetween = delayBetween

// Exemple
const observer = new Rx.Subject()

observer
  .delayBetween(1000)
  .subscribe(
    next => console.log('next', next, Date.now()),
    err => console.log('err'),
    () => console.log('done')
  )

setTimeout(() => {
  observer.next(1)
  observer.next(1)
  observer.next(1)
}, 0E3)

setTimeout(() => {
  observer.next(2)
  observer.next(2)
  observer.next(2)
}, 2E3)

setTimeout(() => {
  observer.next(3)
  observer.next(3)
  observer.next(3)
}, 10E3)

setTimeout(() => {
  observer.next(4)
  observer.next(4)
  observer.next(4)
  observer.complete()
}, 20E3)