"Purely" functional observable in JavaScript

purely-functional-observable.js

var ones$ = Repeat(1);
var naturals$ = foldPast(sum, 0, ones$)
applyEach(
  n => console.log(n),
  naturals$
)

// 1 2 3 4 5 ...

function sum(x, y) {
  return x + y
}

function foldPast(operator, initial, s) {
  return function(callback) {
    s(function(x) {
      initial += x
      callback(initial)
    })
  }
}

function Repeat(x) {
  return function(callback) {
    setInterval(function() {
      callback(x);
    }, 3000);
  }
}

function applyEach(f, s) {
  s(f)
}