Observer.js

function Observer() {
  let head = { prev: null, next: null }
  let tail = { prev: null, next: null }
  head.next = tail
  tail.prev = head

  function subscribe(callback) {
    let node = { callback, prev: tail.prev, next: tail }
    tail.prev.next = node
    tail.prev = node
    return {
      dispose() {
        node.prev.next = node.next
        node.next.prev = node.prev
      }
    }
  }

  function publish(value) {
    let cursor = head
    while ((cursor = cursor.next) !== tail) {
      let callback = cursor.callback
      callback(value)
    }
  }

  return { subscribe, publish }
}