promise.js

//check if this call is a get.
function isGet(val) { return undefined === val }

//check if this call is a set, else, it's a listen
function isSet(val) { return 'function' !== typeof val }

function promise() {
  var value, listeners = []

  return function (listener) {
    if (isGet(listener)) {
      return value
    } else if (isSet(listener)) {
      if (value !== undefined) {
        throw new Error("promise value already delivered")
      } else {
        value = listener
        listeners.forEach(function (l) { l(value) })
      }
    } else {
      listeners.push(listener)
    }
  }
}