when.js

Promise.prototype.spread = function (fn) {
  this.then(function (values) {
    fn.apply(this, values)
  })
}

function when () {
  var requirements = Array.prototype.slice.call(arguments, 0)
  return Promise.all(requirements.map(function(name){
    console.log(name)
    switch (typeof name) {
      case 'string'   : return when.conditions[name].call(this)
      case 'function' : return fn.call(this)
      default         : return name
    }
  }))
}

when.conditions = {}

when.define = function (name, fn) {
  when.conditions[name] = fn
}