< 35 line promisify that works in browsers and node.js

pify.js

function pify (method, context) {
  var resolver, rejector
  var error, success
  var done = false

  var promise = {
    then: function (a) {
      if (!done && success != null) done = true, a(success); else resolver = a
      return { catch: promise.catch }
    },
    catch: function (a) {
      if (!done && error != null) done = true, a(error); else rejector = a
      return { then: promise.then }
    }
  }

  var next = function (a, b) {
    if (resolver && a != null) return (done = true) && resolver(a); else error = a
    if (rejector && b != null) return (done = true) && rejector(b); else success = b
    if (resolver && rejector) return resolver(b)
  }

  return function wrapper () {
    var args = cloneArguments(arguments)
    args.push(next)
    method.apply(context || null, args)
    return promise
  }
}

function cloneArguments (arg, begin, end) {
  for (var i = 0, len = arg.length, args = new Array(len); i < len; ++i) args[i] = arg[i]
  return args = args.slice(begin || 0, end || undefined)
}