node-apply.coffee

###
# Call a Node-style async function and return a promise.
#
# @param {function} fn A function that accepts a Node-style callback.
# @param {...*} var_args A variable number of arguments to pass to the Node function.
# @return {Promise}
###
q = (fn, args...) -> new Promise (resolve, reject) ->
  cb = (err, vals...) ->
    if (err)
      reject(err)
    else if (vals.length > 1)
      resolve(vals)
    else
      resolve(vals[0])

  args.push(cb)
  fn.apply(null, args)

node-apply.js

/**
 * Call a Node-style async function and return a promise.
 *
 * @param {function} fn A function that accepts a Node-style callback.
 * @param {...*} var_args A variable number of arguments to pass to the Node function.
 * @return {Promise}
 */
function q(fn, var_args) {
  var args = [].slice.call(arguments, 1)

  return new Promise(function (resolve, reject) {
    function cb(err, val) {
      if (err) {
        reject(err)
      } else if (arguments.length > 2) {
        resolve([].slice.call(arguments, 1))
      } else {
        resolve(val)
      }
    }

    args.push(cb)
    fn.apply(null, args)
  }
}