turn an async function sync

sync.js

// Usage:
// function asyncAdd(a, b, cb){ cb(a + b ) }
//
// var syncAdd = sync(asyncAdd)
//   , sum = syncAdd(2, 2)
//
// assert(sum == 4)
//
// NOTE:
// this only works for functions that execute their callback before returning.
// an error will be thrown if the function does not callback in time.

function sync(fn, cb) {
  function synced() {
    var err, data, done

    arguments[arguments.length++] =
      function(e, d){ err = e; data = d; done = true }

    fn.apply(this, arguments)

    if (!done) throw new Error(fn.name + " did not respond in time.")

    if (err) throw err

    return data
  }

  if (cb) cb(null, synced)

  else return synced
}