lock.js

// Lock async function until complete.
function lock(fn) {
  return function() {
    var args = Array.prototype.slice.call(arguments, 0);
    var cb   = args.pop() || function(){};
    var that = this;

    if (fn.locked)
      return cb(new Error('locked!'))

    args.push(function() {
      fn.locked = false;
      cb.apply(that, arguments);
    })

    fn.locked = true;
    fn.apply(that, args);
  }
}

// Calls back with 'unlocked!' if there is no error.
var lockedFn = lock(function(cb) {
  setTimeout(function() {
    cb(null, 'unlocked!')
  }, 100)
})

// Our generic callback
function complete(err, res) {
  if (err)
    console.log('locked!')
  else
    console.log(res)
}

// Try to run multiple times!
lockedFn(complete)
lockedFn(complete)
lockedFn(complete)

// Wait til unlocked again
setTimeout(function() {
  lockedFn(complete)
}, 200)