Because callbacks are hard.

synchonously.js

// Because callbacks are hard.

synchronously = {};

synchronously.Mutex = function() {
  this.locked_ = false;
};

synchronously.Mutex.prototype.lock = function() {
  this.locked_ = true;
};

synchronously.Mutex.prototype.unlock = function() {
  this.locked_ = false;
};

synchronously.Mutex.prototype.locked = function() {
  return !!this.locked_;
};

synchronously.Do = function(callback) {
  var mutex = new Mutex();
  mutex.lock();
  callback.call(mutex);
  while (mutex.locked()) {}
};