gistfile1.js

// Initially zero. When decremented to zero, fires callback
function semaphore( callback ) {
  var fn = callback,
      count = 0;
  return {
    increment: function() {
      count++;
    },
    decrement: function() {
      if( count > 0 ) {
        if(--count == 0 && fn) {
          fn();
          fn = null;
        }
      } else {
        throw new Error("Semaphore decremented more than incremented!");
      }
    }
  };
}