Are there any atomic javascript operations to deal with Ajax's asynchronous nature?

Question

I am dynamically loading code (functions) from a server and executing it as javascript code then storing it in an array and executing. All these snippets of code must be ex

Answer 1

Javascript is essentially single-threaded so you don't need a mutex. Your fetch could set up flags such that subsequent fetch calls could avoid making ajax calls e.g.:

var beingFetched = {};//map onflight -> callbacks
function fetch(foo){
  if (foo in fooArray){
      //Do Nothing
  } else {
      if (beingFetched.foo) { //note empty array is truthy
          //register a callback
          var callback = function(r){
             //anything you need to do wit the return object r
             //maybe even eval it.
          };
          //the callback would more likely be an argument to fetch itself
          //or you could use a promise API instead so that you can at your will
          //register multiple callbacks - for error, for success etc.
          beingFetched.foo.push(callback); 
      } else {
          beingFetched.foo = [];//truthy
          //Fetch foo via Ajax and execute
          $.ajax("getFoo/"+foo).done(function() {
              _.each(beingFetched.foo, function(cb){
                  cb.apply(cb,arguments);
              });
              delete beingFetched.foo;
          });
      }
  }
}

Answer 2

JavaScript is a nice language that works great with asynchronous callbacks, timeouts, intervals and user events, yet not having any concurrency problems. This is possible because JavaScript is essentially single-threaded - given piece of code is always executed atomically and never interrupted by another thread running JavaScript.

Your fetch() function will always be executed without any interruption. If it is executed as part of the AJAX callback and if multiple AJAX callbacks are pending, they will be queued.

Another example: if you have an event handler assigned to an input element and you fire the event multiple times at once, event handlers won't be executed concurrently. Instead they will be queued and executed sequentially. This also applies to multiple events triggered by setTimeout()/setInterval().

As a side-note: this is one of the reasons why node.js is so robust: it uses only single thread and never blocks on I/O but uses callbacks instead when data is ready/event occurs.