sequential.js

// Wraps a promise-returning `fn` so that `fn` will only be invoked
// one after another (no 2 concurrent calls)
function sequential (fn) {
  var lastPromise = $.Deferred().resolve().promise();

  return function () {
    var context = this;
    var args = arguments;
    lastPromise = lastPromise.then(function () {
      return fn.apply(context, args);
    });
    return lastPromise;
  };
}

var get = sequential(function (i) {
  var dfd = $.Deferred();

  setTimeout(function () {
    console.log('done', i);
    dfd.resolve();
  }, 1000);

  return dfd.promise();
});

get(1);
get(2);
setTimeout(function () {
  get(3);
}, 500);
setTimeout(function () {
  get(4);
}, 501);