Chaining and queuing ajax requests using jQuery deferred

Question

I have a clunky ajax queue that uses setTimeout and a global flag:

var InProgress = false;

function SendAjax(TheParameters) {

  if (InProgress) {
     setTimeout(function () { SendAjax(TheParameters) } , 500) 
  }

  InProgress = true;

  $.ajax({
    ...
    data: TheParameters,
    complete: InProgress = false
  });
}

How can I rewrite this using a queuing mechanism so that the requests trigger one after the other in the order they're received?

Answer 1

By using then we can chain each request sequentially as they come in.

var previousPromise;

// This actually sends the request
function actualSender(params) {
  return $.ajax(...);
}

// This will make sure that the next request will be fired
// when the previous one finishes.
function SendAjax(TheParameters) {
  if (previousPromise) {
    // Even if the previous request has finished, this will work.
    previousPromise = previousPromise.then(function () {
      return actualSender(TheParameters);
    });
    return previousPromise;
  }

  // first time
  previousPromise = actualSender(TheParameters);
  return previousPromise;
}

I didn't test this but the idea should work