How to fake jquery.ajax() response?

Question

I am writing some QUnit tests for a JavaScript that makes AJAX calls.

For isolation I overwrite $.ajax to write the parameter array of an AJAX call to a

Answer 1

Use a closure to override $.ajax with a dummy response

After trying the accepted answer and the answer posted by user1634074, I devised this simple and flexible blend of the two.

In its most basic form…

function ajax_response(response) {
  return function (params) {
    params.success(response);
  };
}
$.ajax = ajax_response('{ "title": "My dummy JSON" }');

In the above example, define a function ajax_response() that accepts some JSON string as an argument (or any number of custom arguments useful for simulating a response) and returns an anonymous closure function that will be assigned to $.ajax as an override for unit testing.

The anonymous function accepts a params argument which will contain the settings object passed to the $.ajax function. And it uses the argument(s) passed to the outer function to simulate a response from the server. In this example, it always simulates a successful response from the server, by simply invoking the success callback and supplying it with the dummy JSON.

It is easy to reconfigure…

function ajax_response(response, success) {
  return function (params) {
    if (success) {
      params.success(response);
    } else {
      params.error(response);
    }
  };
}

// Simulate success
$.ajax = ajax_response('{ "title": "My dummy JSON." }', true); 
doAsyncThing(); // Function that calls $.ajax

// Simulate error
$.ajax = ajax_response('{ "error": "Who is the dummy now?" }', false); 
doAsyncThing(); // Function that calls $.ajax

Below we can see it in action…

/* FUNCTION THAT MAKES AJAX REQUEST */
function doAsyncThing() {
  $.ajax({
    type: "POST",
    url: "somefile.php",
    // data: {…},
    success: function (results) {
      var json = $.parseJSON(results),
          html = $('#ids').html();
      $('#ids').html(html + '
' + json.id);
    }
  });
}

/* BEGIN MOCK TEST */
// CREATE CLOSURE TO RETURN DUMMY FUNCTION AND FAKE RESPONSE
function ajax_response(response) {
  return function (params) {
    params.success(response);
  };
}

var n = prompt("Number of AJAX calls to make", 10);

for (var i = 1; i <= n; ++i) {
  
  // OVERRIDE $.ajax WITH DUMMY FUNCTION AND FAKE RESPONSE
  $.ajax = ajax_response('{ "id": ' + i + ' }');
  doAsyncThing();
}
/* END MOCK TEST */

IDs: