Question

I’m having problems getting the returned array from a return ajax callback function which is inside of another function. The problem being no array data is being received during the first cycle…..

              var testFile = $("#selection").val();
              var testData = getTestData(testFile); 
              alert(testData); // not working during the first time I run the function, empty


              function getTestData(testF)
              {
                  var testArray = [];

                  $.getJSON("test.php",{fileTest: testF}, function(data)
                  {
                         $.each(data, function(index, value)
                         {
                              if(value == "")
                              {
                              }else
                              {
                                 testArray[index] = value;
                              }
                         }
                   });
                   alert(testArray); // working I see the values
                   return testArray; // not working the first time running this function
              }

Answer 1

Ajax calls are asynchronous. What does that mean?

When you call $.getJson just starts the HTTP request, it does not immediately fill in testArray. Instead, it fills in testArray at some unspecified point in the future after the HTTP request has completed.

The reason you see one of the alerts but not the other is simple: By the time you click on the ‘OK’ button for the first alert, the request has finished and your answer is ready. I’m willing to bet that the first alert that you’re seeing (that fails) is actually the one inside getTestData. Here’s the likely sequence of events:

1. You call getTestData()
2. You start an HTTP request with $.getJSON()
3. the alert(testArray) inside getTestData is called, displaying an alert. Since testArray hasn’t been filled in yet, you have an empty array.
4. While you’re sitting there looking at the alert, the $.getJSON() call completes and fills out testArray.
5. You click OK on the alert box.
6. return testArray; is run, returning the filled-in values
7. Your alert on line 3 is run, showing the array