How do I assign a value to my javascript object using another function?

Question

I have a javascript object that I instantiate called vacationDayRequested.

    function vacationDayRequested(_requestedDate,_clockNo) {
    this.status = "unknowm";
    this.ClockNo = _clockNo;
    this.DayRequested = _requestedDate;
    this.Selectable = IsSelectableBASIC(this);
}

I want to assign the value of the Selectable property via an ajax function called IsSelectableBASIC:

function IsSelectableBASIC(vacationDayRequested)
{
    var returnValue = false;

    $.ajax({
        url: "VacationRequest/IsDaySelectableBasic",
        type: 'POST',
        contentType: 'application/json; charset=utf-8',
        data: JSON.stringify(vacationDayRequested),
        success: function (data) {
            if (data.OperationMessage == "yes") {
                returnValue = true;
            }
            else {
                returnValue = false;
            }
        }
    });

    return returnValue;
}

I checked using the developer tools in Chrome that my ajax is returning JSON with an OperationMessage value of "yes" and yet my vacationDayRequested.Selectable comes back as "false". Do I have a design flaw in my javascript ?

Below is how I instantiate the object and use it in my client code:

var vdr = new vacationDayRequested(requestedDate, clockNo);              
alert(vdr.Selectable);

UPDATE: 9/11/2014

Callbacks are becoming more clear to me now. Do I have it correct when describing what is happening in the code below ? Also, please let me know if I am using the term callback correctly throughout.

• The button is pressed
• The button click event calls the GetValueOfX function and passes it a callback function named DoSomething.

• GetValueOfX executes and has an ajax function where success calls DoSomething because we passed in DoSomething.

  Press

  $("#btn").click(function () {

      GetValueOfX(DoSomething);
  });
  
  
  function DoSomething(result)
  {
      ***//This is the correct place for code that depends on the ajax return***
      x = result;
  
      debugger;
      alert(x);
  }
  
  function GetValueOfX(callback)
  {
      $.ajax({
          url: "/JSTesting/GetSomeTestData",
          type: 'POST',
          contentType: 'application/json; charset=utf-8',
          success: callback
      })
      ***//if we had code here it might fire before the ajax request is complete***
  }