Question

 var AppPatientsList = JSON.parse(JSON RESPONSE);   
  var AppPatientsListSort = AppPatientsList.sort(function(a,b){
     return   a.firstName.toLowerCase() <b.firstName.toLowerCase()
            ? -1
            : a.firstName.toLowerCase()>b.firstName.toLowerCase()
            ? 1 : 0;
  });    
    var DataArray = [];
      for (var i = 0; i < AppPatientsListSort.length; ++i) {
        if (AppPatientsListSort[i].firstName === search.value) {
        var appointment = {};
        appointment.PatientID = AppPatientsListSort[i].PatientID;
        appointment.ScheduleDate = AppPatientsListSort[i].ScheduleDate;
        alert(appointment.ScheduleDate); // Works fine, i get the date... 
        }   
        DataArray[i] = appointment;
      } 
      var RowIndex = 0;
      var ScheduleDate = "";
      for (i = 0, len = DataArray.length; i < len; i++) {
        // Throws me error in this place... WHY?
        if (ScheduleDate != DataArray[i].ScheduleDate) {
          ScheduleDate = DataArray[i].ScheduleDate;        
        }      
      }

What’s wrong with this code, why i am not able to access the ScheduleDate?

Answer 1

You are only initializing the appointment variable when you are inside the if clause, but you are adding it to the array on every iteration.

If the first element of AppPatientsListSort does not have the value you search for, DataArray[0] will contain undefined.

In the second loop you then try to access DataArray[0].ScheduleDate which will throw an error.

Update:
Even more important, as JavaScript has no block scope, it might be that several entries in DataArray point to the same appointment object.

Depending on what you want to do, everything it takes might be to change

DataArray[i] = appointment;

to

DataArray.push(appointment);

and move this statement inside the if clause so that only appointments are added that match the search criteria.

---

Further notes: To have a look what your DataArray contains, make a console.dir(DataArray) before the second loop and inspect the content (assuming you are using Chrome or Safari, use Firebug for Firefox).