I think it should be like,

function onChange(control, oldValue, newValue, isLoading, isTemplate) {

  if (isLoading || newValue == '') {

          return;

    }

var employee = g_form.getReference('u_emp_name', setemployeedetails );

function setemployeedetails(employee) {

alert(employee);

g_form.setValue('u_emp_id', employee.user_name);

g_form.setValue('u_emp_bg', employee.u_segment);

}

}