Question

How to change the following code to make use of all my 4 textboxes with id extra1, extra2,persons and tables?

$(document).ready(function () {
  $('#extra1').blur(function () {
        if ($.trim(this.value) == "") {
          $('#btnUpdate').attr("disabled", true);
        }
      else {
            $('#btnUpdate').removeAttr("disabled");
      }
    });
});

Answer 1

Use jQuery delegate which will bind only one event handler but will act only on the element which triggers the event. Try this

Wokring demo

$(document).ready(function () {
  $('formSelector').delegate('input[type=text]', 'blur', function () {
      if ($.trim(this.value) == "") {
          $('#btnUpdate').attr("disabled", true);
      }
      else {
            $('#btnUpdate').removeAttr("disabled");
      }
    });
});