$(document).delegate('textarea', 'keydown', function(e) {
  var keyCode = e.keyCode || e.which;

  if (keyCode == 9) {
    e.preventDefault();
    var start = $(this).get(0).selectionStart;
    var end = $(this).get(0).selectionEnd;

    // set textarea value to: text before caret + tab + text after caret
    $(this).val($(this).val().substring(0, start)
                + "\t"
                + $(this).val().substring(end));

    // put caret at right position again
    $(this).get(0).selectionStart =
    $(this).get(0).selectionEnd = start + 1;
  }
});

// http://stackoverflow.com/questions/6140632/how-to-handle-tab-in-textarea

$(document).delegate('textarea', 'keydown', function(e) {
  var keyCode = e.keyCode || e.which;

  if (keyCode == 9) {
    e.preventDefault();
    var start = $(this).get(0).selectionStart;
    var end = $(this).get(0).selectionEnd;

    // set textarea value to: text before caret + tab + text after caret
    $(this).val($(this).val().substring(0, start)
                + "\t"
                + $(this).val().substring(end));

    // put caret at right position again
    $(this).get(0).selectionStart =
    $(this).get(0).selectionEnd = start + 1;
  }
});

Add source ref