Html5 string

How to get the specific value of a string in textbox? for example; The value of an input="Gaisano"  i want only to get the letters "Gai" or the first letter of the word. i used this example but it doens't work.

                      var fieldLength = document.getElementById('myText').value.length;
                    var str = document.getElementById('myText').value;
                    str = str.substring(1, str.length + 5);