"00001" or "00002" are not numbers, but if you still wanna do it, think something like:

Function to add leading zeros at left (num is value, size is the size of the string)

function padLeadingZeros(num, size) {
    var s = num+"";
    while (s.length < size) s = "0" + s;
    return s;
}

Then:

$(function() {
      $("input").change(function(e){ // If the value changes
            e.preventDefault();
            $(this).val(padLeadingZeros($(this).val(), 5)); //replace the value with the returned value of the function, 5 is size.
      });
});