How to make only limited letters can be typed into <input type="text" /> with jQuery?

Question

For example,to allow only digital numbers to be typed in:

<input type="text" id="target" />

Answer 1

var value = $('#target').val();

function isNumber ( n ) { return !isNaN( n ) }

isNumber(value) // if its '3' then true
isNumber(value) // if 3 then true
isNumber(value) // if '' then false

Answer 2

Use the keypress() event. This example prevents characters that aren't digits (0 = 48, 9 = 57).

$(function() {
  $("#target").keypress(function(evt) {
    if (evt.which < 48 || evt.which > 57) {
      return false;
    }
  });
});

See this list of Javascript key codes.