JAVASCRIPT: how to disable letters and only type numbers. ....?

How about having a clear div tag that sits on top of your text field. You can detect the letters on key up and transfer then to the text box but not the characters you don't want. See the post above for regular expression info to exclude characters or patterns of characters. You will have to emulate everything keep in mind, in case they make a mistake and use the delete or backspace. A good exercise IMO.