JavaScript 2.0-The Complete Reference, Second ... - freecodingtutorial

Abstracting Form Validation
The previous example illustrated how writing generic input validation routines can be useful.
Instead of having to recode the same or similar field checking functions for each form on your
site, you can write a library of validation functions that can be easily inserted into your pages. In
order to be reusable, such functions should not be hardcoded with form and field names. The
validation functions should not pull the data to be validated out of the form by name; rather, the
data should be passed into the function for checking. This allows you to drop your functions into
any page and apply them to a form using only a bit of event handler ―glue‖ that passes them the
appropriate fields.
Form checking functions should go beyond checking that fields are non-empty. Common
checks include making sure a field is a number, is a number in some range, is a number of
some form (such as a U.S. ZIP code or Social Security number), is only a range of certain
characters like just alpha characters, and whether input is something that at least looks like an
e-mail address or a credit card number. Many of the checks, particularly the e-mail address and
credit card number checks, are not really robust. Just because an e-mail address looks valid
doesn‘t mean it is. We‘ll present e-mail and numeric checks here as a demonstration of
common validation routines in action.
Note Regular expressions are an invaluable tool for form validation because they let you check
input strings against a pattern using very little code. Without them, you’d be stuck writing
complex string parsing functions manually. We’ll use a combination of manual techniques
and regular expressions. Observe how much easier it is to use regexps.
Many forms are used to collect e-mail addresses, and it is nice to ferret out any problems with
addresses before submission. Unfortunately, it is difficult to guarantee that addresses are even
in a valid form. In general, about the best you can say quickly about an e-mail address is that it
is of the form userid@domain, where userid is a string and domain is a string containing a dot.
The ―real‖ rules for what constitutes a valid e-mail address are actually quite complicated, and
take into consideration outdated mail addressing formats, IP addresses, and other corner
cases. Because of the wide variation in e-mail address formats, many validation routines
generally look simply for something of the form string@string. If you want to be extremely
precise, it is even possible not to have a dot (.) on the right side of an e-mail! The function here
checks the field passed in to see if it looks like a valid e-mail address.
function isEmail(field)
{
var positionOfAt;
var s = field.value;
if (isEmpty(s))
{
alert("Email may not be empty");
field.focus();