Validating Form Entries with JavaScript

I use HTML forms quite a lot on the web sites I build for a variety of purposes. Most of the ones I build these days are self-handling PHP forms. This means that the form and the PHP that handles the form are on the same page. This is easy to do. You just put a hidden value in the form that gets set when the visitor submits the form. The PHP code checks for that value. If it's set, it does the processing. If it's not set, it displays the form. The basic code layout looks something like this:

In the first part of the PHP, we check to see if the hidden value "test_val" is set. If it is, process_form(), otherwise print_form(). Nothing to it.

We can have process_form() do just about anything with the submitted data. What we don't want it doing is trying to process form data that cannot, or should not, be processed. We may have certain fields that are required. We may have fields where the input has to be in a certain format or match certain criteria. Of course, we also don't want to be processing anything that is clearly the work of a spambot. We could handle all of this on the server side and in some cases where more complex validation is needed, this might be appropriate. But why have our server waste processing cycles doing simple checks for missing or garbage data? Let the requesting browser do it by giving the data a quick once-over with client-side JavaScript first.

The first thing that we have to do is intercept the form data before it gets sent to the server. To do this, we will replace the usual HTML form SUBMIT button with a generic button that launches our JavaScript when clicked, like this:

For illustration, let's plug this into a simple form that collects an email address:

Now let's assume that we want to do a simple validation to make sure that the email field is not blank when submitted. The basic layout of our JavaScript function (placed in the HEAD) could look something like this:



We check to see if the email value is blank. If it is, we throw an alert and exit the function. Otherwise, we call the JavaScript submit function, which sends the validated form data on its merry way.

Of course, the validation we have done here is very simple. We could check to see that the value of the email field matches the proper format for an email address. We could write a loop to systematically check multiple required fields. We could check to see that multiple fields do not contain the same value - an annoying spambot symptom. We could even dynamically add additional fields to the form based on the visitor's initial inputs. There is loads more we could do.

In our simple example, we could help the visitor even more by sending their cursor directly to the email field:

We could even highlight the field in yellow to further alert the visitor to the problematic field:

This is especially useful if you are checking and flagging multiple fields. You can focus the visitor's cursor on the first problem field, but highlight them all so that they see all the problems. Just remember to reset the highlight color back to the default for all fields at the start of your function, otherwise the fields that your visitor did fix will still be highlighted.

Certainly nothing groundbreaking here, but useful stuff. There are tons of examples of this out there and the complexity of your validation is really only limited by your knowledge of JavaScript.