Advanced Form Validation in JavaScript

I'm already familiar with how to use onSubmit to evaluate form content against RegEx to ensure it meets static parameters for acceptable content. What I'm wondering is if there is a way to further provide validation against a MySQL database, such as if you want to make sure an e-mail address hasn't be开发者_Python百科en used yet before submitting a form and having to re-load the field data back into the proper places for correction.

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Unless you sent all the valid email addresses to the client (you wouldn't) then you're going to have to do some sort of server roundtripping to validate the field, which is equivalent to submitting the form. But using AJAX you can fully validate the form without the user experience being compromised; to them it will look like any other client-side validation. There are JQuery plugins to help make AJAX-based validation easier to code, but it isn't too hard to roll one up yourself.

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you have to use a serverside language for that: php, perl, python... as javascript (client-side technology) cannot communicate with a mysql server (serverside technology) directly.

In javascript you could use the AJAX methodology if you want to avoid the full page refresh. Try using jquery, (a javascript library that take care of crossbrowser issues and simplifies greatly rich interaction programming) and its $.ajax() function. This has been made really easy through the use of the jquery form plugin.

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Most examples of this kind of validation use some kind of asynchronous request back to the server to do the lookup in the database. Either by using XMLHttpRequest or some other Ajax style method.

I'd recommend creating a small URL end-point that you can make a quick HTTP request to sending the email address to check. It can return either true or false in a simple JSON response.