Inline form validation is the way forward with any web site today, and I've found I've been able to extract this process out in to a simple plugin that can be applied anywhere throughout my code using the the following pattern.

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Once the plugin is applied, it will either create an span, or use any adjacent span directly next to the input element.

On keyup the value of the focused input element will be submitted to the form, where the server side will need to handle the request.

You can see a working demo here (from jQuery for Designers)


<form action="/profile" method="post">
    <legend>Change your username</legend>
      <label for="username">Username:</label>
      <input type="text" name="username" value="" id="username" />


Download the liveCheck plugin

Include jQuery and the live check plugin above the following code:

$(function () {
    $('#username').liveCheck({ 'image' : '/images/ajax-loader.gif' });

The plugin takes a single options variable which can be omitted. The available values are:

  • error - the class applied to the loading message span, defaults to 'error'
  • image - the loading image, defaults to blank
  • text - the text shown during the Ajax request, defaults to 'validating...'

JSON response

The response from the Ajax only needs to be two fields, one indicating success and one with a message for the user:

{ 'ok' : true, 'message' : 'The username selected is available' }

..or (for example):

{ 'ok' : false, 'message' : 'Username must be more than 3 characters' }

If you particularly need change the JSON response structure, you can do so via the following two options which are passed in to the liveCheck plugin call:

  • ok - the JSON response field indicating the check passed, defaults to 'ok'
  • message - the JSON response field offering the user some information as to the pass or fail. This text will replace the loading message, defaults to 'message'

Server side

Whatever your language of choice, the server side is pretty much the same. The logic breaks down to this:

  1. If header HTTP_X_REQUESTED_WITH exists, treat it as an Ajax request instead of normal
  2. Run the existing validation against the individual field
  3. Respond with a single JSON object and exit

For example, if my only validation check was the length of the username, before the Ajaxifying the code, my PHP would look like this:

  if ($Action == 'save') {
    $username_errors = User::validateUsername($_POST['username']);
    $password_errors = User::validatePassword($_POST['password']);
    $email_errors = User::validateEmail($_POST['email']);

    if (!count($username_errors) && !count($password_errors) && !count($email_errors)) {
      // save...
    } else {
      // handle errors

The modification is simple to handle the live check:

  if ($Action == 'save') {
    // existing code...
  } else if ($_SERVER['HTTP_X_REQUESTED_WITH']) {
    if (@$_POST['username']) { // silence warn
      $username_errors = User::validateUsername($_POST['username']);
      $json = array('ok' => true, 'message' => 'Selected username is fine');

      if (count($username_errors)) {
        // $username_errors is an array of errors, which we'll join together
        $json = array('ok' => false, 'message' => join(', ', $username_errors));

      echo json_encode($json);

Wrap up

This pattern can be applied pretty much anywhere you need to validate a single piece of information with very little coding overhead.