Javascript

$.fn.checkAvailability = function() {
    $("#check-username").click(function() {
    if ( $('#username').attr("value") != '') {
        $('.loading').show();
        var username = $('#username').val().toLowerCase();
        $.get("check-username.php", { username:username } , function(data) { 
            //if username is already taken (FALSE)
             if (data == 0) {
                $('.loading').hide();
                $('.error').remove();
                $('.available').remove();
                $('#check-username').after('<span class="error"></span>');
                $('.error').text('Username is already taken.');
             } else {
                $('.loading').hide();
                $('.error').remove();
                $('.available').remove();
                $('#check-username').after('<span class="available"></span>');
                $('span.available').text('Username is available.');       
            }
        });
    }
});
 
$(document).ready(function() {
    $("#registration").checkAvailability();
});

xHTML

<form action="#" method="post" id="registration">
    <label for="username">Username</label>
    <input type="text" name="username" id="username" maxlength="30" />
    <input type="button" value="Check Availability" id="check-username" />
    <span class="loading"><img src="images/loading.gif" alt="Loading" /></span>
</form>

CSS

.loading {
    display: none;
}
 
.available {
    color: #060;
}
 
.error {
    color: #f00;
}

Backend

In this case, we are using a php script called check-username.php. However, the backend doesn't have to be in php, it can be in any language .aspx (.NET), .do (Java), .cfm (ColdFusion), etc. The backend script just needs to return either 1 or 0 (True or False). 1 when the username is available and 0 when the username is already taken.

Recommended Books on jQuery

jQuery Cookbook: Solutions & Examples for jQuery Developers

jQuery in Action