javascript - how to use jquery validate .js in wordpress for user name already exist?

Hi I am making a registration form in myWordPress site

this is the code:

inregister.php

 php starting 
 if(isset($_POST['submit'])){ $reg_result = reg_register();
  php ending

<form method="post" action="" name="registerform" id="register-form">
<label >Name</label>
<input name="fname" type="text">
<label >email</label>
<input name="email" id="email" type="text" >
<label >password</label>
<input name="psd" id="psd" type="text" >
<input type="submit" name="submit" />
</form>
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infunctions.php

function reg_register(){

$user_name=esc_attr(stripslashes($_POST['uname']));
$user_email=esc_attr(stripslashes($_POST['email']));
$pass=$_POST['psd'];
$userdata = array(
     'user_login'    =>  $user_name,
        'user_email'  => $user_email,
        'user_pass' =>  $pass
);

 $user_id = wp_insert_user( $userdata ) ;

}
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infooter.php

<script type="text/javascript" language="javascript" src="<?php bloginfo('stylesheet_directory');?>/js/jquery.validate.js"></script> 
  <script type="text/javascript">
  $('#register-form').validate();
   </script>
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My registration form is working correctly with validation. (i.e. this field is required, please enter a valid emial etc) Now i need to make a validation that this username is already exists. How to do that. Please help