javascript - How to check if username exists without refreshing page using wordpress -

i want check text field in form if username exists in database or not.i want without refreshing page , using wordpress.i know possible through ajax have tried ajax in wordpress , ajax code didn't run on it. kindly provide piece of code or helpful link. last time have tried didn't work:

<?php if(!empty($user_name)){   $usernamecheck = $wpdb->get_results("select id wp_teacher_info      user_name='$user_name'"); if(!empty($usernamecheck)){ echo'username not available'; } else {  } }?>  <label for="user_name" id="user_name">username: </label> <input type="text" name="user_name" id="user_name" required/> <span id="user-result" ></span>   <script type="text/javascript">  jquery("#user_name").keyup(function (e) { //user types username on   inputfiled  var user_name = jquery(this).val(); //get string typed user jquery.post('teacher_form.php', {'user_name':user_name}, function(data) {  jquery("#user-result").html(data); //dump data received php page }); }); </script> 

use

if(!empty($_post['user_name']){    $user_name = $_post['user_name']; 

to be

<?php if(!empty($_post['user_name']){        $user_name = $_post['user_name'];       $usernamecheck = $wpdb->get_results("select id wp_teacher_info user_name='$user_name'");       if(!empty($usernamecheck)){          echo'username not available';       }       else {        } } ?> 

but keyup event call ajax each keyup .. can use **.blur()** instead of **.keyup()**