php - disable button on ajax username request -

i have jquery check if username in use before submitting registration form. username check works fine. however, want disable submit button if username not-available. code. works first time: if username unavailable button disabled. once username unavailable username after available still leaves submit button disabled. how can button become enabled without having refresh page?

<script type="text/javascript"> $(document).ready(function() { $("#username").keyup(function (e) {      //removes spaces username     $(this).val($(this).val().replace(/\s/g, ''));      var username = $(this).val();     if(username.length < 4){$("#user-result").html('');return;}      if(username.length >= 4){         $("#user-result").html('<img src="imgs/ajax-loader.gif" />');         $.post('check_username.php', {'username':username}, function(data) {             if(data == '<img src="imgs/available.png" /> username available'){         $("#sumbit").attr("disabled", false);           $("#user-result").html(data);                 }else{             $("#submit").attr("disabled", true);                      $("#user-result").html(data);                 }         });     } });  }); </script> 

check_username.php looks this:

//sanitize username $username = filter_var($username, filter_sanitize_string, filter_flag_strip_low|filter_flag_strip_high);  //check username in db $results = mysqli_query($connecdb,"select username users username='$username'");  //return  count $username_exist = mysqli_num_rows($results);      if($username_exist) {     die('<img src="imgs/not-available.png" /> username not available'); }else{     die('<img src="imgs/available.png" /> username available'); } 

html submit looks this: html input is

<input type="submit" id="submit" class="special" name="submit"   value="register" /> 

try remove disable using : $("#sumbit").removeattr("disabled");

try code

<script type="text/javascript"> $(document).ready(function() { $("#username").keyup(function (e) {      //removes spaces username     $(this).val($(this).val().replace(/\s/g, ''));      var username = $(this).val();     if(username.length < 4){$("#user-result").html('');return;}      if(username.length >= 4){         $("#user-result").html('<img src="imgs/ajax-loader.gif" />');         $.post('check_username.php', {'username':username}, function(data) {             if(data == 'available'){           $("#submit").attr("disabled", false);           $("#user-result").html('<img src="imgs/available.png" /> username available');                 }else{             $("#submit").attr("disabled", true);                      $("#user-result").html('<img src="imgs/not-available.png" /> username not available');                 }         });     } });  }); </script> 

check_username.php looks this:

//sanitize username $username = filter_var($username, filter_sanitize_string, filter_flag_strip_low|filter_flag_strip_high);  //check username in db $results = mysqli_query($connecdb,"select username users username='$username'");  //return  count $username_exist = mysqli_num_rows($results);      if($username_exist) {     die('notavailable'); }else{     die('available'); }