I'm a beginner with JavaScript, what I need to do is to show a div with an alert in case of a certain string in the results data after a JavaScript function has been executed.

Here's my code (I'm working with Parse PHP SDK):

comments.php

echo '
 <h5>'.$topic.'</h5>
 <div style="display:none;" id="anAlert" class="alert alert-danger"> 
 <button type="button" class="close" data-dismiss="alert"&g×ばつ</button> <strong>Ouch!</strong>
 You already upvoted this Topic!
 </div>
 <br>
 ';

Javascript code into comments.php:

// UPVOTE A TOPIC ------------------- 
function upvoteTopic() {
 var topicID = "<?php echo $topicID ?>";
 var votesNr = Number("<?php echo $votes ?>"); 
 $.ajax({
 url:"vote-topic.php?isUpvoted=true&topicID=" + topicID, // Call vote-topic.php 
 type: "GET", 
 success:function(data) {
 var results = data
 console.debug(results);
 if (results !== 'You already upvoted this Topic') {
 // Show updated votes
 document.getElementById('votesTxt').innerHTML = votesNr;
 // Change buttons style
 $( "#upvoteTopicButt" ).removeClass( "btn-default" ).addClass( "btn-primary" );
 $( "#downvoteTopicButt" ).removeClass( "btn-primary" ).addClass( "btn-default" );
 } else {
 document.getElementById("anAlert").style.display = 'block';
 }
 }});
}

vote-topic.php

// UPVOTE ------------------------------------------
if ($isUpvoted == "true") {
 // CANNOT VOTE YOUR OWN TOPIC
 if($userPointer->getObjectId() == $currUser->getObjectId()) {
 echo 'You cannot vote your own Topic!';
 // VOTE TOPIC
 } else {
 // Get votes array
 $upvotedBy = $tObj->get('upvotedBy');
 $downvotedBy = $tObj->get('downvotedBy');
 // You already upvoted this Topic
 if (in_array($userID, $upvotedBy)) {
 echo 'You already upvoted this Topic'; // HERE'S THE LINE I NEED TO COMPARE IN THE IF STATEMENT OF MY JAVASCRIPT FUNCTION!
 // UPVOTE TOPIC
 } else {
 array_push( $upvotedBy, $currUser->getObjectId() );
 $downvotedBy = array_diff( $downvotedBy, array($currUser->getObjectId() ) );
 // Increment votes
 $tObj->increment("votes", 1);
 $tObj->setArray('upvotedBy', $upvotedBy);
 $tObj->setArray('downvotedBy', $downvotedBy);
 $tObj->save();
 // echo 'You upvoted this Topic!';
 }
 }

So, when I click the UPVOTE button, my Javascript function successfully calls the vote-topic.php script and performs the saving in there, it changes the style of the upvoteTopicButt and downvoteTopicButt buttons, but it doesn't display my anAlert div.

I've tried to launch an alert of results, and it actually shows this line:

You already upvoted this Topic

So that's why I've placed this IF statement into my Javascript function:

 if (results !== 'You already upvoted this Topic') {
 ...
 } else {
 document.getElementById("anAlert").style.display = 'block';
 }

In theory, when the results message is == You already upvoted this Topic, my function should show my anAlert div. But it doesn't.

What am I doing wrong?