javascript - jquery: Can't read the response from php function -

i'm trying read response php function response undefined value. if use chromes inspect element , go in network tab can read correct value of item.

here code:

javascript function

function chatheartbeat(){        $.ajax({         url: 'chat.php?action=chatheartbeat', type: 'post', datatype: 'json',     success: function(response) {            $.each(response.items, function(item){         //alert(item.m);                         $(".chatboxcontent").append('<div class=chatboxmessage><span class="chatboxmessagefrom">'+item.f+': </span><span class="chatboxmessagecontent">'+item.m+'</span></div>');     });              settimeout('chatheartbeat();',3000); }, error: function(){     $(".chatboxcontent").append('<div class=chatboxmessage><span class="chatboxmessagefrom">error!</span><span class="chatboxmessagecontent">unknown</span></div>');     alert("failed"); }  }); } 

here php code:

function chatheartbeat() { if ($idchat!=""){     $sql = "select * chat idchat=".$idchat." , recd=0 order id asc";                     $result=$mysqli->query($sql);     if ($result->num_rows>0){         while($row=$result->fetch_array()){             $sqlup="update chat set recd = '1' idchat = ".$row['id'];             if ($mysqli->query($sqlup)=== false){                 echo "error updating record" . $mysqli->error;             }             $item.=<<<eod                        {                         "f": "{$row['from']}",             "m": "{$row['message']}"        }, eod; }      } }else{     $item.=$item.=<<<eod                        {                         "f": "{$mysqli->connect_errno}",             "m": "{$mysqli->connect_error}"        }, eod; }     } }else{     $item.=$item.=<<<eod                        {                         "f": "{$mysqli->connect_errno}",             "m": "{$mysqli->connect_error}"        }, eod; } ?> {     "items": [           <?php echo $item;?>     [ } <?php } ?> 

this might happen if respond malformed json format. try putting data in array/object , use json_encode on it:

<?php $data = []; $data["f"] = "value1"; $data["m"] = "value2"; echo json_encode($data); ?>