如何解决此错误：无法设置 null 的属性（设置“innerHTML”）

Question

I recently live hosted my website and got this error. Uncaught TypeError: Cannot set properties of null (setting 'innerHTML') at response (home:2593) at XMLHttpRequest. (home:2560)

This error did not occur on the local server but it occurred on a public server. I saw a few threads regarding this error and I heard that you should use addEventListener instead of onclick in your code. However, I'm not sure how to implement it into my code so it would be great if you could help me.

This is the line where the error occurred:

info_element.innerHTML = obj.info;

This is the JS:

<script type="text/javascript">
        function ajax_send(data, element) {

            var ajax = new XMLHttpRequest();

            ajax.addEventListener('readystatechange', function() {

                if (ajax.readyState == 4 && ajax.status == 200) {

                    response(ajax.responseText, element);
                }

            });

            data = JSON.stringify(data);

            ajax.open("post", "<?= ROOT ?>ajax.php", true);
            ajax.send(data);

        }

        function response(result, element) {

            if (result != "") {

                var obj = JSON.parse(result);
                if (typeof obj.action != 'undefined') {

                    if (obj.action == 'like_post') {

                        var likes = "";

                        if (typeof obj.likes != 'undefined') {
                            likes =
                                (parseInt(obj.likes) > 0) ?
                                '<svg fill="#1877f2" width="22" height="22" viewBox="0 0 24 24"><path d="M21.216 8h-2.216v-1.75l1-3.095v-3.155h-5.246c-2.158 6.369-4.252 9.992-6.754 10v-1h-8v13h8v-1h2l2.507 2h8.461l3.032-2.926v-10.261l-2.784-1.813zm.784 11.225l-1.839 1.775h-6.954l-2.507-2h-2.7v-7c3.781 0 6.727-5.674 8.189-10h1.811v.791l-1 3.095v4.114h3.623l1.377.897v8.328z"/></svg>' :
                                '<svg fill="#626a70cf" width="22" height="22" viewBox="0 0 24 24"><path d="M21.216 8h-2.216v-1.75l1-3.095v-3.155h-5.246c-2.158 6.369-4.252 9.992-6.754 10v-1h-8v13h8v-1h2l2.507 2h8.461l3.032-2.926v-10.261l-2.784-1.813zm.784 11.225l-1.839 1.775h-6.954l-2.507-2h-2.7v-7c3.781 0 6.727-5.674 8.189-10h1.811v.791l-1 3.095v4.114h3.623l1.377.897v8.328z"/></svg>';
                            element.innerHTML = likes;
                        }

                        if (typeof obj.info != 'undefined') {
                            var info_element = document.getElementById(obj.id);
                            info_element.innerHTML = obj.info;
                        }
                    }
                }
            }
        }

        function like_post(e) {

            e.preventDefault();
            var link = e.currentTarget.href;

            var data = {};
            data.link = link;
            data.action = "like_post";
            ajax_send(data, e.currentTarget);
        }
    </script>

This is where I implemented like_post in my HTML:

<a onclick="like_post(event)" href="<?= ROOT ?>like/post/<?php echo $ROW['postid'] ?>" style="text-decoration:none;float:left;position:relative;top:2px;">
                <svg id="icon_like" fill="<?= $Like_color ?>" width="22" height="22" viewBox="0 0 24 24">
                    <path d="M21.216 8h-2.216v-1.75l1-3.095v-3.155h-5.246c-2.158 6.369-4.252 9.992-6.754 10v-1h-8v13h8v-1h2l2.507 2h8.461l3.032-2.926v-10.261l-2.784-1.813zm.784 11.225l-1.839 1.775h-6.954l-2.507-2h-2.7v-7c3.781 0 6.727-5.674 8.189-10h1.811v.791l-1 3.095v4.114h3.623l1.377.897v8.328z" />
                </svg>
            </a>

I also did some debugging using console.log. This is what I received when I console.log(obj.id):

info_

It's supposed to return some values after info_ eg.143884

This is the code in ajax.php:

<?php

include("classes/autoload.php");

$data = file_get_contents("php://input");
if($data != ""){
  $data = json_decode($data);
}

if(isset($data->action) && $data->action == "like_post"){

  include("ajax/like.ajax.php");
}

This is the code in like.ajax.php:

<?php

if(!empty($data->link)){
    $URL = split_url_from_string($data->link);
}

$_GET['type'] = isset($URL[5]) ? $URL[5] : null;
$_GET['id'] = isset($URL[6]) ? $URL[6] : null;


$_SESSION['mybook_userid'] = isset($_SESSION['mybook_userid']) ? $_SESSION['mybook_userid'] : 0;
$login = new Login;
$user_data = $login->check_login($_SESSION['mybook_userid'],false);

//check if not logged in
if($_SESSION['mybook_userid'] == 0){

        $obj = (object)[];
        $obj->action = "like_post";

        echo json_encode($obj);
        die;
}

/*
 $query_string = explode("?", $data->link);
 $query_string = end($query_string);

$str = explode("&", $query_string);

foreach ($str as $value) {
    # code...
    $value = explode("=", $value);
    $_GET[$value[0]] = $value[1];
}
*/

$_GET['id'] = addslashes($_GET['id']);
$_GET['type'] = addslashes($_GET['type']);

    if(isset($_GET['type']) && isset($_GET['id'])){

        $post = new Post();

        if(is_numeric($_GET['id'])){

            $allowed[] = 'post';
            $allowed[] = 'user';
            $allowed[] = 'comment';

            if(in_array($_GET['type'], $allowed)){


                $user_class = new User();
                $post->like_post($_GET['id'],$_GET['type'],$_SESSION['mybook_userid']);

                if($_GET['type'] == "user"){
                    $user_class->follow_user($_GET['id'],$_GET['type'],$_SESSION['mybook_userid']);

                }

            }

        }

        //read likes
        $likes = $post->get_likes($_GET['id'],$_GET['type']);

        //create info
        /////////////////
        $likes = array();
        $info = "";

                $i_liked = false;
                if(isset($_SESSION['mybook_userid'])){

                    $DB = new Database();

                    $sql = "select likes from likes where type='post' && contentid = '$_GET[id]' limit 1";
                    $result = $DB->read($sql);
                    if(is_array($result)){

                        $likes = json_decode($result[0]['likes'],true);

                        $user_ids = array_column($likes, "userid");

                        if(in_array($_SESSION['mybook_userid'], $user_ids)){
                            $i_liked = true;
                        }
                    }

                }

                $like_count = count($likes);

                if($like_count > 0){

                    $info .= "<br/>";

                    if($like_count == 1){

                        if($i_liked){
                            $info .= "<div style='text-align:left;'>You liked this post </div>";
                        }else{
                            $info .= "<div style='text-align:left;'> 1 person liked this post </div>";
                        }
                    }else{

                        if($i_liked){

                            $text = "others";
                            if($like_count - 1 == 1){
                                $text = "other";
                            }
                            $info .= "<div style='text-align:left;'> You and " . ($like_count - 1) . " $text liked this post </div>";
                        }else{
                            $info .= "<div style='text-align:left;'>" . $like_count . " others liked this post </div>";
                        }
                    }


                }

        /////////////////////////
        $obj = (object)[];
        $obj->likes = count($likes);
        $obj->action = "like_post";
        $obj->info = $info;
        $obj->id = "info_$_GET[id]";

        echo json_encode($obj);


    }

Answer 1

Probably it's related to a syntax error in your $_GET['id'] variable.
That's why console.log(obj) returned an empty id.

<?php echo $_GET[id]; // id is not enclosed by quotes ?>

Won't work on most servers running php7.x.
Make sure to always enclose your variable name in quotes (most of the time you did this, anyway).

<?php echo $_GET['id']; ?>

To simplify escaping, save your $_GET['id'] to a variable and sanitize it before you pass it to a sql query.
Try this:

Setting $_GET['id']
Replace null with '' – this way you save a lot of additional isset() checking eg

$_GET['id'] = isset($URL[6]) ? $URL[6] : '';
$_GET['type'] = isset($URL[5]) ? $URL[5] : '';

Saving to a sanitized variable:
You can now operate with this sanitized $_id variable instead of $_GET['id'].

$_id = $_GET['id'] ? htmlspecialchars( $_GET['id'], ENT_QUOTES) : '';
$_type = $_GET['type'] ? htmlspecialchars( $_GET['type'], ENT_QUOTES) : '';

Don't sanitize $_GET/$_POST (and other superglobals) variables by redefining them. Better leave them as the are but sanitize variables before any further usage (like echoeing, returning, passing to other functions etc.)

See also this thread: Sanitizing user's data in GET by PHP

DB query (the previous code also contained the enclosing issue):

$sql = "select likes from likes where type='post' && contentid = '$_id' limit 1";

Create your object data

$obj->id = 'info_'.$_id;

Code simplification and tweaks
As our ids and type variables are always defined (but maybe empty), you can focus on checking the values.

if( $_id && $_type ){ }

instead of

if(isset($_GET['type']) && isset($_GET['id'])){

The 'allowed array' might also cause some php warnings (depending on your php version etc.) – since you add array items to an array that hasn't been defined before. Better write it like this:

$allowed = array('post', 'user', 'comment');

JS part

if(info_element){
    info_element.innerHTML = obj.info
}

You should keep this condition as it just prevents the 'Cannot set properties of null' console.error.

The main problem should be solved by the aforementioned php fixes.