AJAX not Working with php as supposed

I am creating a like system in PHP . My code looks like this .

?php 
class Like_System {
    private $userid;
    private $postid;
    private $user_ko_like_count;
    private $post_ko_like_count;
    private $user_ko_dislike_count;
    private $post_ko_dislike_count;
    private $user_ip;

    public function __construct(){
    }
    public function our_ajax_script(){
        wp_enqueue_script( 'sb_like_post', get_template_directory_uri().'/data/js/post-like.min.js', false, '1.0', 1 );
        wp_localize_script( 'sb_like_post', 'ajax_var', array(
            'url' = admin_url( 'admin-ajax.php' ),
            'nonce' = wp_create_nonce( 'like_system' )
            )
        );
    }
    public function load_our_ajax_script(){
        add_action( 'init', array($this,"our_ajax_script") );
        add_action( 'wp_ajax_nopriv_like_system',array($this,"like_dislike_kernal"));
        add_action( 'wp_ajax_like_system',array($this,"like_dislike_kernal"));
    }
    public function verify_nonce($nonce){
        if ( ! wp_verify_nonce( $nonce, 'like_system' ) )
        die ();
    }
    public function like_dislike_kernal(){
        extract($_POST);
        $this-verify_nonce($nonce);

        $this-postid=$postid;
        $this-userid=get_current_user_id();
        $this-post_ko_like_count = get_post_meta( $postid, "_post_ko_like_count", true );

        if($system=="like"){
            if(is_user_logged_in()){
                $this-logged_in_user_like_kernal();
            }else{
                $this-anonymous_user_like_kernal();
            }
        }
        die();
    }
    private function make_array_if_not_exist($var){
        if(!is_array($var)){
            $var=[];
            return $var;
        }
        return $var;
    }
    private function already_liked_or_disliked($whattocheck="liked"){
        if(is_user_logged_in()){
            $post_ma_like_garney_user=$this-make_array_if_not_exist(get_post_meta( $this-postid, "_post_ma_like_garney_user", true ));
            if(in_array($this-userid,$post_ma_like_garney_user)) return true;
        }
        return false;
    }
    private function logged_in_user_like_kernal(){  
        $user_lay_like_gareko_posts=$this-make_array_if_not_exist(get_user_option( "_user_lay_like_gareko_posts", $this-userid  ));
        $post_ma_like_garney_user=$this-make_array_if_not_exist(get_post_meta( $this-postid, "_post_ma_like_garney_user", true ));

        $user_lay_like_gareko_posts["Post_ID_".$this-postid]=$this-postid;
        $post_ma_like_garney_user["User_ID_".$this-userid]=$this-userid;

        if($this-already_liked_or_disliked()==false){
            update_post_meta( $this-postid, "_post_ko_like_count", $this-post_ko_like_count + 1 ); 
            echo $this-post_ko_like_count + 1; // update count on front end
        }else{
            $ukey= array_search( $this-userid,  $post_ma_like_garney_user);
            $pkey= array_search( $this-postid, $user_lay_like_gareko_posts );

            unset( $user_lay_like_gareko_posts[$pkey] ); // remove from array
            unset( $post_ma_like_garney_user[$ukey] ); // remove from array
            update_post_meta( $this-postid, "_post_ko_like_count", --$this-post_ko_like_count );
            echo $this-post_ko_like_count; 
        }

    }

}
$like_system=new Like_System();
$like_system-load_our_ajax_script();
?

Basically, when user click like button the ajax works and this like_system class works will add like count on data base by +1

Problem Everything works fine if user click like button slowly . For instance if i click like button and it showed like count 5. When i clicked like after 5 second(suppose) it will unlike and show like count 4 and as we expect if i click the like count will show 5.Like this here is one of test where i consoled log the like counter which toogle correctly when i click button after some interval.

But supposed i clicked like button very fast say 5 times in a second . We might expect like count should toggle between 4 and 5 but it doesn't sometimes it shows -1 or -3 and sometime even 8 . Why is this happening ?

Here is the test when i click the like button very fast. Digged in to code for hours but still cannot find any problem :(

I even don't know is ajax a problem or php problem :( Thanks.