function longPolling() {
        $.ajax({
            url:'data.json',
            type:'POST',
            dataType: "json",            
            success:function(data){
                if (data){
                    var date_len = data.length;
                    for (var i=0;i<date_len;i++){
                        var dom = '<li><p id="'+i+'"><p class="num">'+data[i].Num+'</p><p class="a">'+data[i].Time+'<p class="C">'+data[i].Description+'</p></p></p></li>';
                        var tag = '#'+i;
                        console.log(!$(tag).length);
                        if(!$(tag).length){
                            $('.tb-con ul').append(dom);
                        }
                    }
                }
            }
        });
    }
    
window.setInterval('longPolling()',1000);   

Adding data to json can be displayed immediately, but it cannot be updated immediately after deletion.