使用AJAX时出现错误

Question

when i send data to server using ajax its outputting an error rather than success despite the fact that the data is being received by the server. Im just wondering what i'm doing wrong?

client side

$( "#PlotBtn" ).click(function() { 
  $.ajax({
    type: 'POST',
    url: '/data',
    dataType: 'JSON',
    data: data0,
    success: function() { 
    console.log('success'); 
    $("#graph1").attr('src', $("#graph1").attr('src') + '?' + Math.random() );
    $("#graph1").css('visibility', 'visible');},
    error: function() { console.log('error');}
    });
});

server side

    pp.post("/data", function (req, res) {
       console.log('data: ');
       var data2 = req.body;
       console.log(data2); here
});