Javascript 无法读取未定义的属性-Ajax POST

我正在尝试从ajax上传数据（上传音乐文件），当我点击“上传”按钮时，它会将数据保存到我的mongodb中。然而，我不断发现fieldname是未定义的。我认为在我的代码中的某个地方，它没有拾取数据

$('#upload').on('click', function(){ 
          var formData = new FormData($('#file')[0].files[0]);

            $.ajax({
              url: 'http://localhost:3000/api/',
              method: 'POST',
              // data: {
              //    file: $('#file')[0].files[0]
              //  },
              data: formData,
              contentType: false,
              processData: false,
              mimeType: "multipart/form-data",
              success: function(data){  //if successful upon grabbing data
                console.log(data);
                console.log('Created music')
                var id = $('<p>').text("Id:" + data[i]._id);
                var title = $('<p>').text("Title:" + data[i].originalname);
                var play = $('<button>').data('Data-id', data[i]._id).text('Play').on('click', playSong); //creates edit button with donut id and carries a function editDonut in which we will define later

                var del = $('<button>').data('Data-id', data[i]._id).text('Delete').on('click', deleteMusic);  
                var container = $('<div>').attr('Data-id', data[i]._id);
                $(container).append(id, title, play, del); 
                $('body').append(container) 
                // $('#new-form').hide();  
              }
          })
        })

在Express-app.js中

app.use('/api', apiRouter);

终端中的错误

TypeError: Cannot read property 'fieldname' of undefined
    at /Users/will/pulse-express2-passport/config/apiRouter.js:72:24
    at Layer.handle [as handle_request] (/Users/will/pulse-express2-passport/node_modules/express/lib/router/layer.js:95:5)
    at next (/Users/will/pulse-express2-passport/node_modules/express/lib/router/route.js:131:13)
    at Array.<anonymous> (/Users/will/pulse-express2-passport/node_modules/multer/lib/make-middleware.js:52:37)
    at listener (/Users/will/pulse-express2-passport/node_modules/on-finished/index.js:169:15)
    at onFinish (/Users/will/pulse-express2-passport/node_modules/on-finished/index.js:100:5)
    at callback (/Users/will/pulse-express2-passport/node_modules/ee-first/index.js:55:10)
    at IncomingMessage.onevent (/Users/will/pulse-express2-passport/node_modules/ee-first/index.js:93:5)
    at emitNone (events.js:86:13)
    at IncomingMessage.emit (events.js:185:7)
    at endReadableNT (_stream_readable.js:934:12)
    at _combinedTickCallback (internal/process/next_tick.js:74:11)
    at process._tickCallback (internal/process/next_tick.js:98:9)

TypeError:无法读取未定义的属性“fieldname”
at/Users/will/pulse-express2-passport/config/apiRouter.js:72:24
在Layer.handle[作为handle_请求]（/Users/will/pulse-express2-passport/node_modules/express/lib/router/Layer.js:95:5）
下一步（/Users/will/pulse-express2-passport/node_modules/express/lib/router/route.js:131:13）
在阵列上。（/Users/will/pulse-express2-passport/node_modules/multer/lib/make middleware.js:52:37）
在侦听器上（/Users/will/pulse-express2-passport/node_modules/on finished/index.js:169:15）
在onFinish（/Users/will/pulse-express2-passport/node_modules/on finished/index.js:100:5）
回调时（/Users/will/pulse-express2-passport/node_modules/ee first/index.js:55:10）
在IncomingMessage.onevent（/Users/will/pulse-express2-passport/node_modules/ee first/index.js:93:5）
在emitNone（events.js:86:13）
在IncomingMessage.emit（events.js:185:7）
在endReadableNT（_stream_readable.js:934:12）
at _combinedTickCallback（内部/流程/下一步_tick.js:74:11）
在进程中。_tick回调（内部/process/next_tick.js:98:9）
构造函数需要一个表单，从中提取键（元素名称）和值。将表单而不是文件传递给构造函数

var formData = new FormData($('#new-form')[0]);

或者，您可以创建一个空的FormData对象，然后向其中添加值

var formData = new FormData();
formData.append('uploads', $('#file')[0].files[0]);

您是否尝试监视来自浏览器工具的请求？您是否看到ajax请求确实在提交数据？确实如此。谢谢
var formData = new FormData($('#new-form')[0]);

var formData = new FormData();
formData.append('uploads', $('#file')[0].files[0]);