Callback must be a function. Received undefined

fs.js:143
    throw new ERR_INVALID_CALLBACK(cb);
    ^

TypeError [ERR_INVALID_CALLBACK]: Callback must be a function. Received undefined
    at makeCallback (fs.js:143:11)
    at Object.unlink (fs.js:1024:14)
    at D:\node\littleAlbum\controller\router.js:88:16
    at IncomingForm.<anonymous> (D:\node\littleAlbum\node_modules\formidable\lib\incoming_form.js:107:9)
    at IncomingForm.emit (events.js:210:5)
    at IncomingForm._maybeEnd (D:\node\littleAlbum\node_modules\formidable\lib\incoming_form.js:557:8)
    at D:\node\littleAlbum\node_modules\formidable\lib\incoming_form.js:238:12
    at WriteStream.<anonymous> (D:\node\littleAlbum\node_modules\formidable\lib\file.js:79:5)
    at Object.onceWrapper (events.js:299:28)
    at WriteStream.emit (events.js:210:5) {
  code: 'ERR_INVALID_CALLBACK'
}

 原因:

正确应该为：