在 fs.createReadStream 之后没有执行代码块

Question

我正在尝试读取上传的.csv 文件并将其存储在一个数组中，以便将数据批量插入到我的数据库中，我使用csv-parser 包和fs 用于文件流。

router.post('/fileupload', function(request, response, next){
var bulk_emp_data = [];
if(request.files){
  var sampleFile = request.files.filename;
  console.log(sampleFile.name);
  sampleFile.mv('C:\\Users\\QPS-AUDRICK\\Desktop\\QBOS Timekeeping\\qbos\\public\\temp\\'+sampleFile.name, function(err){
    if(err){
      console.log('Error moving ' + err);
    }else{
      fs.createReadStream('C:\\Users\\QPS-AUDRICK\\Desktop\\QBOS Timekeeping\\qbos\\public\\temp\\'+sampleFile.name)
        .pipe(csv())
        .on('data', (data) => results.push(data))
        .on('end', () => {
          console.log(results);
          bulk_emp_data = results;
          console.log('CSV file successfully processed');
          console.log(bulk_emp_data.length);
        });
        console.log('sample output');
    }
  });
}

var data = {
    success: 1,
    msg: 'Successfully parsed the sample file.',
    data: bulk_emp_data.length
};

response.json(data);
});

通过fs.createReadStream 读取文件后，我的console.log 似乎没有被执行。这是终端的最后两行。

CSV file successfully processed
63

此外，bulk_emp_data 变量似乎已被清空。这是response的结果

{"success":1,"msg":"Successfully parsed the sample file.","data":0}

请注意，终端上的最后一行是 63，而它应该是 console.log()，而在 response 上，bulk_emp_data.length 是 0

Answer 1

存在一些执行顺序的误解。

通过 fs.createReadStream 读取文件后，我的 console.log 似乎没有被执行。

实际上console.log 是在使用fs.createReadStream() 启动流后立即执行的。读取流启动，两个on() 仅在流本身触发事件data 或end 时执行，因此您应该在任何'CSV file successfully processed' 之前看到'sample output'。

bulk_emp_data.length 也是在流结束之前打印出来的，所以它是零（理论上它可以是某个值，但实际上永远不是真正的值）。

在您的代码上添加一些注释以更清晰：

router.post('/fileupload', function (request, response, next) {
  var bulk_emp_data = [];
  if (request.files) {
    var sampleFile = request.files.filename;
    console.log(sampleFile.name);

    // sync operation - the execution "waits" here
    sampleFile.mv('C:\\Users\\QPS-AUDRICK\\Desktop\\QBOS Timekeeping\\qbos\\public\\temp\\' + sampleFile.name, function (err) {
      if (err) {
        console.log('Error moving ' + err);
      } else {

        // async operation
        // start a stream and attach some event listener
        fs.createReadStream('C:\\Users\\QPS-AUDRICK\\Desktop\\QBOS Timekeeping\\qbos\\public\\temp\\' + sampleFile.name)
          .pipe(csv())
          // executed when the stream fires the 'data' event
          .on('data', (data) => results.push(data))
          // executed when the stream fires the 'end' event
          .on('end', () => {
            console.log(results);
            bulk_emp_data = results;
            console.log('CSV file successfully processed');
            console.log(bulk_emp_data.length);
          });

        // executed after starting the stream (and pratically before any event that the stream can fire)
        console.log('sample output');
      }
    });
  }

  // executed after "console.log('sample output')"  (and pratically before any event that the stream can fire)
  var data = {
    success: 1,
    msg: 'Successfully parsed the sample file.',
    data: bulk_emp_data.length
  };

  // executed pratically before any event that the stream can fire
  response.json(data);
});

执行顺序回顾：

1. sampleFile.mv()
2. fs.createReadStream()
3. console.log('样本输出')
4. var 数据 = { ... }
5. response.json(数据)
6. fs.createReadStream().on('data') X 次
7. fs.createReadStream().on('end') 1 次

解决方案：您应该将点 3 4 5 放入 .on('end') 处理程序中，例如

.on('end', () => {
  console.log(results);
  bulk_emp_data = results;
  console.log('CSV file successfully processed');
  console.log(bulk_emp_data.length);

  console.log('sample output');

  var data = {
    success: 1,
    msg: 'Successfully parsed the sample file.',
    data: bulk_emp_data.length
  };

  response.json(data);
});