并行读取多个文件，并相应地将数据写入新文件node.js [英] Reading multiple files in parallel and writing the data in new files accordingly node.js

问题描述

我尝试处理一个异步操作，从文件夹同时读取多个文件，并在不同的文件夹中写入新的文件。我读的文件是通过配对。一个文件是数据模板，另一个是关于数据。根据模板，我们处理来自相关数据文件的数据。我从这两个文件中获得的所有信息都插入到一个对象中，我需要将其写入一个新文件。如果只有一对这些文件（1个模板和1个数据），下面的代码工作完美：

I'm trying to handle a asynchronous action that read multiple files from a folder at a same time and writes new ones in a different folder. The files that I read are by pair. One file is the data template and the other one is about the data. According to the template, we process the data from the related data file. All the information that I got from the both files are inserted into an object that I need to write in JSON into a new file . The code below works perfectly if there are only one pair of these files (1 template and 1 data):

for(var i = 0; i < folderFiles.length; i++)
{
    var objectToWrite = new objectToWrite();
    var templatefileName = folderFiles[i].toString();
    //Reading the Template File
    fs.readFile(baseTemplate_dir + folderFiles[i],{ encoding: "utf8"},function(err, data)
    {
      if(err) throw err;
      //Here I'm manipulating template data

      //Now I want to read to data according to template read above
      fs.readFile(baseData_dir + folderFiles[i],{ encoding: "utf8"},function(err, data)
      {
        if(err) throw err;
        //Here I'm manipulating the data
        //Once I've got the template data and the data into my object objectToWrite, I'm writing it in json in a file
        fs.writeFile(baseOut_dir + folderFiles[i], JSON.stringify(objectToWrite ),{ encoding: 'utf8' }, function (err) 
        {
            if(err) throw err;
            console.log("File written and saved !");
        }
      }
    }
}

文件所以两个模板文件和两个相关的数据文件，它崩溃了。所以我相信我有回调的问题...也许有人可以帮助我搞清楚！提前感谢！

Since I have 4 files so two template files and two related data files, it crashed. So I believe I have a problem with the callbacks... Maybe someone could help me to figure it out ! Thanks in advance !

推荐答案

这是因为 readFile 因此对于循环不等待它被执行，并继续下一次迭代，它最终完成所有迭代真快，所以到时间 readFile 回调被执行， folderFiles [i] 将包含最后一个文件夹的名称=>所有回调将只操作这最后一个文件夹名称。解决方案是将所有这些东西移动到循环中的一个单独的函数，所以闭包将派上用场。

It is happening because readFile is asynchronous, so for loop does not wait for it to be executed and goes on with the next iteration, and it eventually finishes all iterations really fast, so by the time readFile callback is executed, folderFiles[i] will contain a name of the last folder => all callbacks will operate only this last folder name. The solution is to move all this stuff to a separate function out of the loop, so closures will come in handy.

function combineFiles(objectToWrite, templatefileName) {
  //Reading the Template File
  fs.readFile(baseTemplate_dir + templatefileName,{ encoding: "utf8"},function(err, data)
  {
    if(err) throw err;
    //Here I'm manipulating template data

    //Now I want to read to data according to template read above
    fs.readFile(baseData_dir + templatefileName,{ encoding: "utf8"},function(err, data)
    {
      if(err) throw err;
      //Here I'm manipulating the data
      //Once I've got the template data and the data into my object objectToWrite, I'm writing it in json in a file
      fs.writeFile(baseOut_dir + templatefileName, JSON.stringify(objectToWrite ),{ encoding: 'utf8' }, function (err) 
      {
          if(err) throw err;
          console.log("File written and saved !");
      }
    }
  }
}

for(var i = 0; i < folderFiles.length; i++)
{
    var objectToWrite = new objectToWrite();
    var templatefileName = folderFiles[i].toString();

    combineFiles(objectToWrite, templatefileName);
}

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