如何避免node.js的异步行为？

Question

I am uploading multiple files and saving in some directory. My code is as below

app.post('/file_upload', function (req, res) {
  var msgs = '';

  req.files.forEach(function(element) {
   var fileNameToWrite = __dirname + "\\uploads\\" + element.originalname;
   fs.readFile( element.path, function (err, data) {
        fs.writeFile(fileNameToWrite, data, function (err) {
         if( err ){
              msgs += element.originalname + " Uploaded Failed Error: '"+ err +"' ";
         }
         else{
              msgs += element.originalname + " Uploaded Successfully ";
          }
       });
   });
  },this);
  console.log("Final Msgs: " + msgs);
  res.end( JSON.stringify( msgs ) );
});

The problem is msgs is populated asynchronously, I want to get msgs once forEach is complete. How can I achieve this?

Answer 1

If you really want to get rid of the asynchronous code, you can use writeFileSync and readFileSync . But it is not a good pratice.

A simple way is to use a callback like this :

app.post('/file_upload', function (req, res) {
  var msgs = '';

  function finish() {
    console.log("Final Msgs: " + msgs);
    res.end( JSON.stringify( msgs ) );
  }

  var counter = 0;

  req.files.forEach(function(element) {
   var fileNameToWrite = __dirname + "\\uploads\\" + element.originalname;
   fs.readFile( element.path, function (err, data) {
        fs.writeFile(fileNameToWrite, data, function (err) {
         if( err ){
              msgs += element.originalname + " Uploaded Failed Error: '"+ err +"' ";
         }
         else{
              msgs += element.originalname + " Uploaded Successfully ";
          }

         // We call the finish when we write the last file
         counter += 1;
         if (counter == req.files.length) {
             finish();
         }
       });
   });
  },this);
});

Answer 2

What I would go for to make the asynchronous functions not seem so asynchronous are Promises . In particular the Bluebird promise library is excellent. In essence, your function could look like this:

var Promise = require('bluebird')
var readFile = Promise.promisify(require('fs').readFile);

app.post('/file_upload', function (req, res) {
  var msgs = '';

  req.files.forEach(function(element) {
    var fileNameToWrite = __dirname + "\\uploads\\" + element.originalname;
    readFile(element.path).then(function (data) {
      return writeFile(filenameToWrite, data);
    }).then(function () {
      msgs += element.originalname + " Uploaded Successfully ";
    }).catch(function () {
      msgs += element.originalname + " Uploaded Failed Error: '"+ err +"' ";
    }).then(function () {
      console.log("Final Msgs: " + msgs);
      res.end( JSON.stringify( msgs ) );
    });
  },this);
});

This keeps all the goodiness of asynchronous calls (eg not locking the running thread), making your API quick and responsive. Still, it let's you write code "as if" if was synchronous by just chaining then .

Answer 3

You can use several approaches,

1.Use writefilesync instead of fs.writeFile , like this

req.files.forEach(function(element) {
  var fileNameToWrite = __dirname + "\\uploads\\" + element.originalname;

  try {
    fs.writeFileSync(fileNameToWrite, data);
    msgs += element.originalname + " Uploaded Successfully ";
  } catch(err) {
    msgs += element.originalname + " Uploaded Failed Error: '"+ err +"' ";
  }
}, this);

2.Or use libraries like async ( npm i async --save-dev ), like so

var async = require('async');
var msgs = '';

async.eachSeries(req.files, function (element, next) {
  var fileNameToWrite = __dirname + "\\uploads\\" + element.originalname;

  fs.readFile(element.path, function (err, data) {
    if (err) {
      msgs += element.originalname + " Uploaded Failed Error: '"+ err +"' ";
    } else {
      msgs += element.originalname + " Uploaded Successfully ";
    }

    next();
  });  
}, function () {
  console.log("Final Msgs: " + msgs);
  res.end( JSON.stringify( msgs ) );
})

Answer 4

@Nomi I would recommend to change following things which will help you to gain much performance.

1.Change your file upload from single post request to multiple post requests. I meant, you should change your front-end code to send separate post request to each file.

2.Keep a tracker/count variable in each JavaScript request. Basically, the counter value initially same as number of files. Then reduce by 1, when each file fully uploaded. And you can show the success message, when the counter value reaches 0. Probably, you can use promise for this file tracking.

3.Modify your server-side code to handle file upload request in streaming mode. You may want to see the following server-side code.

var fs = require('fs');
app.post('/file_upload', function(req, res) {
  // Read file name and extension from request header and replace 'somefile.someExtension' below.
  var fileNameToWrite = __dirname + "\\uploads\\" + 'somefile.someExtension';
  var wStream = fs.createWriteStream(fileNameToWrite);
  req.pipe(wStream);
});

Hopefully, this will give you an alternate idea.