Node.js loop printing in wrong order

Question

I am trying to show some data from a couple of files in a directory, however there contents are printing out in the wrong order.

fs.readdir('files/sets', function(err, files){  
    for(var i = 0; i < files.length; i++){  
        console.log("_______________________________");
        readFile(files[i]);
    }
});

function readFile(file){
    csv()
    .from.path('files/training_set/'+file, {delimiter: ','})
    .transform(function(row){
        console.log(row);
    });
}

each file contains two lines:

File One:

John:
Hey there!

File Two:

Mary:
Whats up!

I would expect the output to be something like

____________________
John
Hey there!
____________________
Mary
Whats up!

But I actually get

____________________
____________________
John
Mary
Hey there! 
Whats up!

I have been searching and think its something to do with asynchronous function and the loop being to fast. But I cant seem to fix the problem. I have tried the following method, but results are the same.

fs.readdir('files/sets', function(err, files){  
    for(var i = 0; i < files.length; i++){  
        (function(j){  
            console.log("_______________________________");
            readFile(files[j]);
        })(i)
    }
});

Answer 1

readFile is an async operation and you are trying to run it synchronously without waiting for the callback to finish:

var async = require('async'); // npm install async

fs.readdir('files/sets', function(err, files){
  async.eachSeries(files,readFile,function(err){
   // more async stuff ...
  });
});

function readFile(file,callback){
    console.log("_______________________________");
    csv()
    .from.path('files/training_set/'+file, {delimiter: ','})
    .transform(function(row){
        console.log(row);
    }).on('close',function() {
      callback();
    });
}

Also, you have to run it in series if you need to wait to the previous file to finish.

Answer 2

Yes, you are seeing the result of asynchronous processing. You don't want to write a line at a time, since you will generally get the output for the files interspersed. Instead, you only want to write the entire output for a file to the console in a single call to console.log.

I think this should work:

fs.readdir('files/sets', function(err, files){
    for (var i = 0; i < files.length; i++){
        readFile(files[i]);
    }
});

function readFile(file){
    var outStr = "_______________________________";
    csv()
    .from.path('files/training_set/'+file, {delimiter: ','})
    .transform(function(row){
        outStr += '\n' + row;
    })
    .on('end', function(){
        console.log(outStr);
    });
}