Asynchronous issue while uploading multiple files

Here is my guess. Since your calls are deferred to later, your function ends and the file list is probably erased by Node or whatever, you can see a simplified example here :

let myList = [1, 2, 3, 4, 5];

function doWork(list) {
  console.log('Start of function');
  
  for (let i = 0; i < list.length; i++) {
    (function(i) {
      setTimeout(function() {
        console.log('Working on index', i);
        console.log('list[i] :', list[i]);
      }, 100);
    })(i);
  }

  console.log('End of function');
}

doWork(myList);

myList[4] = null;

The solution is to enclose the file (or the list/index couple) itself to keep it alive in your context :

let myList = [1, 2, 3, 4, 5];

function doWork(list) {
  console.log('Start of function');
  
  for (let i = 0; i < list.length; i++) {
    (function(num) {
      setTimeout(function() {
        console.log('Num :', num);
      }, 100);
    })(list[i]);
  }

  console.log('End of function');
}

doWork(myList);

myList[4] = null;

Actually the problem is with the gridFs connection, files are uploading inside loop, the grid connection is up and writing the first file and the loop continues parallel. once connection is on from the gridfs it should close before inserting another file and have to reconnect for the next time properly.

Here is the working code check it :

var files = req.files.fileData[0];
var count = "";

if(files instanceof Array){
    count = files.length;
}
else{
    count = 1;
}

if(!fs.existsSync('./attachments')){
    console.log("Creating the attachments folder.");
    fs.mkdir('./attachments');
}   

function moveToGrid(gridArr,callback){      
    var uniqId = gridArr.uniqId, 
        filePath = gridArr.targetPath, 
        fileName = gridArr.fileName,
        fileType = gridArr.fileType;

        console.log(filePath+' '+fileName+' '+fileType + ' '+uniqId);

    mongoose.connect(configSettings.mongodb.ip, configSettings.mongodb.databasename, configSettings.mongodb.port, function (err, succ){

        var uploadResponse   = function(uniqId){
            // Delete the tmp file
            fs.unlink("./attachments/" + uniqId, function (err) {
                console.log(uniqId + ' ::temp file successfully deleted ');
            });
             mongoose.connection.close();
             console.log("=========request END time==========::::" + (new Date()).toString() + "::" +(new Date()).getMilliseconds() );
             callback();
        };
        var id = new ObjectID(uniqId);
        new GridStore(mongoose.connection.db,id,fileName, 'w',{'content_type':'application/octet-stream'}).open(function(err, gs) {
            gs.writeFile(filePath, function(err) {
                if(err){
                console.log("err : "+err);
                }
                gs.close(function(err) {
                    uploadResponse(uniqId);
                });
            });
        });
    });
}

var recursive = function(gridArr){  
    function uploader(i){
      if( i < gridArr.length ){
        moveToGrid(gridArr[i],
             function(){
              uploader(i+1)
             });
       }
       else{
        res.send("success");
       }
    }
    uploader(0); 
}

if(count > 1){
    var gridArr = [];
    for(var i=0; i<files.length; i++){
        (function(i){
            var singleFile = files[i];
                var fileData = JSON.parse(singleFile.name);
                var fileName = fileData.fileName;
                var fileType = singleFile.type;
                var uniqId   =  fileData.attachmentId;
                var targetPath =  "./attachments/" + uniqId;
                var tmp_path = singleFile.path;

                fs.readFile(tmp_path,function(err,data){
                    fs.writeFile(targetPath,data,function(err){
                        if(!err){
                            gridArr.push({"uniqId":uniqId, "targetPath":targetPath, "fileName":fileName, "fileType":fileType});
                            count --;
                            if(count == 0){
                                recursive(gridArr);
                            }
                        }
                    });
                });
        }(i));                  
    }
}
else{
    var gridArr = [];
    var singleFile = files;
    var fileData = JSON.parse(singleFile.name);
    var fileName = fileData.fileName;
    var fileType = singleFile.type;
    var uniqId   =  fileData.attachmentId;
    var targetPath =  "./attachments/" + uniqId;
    var tmp_path = singleFile.path;

    fs.readFile(tmp_path,function(err,data){
        fs.writeFile(targetPath,data,function(err){
            if(!err){
                gridArr.push({"uniqId":uniqId, "targetPath":targetPath, "fileName":fileName, "fileType":fileType});
                count --;
                if(count == 0){
                    recursive(gridArr);
                }
            }
        });
    });     
}

Using this code, we can insert multiple files to mongo gridFs.. Hope this helps!

short answer learn here how to async function inside loop

(function loop(i) {
if(i==4) return
setTimeout(function() {
  console.log('Working on index', i);
  loop(++i)
 }, 1000);
})(0);

TL;DR https://mongodb.github.io/node-mongodb-native/api-generated/gridstore.html