lambda reading S3 Bucket files and merging Array data from multiple files to one file and storing back to S3

lambdafunction.js

/*
this was a part of a project where we were storing data from AWS IoT
and then triggering AWS lambda to datafiles and make them one single CSV file
So this might be useful later on.
*/
var AWS = require('aws-sdk');
var s3 = new AWS.S3();
var jsonexport = require('jsonexport');
var fs = require('fs');

exports.handler = (event, context, callback) => {
    // TODO implement
    console.log("event is :"+JSON.stringify(event));
    
    var src_bkt = event.Records[0].s3.bucket.name;
    var src_key = event.Records[0].s3.object.key;
    //console.log("bucket :"+JSON.stringify(src_bkt));
    //console.log("key: "+JSON.stringify(src_key));
    
    var fetchFrom;
    var ouputFilePath;
    var s3array =[];
    
    
    var data_x=[];
    var data_y=[];
    var data_z=[];
    var final_xyz=[];
    
    
    // Retrieve the object
    s3.getObject({
        Bucket: src_bkt,
        Key: src_key
    }, function(err, data) {
        if (err) {
            console.log(err, err.stack);
            callback(err);
        } else {
            fetchFrom = "Data/"+JSON.parse(data.Body).DeviceId +"/"+ JSON.parse(data.Body).connectionstamp +"/";
            ouputFilePath= "MergedCSV/"+JSON.parse(data.Body).DeviceId +"/"+ JSON.parse(data.Body).connectionstamp +"/";
        for(var i=0;i<9600;i+=128)
            {
            var file = fetchFrom+`${i}`+".json";
            //console.log("file key:"+ file);
            s3array.push(s3.getObject(
                {
                    Bucket: src_bkt,
                    Key: file
                }).promise()
            )
            }//end of loop 1
            
            
            console.log("S3 Array: "+ s3array);
        Promise.all(s3array).then(
            function (data){
                data.forEach(function(file){
                    var fileJSON = JSON.parse(file.Body);
                    console.log("File Data:"+ file.Body.toString('ascii'));
                    data_x.push.apply(data_x,fileJSON.x);
                    data_y.push.apply(data_y,fileJSON.y);
                    data_z.push.apply(data_z,fileJSON.z);
                });
                
                var len=data_x.length;
                for(i=0;i<len;i++){
                    var temp={"t":i*0.8,"x":data_x[i],"y":data_y[i],"z":data_z[i]};
                    final_xyz.push(temp);
                }
                
                
                jsonexport(final_xyz,function(err, csv){
                    if(err) return console.log(err);
                     var params = {
                         Bucket : src_bkt,
                            Key : ouputFilePath+"out.csv",
                            Body : csv
                        }
                     s3.putObject(params, function(err, data) {
                            if (err) console.log(err, err.stack); // an error occurred
                        else     console.log(data);           // successful response
                            });
                });
            }
            )
        .catch(function(err){
            console.log("catch err: "+ err);
        }
            );

        }
    });
    
//   console.log("Hi shirish here Bucket: "+event.bucket);
//   console.log("&  S3 is: "+event.bucket);
};