Process data from mongodb and upload result to S3

I'm working on an AWS lambda function that take data from two mongodb databases, find the matching records between two collections by 'id' and calculate each record commission by fixed rate. Then export data to a xls file and upload it to s3.

connect.js

const MongoClient = require('mongodb').MongoClient;

const getAllDocument = async (dbString, dbName, collectionName) => {
    console.log('Initializing connection');

    const client = await MongoClient.connect(dbString, { useUnifiedTopology: true }).catch(err => {
        console.error(err);
    });
    if(!client){
        return;
    }
    // Get collection data
    try {
        return await client.db(dbName).collection(collectionName).find().toArray();
    } catch(err) {
        console.error(err);
    } finally {
        client.close();
    }
};

const getDocument = async(dbString, dbName, collectionName, property) => {
    const client = await MongoClient.connect(dbString, {useUnifiedTopology: true }).catch(err => {
        console.error(err);      
    });
    if(!client){
        return;
    }
    try {
        let query = { name: property };
        return await client.db(dbName).collection(collectionName).findOne(query);
    } catch(err) {
        console.error(err);
    } finally {
        client.close();
    }

};

module.exports.getAllDocument = getAllDocument;
module.exports.getDocument = getDocument;

lambda.js

const XLSX = require('xlsx');
const fs = require('fs');
const AWS = require('aws-sdk');
const getAllDocument = require("./connect.js").getAllDocument;
const getDocument = require("./connect.js").getDocument;

var bucketName = process.env.S3_BUCKET;
const db_host_seller = process.env.DB_HOST_SELLER;
const db_name_seller = process.env.DB_NAME_SELLER;
const db_host_retail = process.env.DB_HOST_RETAIL;
const db_name_retail = process.env.DB_NAME_RETAIL;

const sellerName = process.env.SELLER;
const retailerName = process.env.RETAILER;

s3 = new AWS.S3();

loadData = async () => {
    retailData = await getAllDocument(db_host_retail, db_name_retail, "retailerCollection");
    sellerData = await getAllDocument(db_host_seller, db_name_seller, "sellerCollection");
    seller = await getDocument(db_host_seller, db_name_seller, "sellerCollection", sellerName);
};


exports.handler = (event) => {
    // TODO implement
    loadData().then(() => {

        // Find retailer's rate
        let rate = seller.retailers.find(obj => {
          return obj.name === retailerName;
        }).rate;

        // Match records between seller and retailer data
        let matchedItem = [];

        for (var i in sellerData) {
          // Convert date format
          sellerData[i]["Connection Date"] = sellerData[i]["Connection Date"].toDateString()
          delete sellerData[i]['_id'];
          if (sellerData[i]["Disconnection Date"] !== "") {      
            sellerData[i]["Disconnection Date"] = sellerData[i]["Disconnection Date"].toDateString();
          }
          for( var j in retailData){
            if (sellerData[i]["id"] == retailData[j]["id"]) {    
              sellerData[i]['Commission'] = rate * sellerData[i]['TotalPayment'] ; 
              matchedItem.push(sellerData[i]);
            }
          }
        }

        // Export to xls file        
        let worksheet = XLSX.utils.json_to_sheet(matchedItem);
        let workbook = XLSX.utils.book_new();
        XLSX.utils.book_append_sheet(workbook, worksheet, 'Commission')
        XLSX.writeFile(workbook, '/tmp/Commission.xls');

        const file = fs.readFileSync('/tmp/Commission.xls');
        const params = {
            Bucket: bucketName,
            Key: 'Commission.xls', // File name to save in S3
            Body: file
        };

        // Uploading files to the bucket
        s3.upload(params, function(err, data) {
          if (err) {
              throw err;
          }
          console.log(`File uploaded to: ` + data.Location);
      });
    });
};

I would like to find out if there is any way for me to improve the code in terms of readability and performance.