const AWS = require('aws-sdk');

AWS.config.logger = console;

const s3 = new AWS.S3();

exports.handler = async (event) => {
  return await getObject({
    Bucket: "<MY-BUCKET>",
    Key: "<MY-KEY>"
  }).then( (res) => {
    console.log('Retrieved object from S3');
    console.log(res);
    return res.Body.toString('ascii');
  })
};

async function getObject(params){
  return await s3.getObject(params).promise();
}

var requestObject = s3.getObject(params);

requestObject.on('httpHeaders', (statusCode, headers, response, statusMessage) => {
    // your code here.
});

requestObject.promise()
    .then(response => { ... })