polly_s3.js

const writeAudioStreamToS3 = ( audioStream, filename ) =>
  putObject(aws_publicBucket, filename, audioStream,'audio/mp3').then( res => {
    if(!res.ETag) throw res
    else return {
      msg: 'File successfully generated.',
      ETag: res.ETag,
      url: `https://s3-eu-west-1.amazonaws.com/${aws_publicBucket}/${filename}`
    }
  })

const putObject = (bucket, key, body, ContentType) =>
  s3.putObject({
    Bucket: bucket,
    Key: key,
    Body: body,
    ContentType
  }).promise()