temp.js

const getPartsAssemblerStream = s3 => bucket => key => uploadID => {
    const parts = []
    return new stream.Writable({
        objectMode: true,
        write(part, encoding, callback) {    
                                                                                                                                                                                                          
            if(getOr(false, 'isLast', part) === false) {
                parts.push(part)
                callback()
                return true
            }
            debug("Done, uploading all...")
            parts.push(pick(['ETag', 'PartNumber'], part))
            debug("parts: %O", parts)
            s3.completeMultipartUpload({
                Bucket: bucket,
                Key: key,
                UploadId: uploadID,
                MultipartUpload: {
                    Parts: sortBy(['PartNumber'], parts)
                }
            })
            .promise()
            .then(
                result => {
                    debug("final upload result: %O", result)
                    callback(undefined, result)
                }
            )
            .catch(
                error =>
                debug("error completing: %O", error) ||
                    callback(error)
            )
            return false
        },
    })
}