Stolen from DaniD in Mirth Connect Slack, "After looking at too many javadocs, I've put together code that uses the V2 sdk to get a list of keys in a bucket, via Javascript"

Use AWS v2 SDK to call S3 buckets from Mirth.js

var clientBuilder = new Packages.software.amazon.awssdk.services.s3.S3Client.builder();

// Set the AWS account Access Key, AWS account Secret Key
var awsCreds = new Packages.software.amazon.awssdk.auth.credentials.AwsBasicCredentials.create(access_key, secret_key);
var awsCredProvider = new Packages.software.amazon.awssdk.auth.credentials.StaticCredentialsProvider.create(awsCreds)

var east_region = Packages.software.amazon.awssdk.regions.Region.US_EAST_1

// build s3client with options
var s3client = clientBuilder
		.credentialsProvider(awsCredProvider)
		.region(east_region)
		.build();

// I stole this from the mirth github
function createListRequest(bucketName, prefix, delim) {
	return new Packages.software.amazon.awssdk.services.s3.model.ListObjectsV2Request.builder().bucket(bucketName).prefix(prefix).delimiter(delim);
}

// get a result-object
result = s3client.listObjectsV2(createListRequest(bucket_name,prefix_name, '/').build())
// get an array of result-objects
result_items = result.contents();

for(i = 0; i < result_items.length; i++){
	logger.info(result_items[i].key());
}