S3 Mock, using In-Memory key->val store

S3InMemoryClient.js

const co = require('co');
const stream = require('stream');


function S3InMemoryClient() {
	var memFs = {};

	const client = {
		upload: (opts, cb) => co(function*() {
			const data = opts.Body instanceof stream.Readable ?
				yield readStream(opts.Body) : opts.Body;
			const key = [opts.Bucket, opts.Key].join('/');

			memFs[key] = data;
		}).then(() => cb(), err => cb(err)),

		getObject: (opts) => ({
			createReadStream: () => {
				const key = [opts.Bucket, opts.Key].join('/');
				const buf = memFs[key];

				return buf ? ContentStream(buf) : ErrorStream(new Error(`${key} was not mocked`));
			}
		}),

		listObjects: (s3Opts, cb) => {
			const fsPrefix = [s3Opts.Bucket, s3Opts.Prefix].join('/');

			const matchingKeys = Object.keys(memFs)
				.filter(fsKey => fsKey.startsWith(fsPrefix))
				.map(fsKey => fsKey.slice(s3Opts.Bucket.length + 1));

			cb(null, {
				Contents: matchingKeys.map(Key => ({ Key }))
			});
		}
	};

	return client;
}


function ContentStream(buffer) {
  const mockStream = new stream.PassThrough();
  setTimeout(() => mockStream.end(buffer));
  return mockStream;
}

function ErrorStream(err) {
  const mockStream = new stream.PassThrough();
  setTimeout(() => mockStream.emit('error', err), 10);
  return mockStream;
}

/**
 * @param {stream.Readable} stream
 * @returns {Promise<Buffer>}
 */
function readStream(stream) {
  return co(function*() {
    var buf = new Buffer(0);
    yield cb => stream
      .on('data', data => buf = Buffer.concat([buf, data]))
      .on('error', cb)
      .on('end', cb);

    return buf;
  })
}


module.exports = S3InMemoryClient;

Note that this currently only supports returning a ReadableStream from getObject,
but does not support returning a ReadableStream from listObjects

Hence, why I haven't gone through the trouble of make a fill npm package out of it.