• 🚀 Zero config — create().build().start() and you have a live server
• 🎯 The real thing — runs the official nats-server binary, not a mock
• 🔌 Auto free port — no collisions when test suites run in parallel
• 🔒 Checksum-verified downloads — the binary is checked against the release SHA256SUMS
• 🧰 Fluent builder with full TypeScript types
• 🌊 JetStream ready
• 🌐 Proxy aware — httpProxy / httpsProxy / noProxy
• 🧹 Clean teardown — stop() and the process is gone

npm install nats-memory-server
# or
yarn add nats-memory-server

The official nats-server binary is downloaded automatically on postinstall.

const { NatsServerBuilder } = require('nats-memory-server');
const { connect } = require('nats');

// Starts on a random free port and resolves once the server is ready
const server = await NatsServerBuilder.create().build().start();

const nc = await connect({ servers: server.getUrl() });
// ... publish / subscribe ...
await nc.close();

await server.stop();

A fuller publish/subscribe round-trip:

const { NatsServerBuilder } = require('nats-memory-server');
const { connect, StringCodec } = require('nats');

(async () => {
  // Start the server (a free port is picked automatically if none is set)
  const server = await NatsServerBuilder.create().build().start();
  console.log(`NATS server started at ${server.getUrl()}`);

  try {
    const nc = await connect({ servers: server.getUrl() });
    const sc = StringCodec();

    const sub = nc.subscribe('hello');
    (async () => {
      for await (const m of sub) {
        console.log(`[${sub.getProcessed()}]: ${sc.decode(m.data)}`);
      }
    })();

    nc.publish('hello', sc.encode('world'));

    await nc.drain(); // flush in-flight messages, then close
  } catch (err) {
    console.error(err);
  } finally {
    await server.stop();
  }
})();

💡 Runnable version: example.js.

Configuration drives two things:

1. Installation — which nats-server binary to download or build (during postinstall).
2. Runtime — how the server instance behaves (port, ip, args, …).

Provide it via any of: nats-memory-server.json · nats-memory-server.js · nats-memory-server.ts · the natsMemoryServer key in package.json.

🔒 verifyChecksum — a checksum mismatch always aborts the install (only off skips the check entirely). warn (default) additionally just warns when a checksum can't be obtained (custom downloadUrl, buildFromSource, or an unreachable SHA256SUMS); strict aborts in that case too.

{
  "version": "v2.9.16",
  "verbose": false,
  "port": 4222
}

{
  "natsMemoryServer": {
    "version": "v2.9.16",
    "port": 4222
  }
}

{
  "download": true,
  "downloadDir": "node_modules/.cache/nats-memory-server",
  "version": "v2.9.16",
  "buildFromSource": false,
  "binPath": "node_modules/.cache/nats-memory-server/nats-server",
  "verifyChecksum": "warn",
  "verbose": true,
  "ip": "127.0.0.1"
}

Fluent builder for NatsServer instances — every setter returns this, so calls chain.

Enable JetStream with setArgs (or constructor options):

const os = require('os');
const { NatsServerBuilder } = require('nats-memory-server');

await NatsServerBuilder.create()
  .setArgs(['--jetstream', '--store_dir', os.tmpdir()])
  .build()
  .start();

const os = require('os');
const { NatsServer, DEFAULT_NATS_SERVER_OPTIONS } = require('nats-memory-server');

new NatsServer({
  ...DEFAULT_NATS_SERVER_OPTIONS,
  args: ['--jetstream', '--store_dir', os.tmpdir()],
});

Spin up one server per suite — start it in beforeAll, tear it down in afterAll:

const { NatsServerBuilder } = require('nats-memory-server');
const { connect } = require('nats');

let server;
let nc;

beforeAll(async () => {
  server = await NatsServerBuilder.create().build().start();
  nc = await connect({ servers: server.getUrl() });
});

afterAll(async () => {
  await nc.close();
  await server.stop();
});

test('should publish and subscribe', async () => {
  // your test logic here
});

• Node.js ≥ 16
• Go ≥ 1.19 — optional, only needed when buildFromSource is enabled

Contributions are welcome! Found a bug or have an idea? Open an issue or a pull request.

Please follow the Code of Conduct.

MIT © Llirik1337