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1.4.12 • Public • Published

NATS.js - Node.js Client

A Node.js client for the NATS messaging system.

license NATS.js CI npm npm npm JavaScript Style Guide


npm install nats
# to install current dev version: 
npm install nats@next

NATS.js 2.0 Preview

⚠️ We have a preview for a nats.js v2 available. Nats.js v2 is currently hosted in the v2 branch.

Version 2.0 changes existing APIs slightly, and while porting is trivial, it will require careful changes on existing code bases. A description of the changes and migration can be found here.

Nats.js 2.0 will be the underlying engine for ts-nats which provides an async/await API on top of nats.js. You can play with the nats.js v2 by npm install nats@alpha.

Basic Usage

const NATS = require('nats')
const nc = NATS.connect()
// Simple Publisher
nc.publish('foo', 'Hello World!')
// Simple Subscriber
nc.subscribe('foo', function (msg) {
  console.log('Received a message: ' + msg)
// Unsubscribing
const sid = nc.subscribe('foo', function (msg) {})
// Subscription/Request callbacks are given multiple arguments:
// - msg is the payload for the message
// - reply is an optional reply subject set by the sender (could be undefined)
// - subject is the subject the message was sent (which may be more specific
//   than the subscription subject - see "Wildcard Subscriptions".
// - finally the subscription id is the local id for the subscription
//   this is the same value returned by the subscribe call.
nc.subscribe('foo', (msg, reply, subject, sid) => {
  if (reply) {
    nc.publish(reply, 'got ' + msg + ' on ' + subject + ' in subscription id ' + sid)
  console.log('Received a message: ' + msg + " it wasn't a request.")
// Request, creates a subscription to handle any replies to the request
// subject, and publishes the request with an optional payload. This usage
// allows you to collect responses from multiple services
nc.request('request', (msg) => {
  console.log('Got a response in msg stream: ' + msg)
// Request with a max option will unsubscribe after
// the first max messages are received. You can also specify the number
// of milliseconds you are willing to wait for the response - when a timeout
// is specified, you can receive an error
nc.request('help', null, { max: 1, timeout: 1000 }, (msg) => {
  if (msg instanceof NATS.NatsError && msg.code === NATS.REQ_TIMEOUT) {
    console.log('request timed out')
  } else {
    console.log('Got a response for help: ' + msg)
// Replies
nc.subscribe('help', function (request, replyTo) {
  nc.publish(replyTo, 'I can help!')
// Close connection


The json connect property makes it easier to exchange JSON data with other clients.

const nc = NATS.connect({ json: true })
nc.on('connect', () => {
  nc.on('error', (err) => {
  nc.subscribe('greeting', (msg, reply) => {
    // msg is a parsed JSON object object
    if (msg.name && msg.reply) {
      nc.publish(reply, { greeting: 'hello ' + msg.name })
  // As with all inputs from unknown sources, if you don't trust the data
  // you should verify it prior to accessing it. While JSON is safe because
  // it doesn't export functions, it is still possible for a client to
  // cause issues to a downstream consumer that is not written carefully
  nc.subscribe('unsafe', function (msg) {
    // for example a client could inject a bogus `toString` property
    // which could cause your client to crash should you try to
    // concatenation with the `+` like this:
    // console.log("received", msg + "here");
    // `TypeError: Cannot convert object to primitive value`
    // Note that simple `console.log(msg)` is fine.
    if (Object.hasOwnProperty.call(msg, 'toString')) {
      console.log('tricky - trying to crash me:', msg.toString)
    // of course this is no different than using a value that is
    // expected in one format (say a number), but the client provides
    // a string:
    if (isNaN(msg.amount) === false) {
      // do something with the number
    // ...
  // the bad guy
  nc.publish('unsafe', { toString: 'no good' })
  nc.flush(function () {

Wildcard Subscriptions

  // "*" matches any token, at any level of the subject.
  nc.subscribe('foo.*.baz', (msg, reply, subject) => {
    console.log('Msg received on [' + subject + '] : ' + msg)
  nc.subscribe('foo.bar.*', (msg, reply, subject) => {
    console.log('Msg received on [' + subject + '] : ' + msg)
  // ">" matches any length of the tail of a subject, and can only be
  // the last token E.g. 'foo.>' will match 'foo.bar', 'foo.bar.baz',
  // 'foo.foo.bar.bax.22'
  nc.subscribe('foo.>', (msg, reply, subject) => {
    console.log('Msg received on [' + subject + '] : ' + msg)

Queue Groups

  // All subscriptions with the same queue name will form a queue group.
  // Each message will be delivered to only one subscriber per queue group,
  // queuing semantics. You can have as many queue groups as you wish.
  // Normal subscribers will continue to work as expected.
  nc.subscribe('foo', { queue: 'job.workers' }, function () {
    received += 1

Clustered Usage

const servers = ['nats://nats.io:4222', 'nats://nats.io:5222', 'nats://nats.io:6222']
// Randomly connect to a server in the cluster group.
// Note that because `url` is not specified, the default connection is called first
// (nats://localhost:4222). If you don't want default connection, specify one of
// the above the above servers as `url`: `nats.connect(servers[0], {'servers': servers});`
let nc = NATS.connect({ servers: servers })
// currentServer is the URL of the connected server.
nc.on('connect', () => {
  console.log('Connected to ' + nc.currentServer.url.host)
// Preserve order when connecting to servers.
nc = NATS.connect({ noRandomize: true, servers: servers })

Draining Connections and Subscriptions

// Unsubscribing removes the subscription handler for a subscription
// and cancels the subscription. Any pending messages on the client's
// buffer are discarded.
// Draining is similar to unsubscribe, but the client instead
// sends the unsubscribe request followed by a flush. When the flush
// returns, the subscription handler is removed. Thus the client is
// able to process all messages sent by the server before the subscription
// handler is removed.
// Draining is particularly valuable with queue subscriptions preventing
// messages from being lost.
let c1 = 0
const sid1 = nc.subscribe('foo', { queue: 'q1' }, () => {
  if (c1 === 1) {
    nc.drainSubscription(sid1, () => {
      // subscription drained - possible arguments are an error or
      // the sid (number) and subject identifying the drained
      // subscription
// It is possible to drain a connection, draining a connection:
// - drains all subscriptions
// - after calling drain it is impossible to make subscriptions or requests
// - when all subscriptions are drained, it is impossible to publish
// messages and drained connection is closed.
// - finally, the callback handler is called (with possibly an error).
let c2 = 0
nc.subscribe('foo', { queue: 'q1' }, () => {
  if (c2 === 1) {
    nc.drain(() => {
      // connection drained - possible arguments is an error
      // connection is closed by the time this function is
      // called.


const NATS = require('nats')
const fs = require('fs')
// Simple TLS connect
let nc = NATS.connect({ tls: true })
// Overriding and not verifying the server
let tlsOptions = {
  rejectUnauthorized: false
nc = NATS.connect({ tls: tlsOptions })
// nc.stream.authorized will be false
// Use a specified CA for self-signed server certificates
tlsOptions = {
  ca: [fs.readFileSync('./test/certs/ca.pem')]
nc = NATS.connect({ tls: tlsOptions })
// nc.stream.authorized should be true
// Use a client certificate if the server requires
tlsOptions = {
  key: fs.readFileSync('./test/certs/client-key.pem'),
  cert: fs.readFileSync('./test/certs/client-cert.pem'),
  ca: [fs.readFileSync('./test/certs/ca.pem')]
nc = NATS.connect({ tls: tlsOptions })

Basic Authentication

// Connect with username and password in the url
let nc = NATS.connect('nats://foo:bar@localhost:4222')
// Connect with username and password inside object
nc = NATS.connect({ url: 'nats://localhost:4222', user: 'foo', pass: 'bar' })
// Connect with token in url
nc = NATS.connect('nats://mytoken@localhost:4222')
// Connect with token inside object
nc = NATS.connect({ url: 'nats://localhost:4222', token: 'mytoken' })

New Authentication (Nkeys and User Credentials)

See examples for more usage.

const nkeys = require('ts-nkeys')
// Simple connect using credentials file. This loads JWT and signing key
// each time that NATS connects.
let nc = NATS.connect('connect.ngs.global', NATS.creds('./myid.creds'))
// Manually, you need to specify the JWT, and seed and sign the challenge
const jwt = 'eyJ0eXAiOiLN1...'
const sk = nkeys.fromSeed(Buffer.from(nkeySeed))
// Setting nkey and signing callback directly.
nc = NATS.connect('nats://localhost:4222', {
  nonceSigner: function (nonce) {
    return sk.sign(nonce)
// Setting user JWT statically.
nc = NATS.connect({
  userJWT: jwt,
  nonceSigner: function (nonce) {
    return sk.sign(nonce)
// Having user JWT be a function that returns the JWT. Can be useful for
// loading a new JWT.
nc = NATS.connect({
  userJWT: function () {
    return jwt
  nonceSigner: function (nonce) {
    return sk.sign(nonce)

Advanced Usage

// Publish with callback, callback fires when server has processed the message
nc.publish('foo', 'You done?', () => {
  console.log('msg processed!')
// Flush connection to server, callback fires when all messages have
// been processed.
nc.flush(() => {
  console.log('round trip to the server done')
// If you want to make sure NATS yields during the processing
// of messages, you can use an option to specify a yieldTime in ms.
// During the processing of the inbound stream, NATS will yield if it
// spends more than yieldTime milliseconds processing.
nc = NATS.connect({ port: 4222, yieldTime: 10 })
// Timeouts for subscriptions
let sid = NATS.subscribe('foo', () => {
  // do something
// Timeout unless a certain number of messages have been received
// the callback for the timeout. The callback for the timeout
// provides one argument, the subscription id (sid) for the
// subscription. This allows a generic callback to identify
// where the timeout triggered.
nc.timeout(sid, 1000, 1, () => {
  // do something
// Auto-unsubscribe after max messages received
sid = nc.subscribe('foo', { max: 100 })
nc.unsubscribe(sid, 100)
// Multiple connections
const nc1 = NATS.connect()
const nc2 = NATS.connect()
nc1.subscribe('foo', () => {
  // do something
// Encodings
// By default messages received will be decoded using UTF8. To change that,
// set the encoding option on the connection.
nc = NATS.connect({ encoding: 'ascii' })
// PreserveBuffers
// To prevent payload conversion from a Buffer to a string, set the
// preserveBuffers option to true. Message payload return will be a Buffer.
nc = NATS.connect({ preserveBuffers: true })
// Reconnect Attempts and Time between reconnects
// By default a NATS connection will try to reconnect to a server 10 times
// waiting 2 seconds between reconnect attempts. If the maximum number of
// retries is reached, the client will close the connection.
// To change the default behaviour specify the max number of connection
// attempts in `maxReconnectAttempts` (set to -1 to retry forever), and the
// time in milliseconds between reconnects in `reconnectTimeWait`.
nc = NATS.connect({ maxReconnectAttempts: -1, reconnectTimeWait: 250 })


The nats client is an event emitter, you can listen to several kinds of events.

// emitted whenever there's an error. if you don't implement at least
// the error handler, your program will crash if an error is emitted.
nc.on('error', (err) => {
// connect callback provides a reference to the connection as an argument
nc.on('connect', (nc) => {
  console.log(`connect to ${nc.currentServer.url.host}`)
// emitted whenever the client disconnects from a server
nc.on('disconnect', () => {
// emitted whenever the client is attempting to reconnect
nc.on('reconnecting', () => {
// emitted whenever the client reconnects
// reconnect callback provides a reference to the connection as an argument
nc.on('reconnect', (nc) => {
  console.log(`reconnect to ${nc.currentServer.url.host}`)
// emitted when the connection is closed - once a connection is closed
// the client has to create a new connection.
nc.on('close', function () {
// emitted whenever the client unsubscribes
nc.on('unsubscribe', function (sid, subject) {
  console.log('unsubscribed subscription', sid, 'for subject', subject)
// emitted whenever the server returns a permission error for
// a publish/subscription for the current user. This sort of error
// means that the client cannot subscribe and/or publish/request
// on the specific subject
nc.on('permission_error', function (err) {
  console.error('got a permissions error', err.message)

See examples and benchmarks for more information.

Connect Options

The following is the list of connection options and default values.

Option Default Description
encoding "utf8" Encoding specified by the client to encode/decode data
json false If true, message payloads are converted to/from JSON
maxPingOut 2 Max number of pings the client will allow unanswered before raising a stale connection error
maxReconnectAttempts 10 Sets the maximum number of reconnect attempts. The value of -1 specifies no limit
name Optional client name
nkey `` See NKeys/User Credentials
noEcho false Subscriptions receive messages published by the client. Requires server support (1.2.0). If set to true, and the server does not support the feature, an error with code NO_ECHO_NOT_SUPPORTED is emitted, and the connection is aborted. Note that it is possible for this error to be emitted on reconnect when the server reconnects to a server that does not support the feature.
noRandomize false If set, the order of user-specified servers is randomized.
nonceSigner `` See NKeys/User Credentials. A function that takes a Buffer and returns a nkey signed signature.
pass Sets the password for a connection
pedantic false Turns on strict subject format checks
pingInterval 120000 Number of milliseconds between client-sent pings
preserveBuffers false If true, data for a message is returned as Buffer
reconnectTimeWait 2000 If disconnected, the client will wait the specified number of milliseconds between reconnect attempts. See jitter.
reconnectJitter 100 Number of millis to randomize after reconnectTimeWait. See jitter.
reconnectJitterTLS 1000 Number of millis to randomize after reconnectTimeWait when TLS options are specified. See jitter.
reconnectDelayHandler Generated function A function that returns the number of millis to wait before the next connection to a server it connected to. See jitter.
reconnect true If false server will not attempt reconnecting
servers Array of connection urls
timeout node default - no timeout Number of milliseconds the client will wait for a connection to be established. If it fails it will emit a connection_timeout event with a NatsError that provides the hostport of the server where the connection was attempted.
tls false This property can be a boolean or an Object. If true the client requires a TLS connection. If false a non-tls connection is required. The value can also be an object specifying TLS certificate data. The properties ca, key, cert should contain the certificate file data. ca should be provided for self-signed certificates. key and cert are required for client provided certificates. rejectUnauthorized if true validates server's credentials
token Sets a authorization token for a connection
tokenHandler A function returning a token used for authentication.
url "nats://localhost:4222" Connection url
useOldRequestStyle false If set to true calls to request() and requestOne() will create an inbox subscription per call.
user Sets the username for a connection
userCreds `` See NKeys/User Credentials. Set with NATS.creds().
userJWT `` See NKeys/User Credentials. The property can be a JWT or a function that returns a JWT.
verbose false Turns on +OK protocol acknowledgements
waitOnFirstConnect false If true the server will fall back to a reconnect mode if it fails its first connection attempt.
yieldTime If set, processing will yield at least the specified number of milliseconds to IO callbacks before processing inbound messages


The settings reconnectTimeWait, reconnectJitter, reconnectJitterTLS, reconnectDelayHandler are all related. They control how long before the NATS client attempts to reconnect to a server it has previously connected.

The intention of the settings is to spread out the number of clients attempting to reconnect to a server over a period of time, and thus preventing a "Thundering Herd".

The relationship between these is:

  • If reconnectDelayHandler is specified, the client will wait the value returned by this function. No other value will be taken into account.
  • If the client specified TLS options, the client will generate a number between 0 and reconnectJitterTLS and add it to reconnectTimeWait.
  • If the client didn't specify TLS options, the client will generate a number between 0 and reconnectJitter and add it to reconnectTimeWait.


The examples, node-pub, node-sub, node-req, node-reply are now bound to bin entries on the npm package. You can use these while developing your own tools. After you install the nats npm package, you'll need to add a dependency on minimist before you can use the tools:

npm install nats
npm install minimist
% npx node-sub hello &
[1] 9208
% Listening on [hello]
% npx node-pub hello world
Received "world"
Published [hello] : "world"

Supported Node Versions

Our support policy for Nodejs versions follows Nodejs release support. We will support and build node-nats on even-numbered Nodejs versions that are current or in LTS.

Running Tests

To run the tests, you need to have a nats-server executable in your path. Refer to the server installation guide in the NATS.io documentation. With that in place, you can run npm test to run all tests.


Unless otherwise noted, the NATS source files are distributed under the Apache Version 2.0 license found in the LICENSE file.


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