@nats-io/nats-core

Core

The core module implements the core functionality for JavaScript clients:

• Connection, authentication, connection lifecycle
• NATS protocol handling - messaging functionality (publish, subscribe and request reply)

A native transports (node, deno) modules are a peer module that export a connect function which returns a concrete instance of a NatsConnection. The transport library re-exports all the functionality in this module, to make it the entry point into the NATS JavaScript ecosystem.

You can use this module as a runtime agnostic dependency and implement functionality that uses a NATS client connection without binding your code to a particular JavaScript runtime. For example, the @nats-io/jetstream library depends on @nats-io/nats-core to implement all of its JetStream protocol.

WebSocket Support

The core module also offers a for W3C Websocket transport (aka browser, Deno, and Node v22) via the exported wsconnect function. This function is semantically equivalent to the traditional connect, but returns a NatsConnection that is backed by a W3C WebSocket.

Installation

If you are not implementing a NATS client compatible module, you can use this repository to view the documentation of the NATS core functionality. Your NATS client instance already uses and re-exports the module implemented here, so there's no need for you to directly depend on this library.

Note that this module is distributed in two different registries:

• npm a node-specific library supporting CJS (require) and ESM (import) for node specific projects
• jsr a node and other ESM (import) compatible runtimes (deno, browser, node)

If your application doesn't use require, you can simply depend on the JSR version.

NPM

The NPM registry hosts a node-only compatible version of the library @nats-io/nats-core supporting both CJS and ESM:

npm install @nats-io/nats-core

JSR

The JSR registry hosts the ESM-only @nats-io/nats-core version of the library.

deno add @nats-io/nats-core

npx jsr add @nats-io/nats-core

yarn dlx jsr add @nats-io/nats-core

bunx jsr add @nats-io/nats-core

Referencing the library

Once you import the library, you can reference in your code as:

import * as nats_core from "@nats-io/nats-core";
// or in node (only when using CJS)
const nats_core = require("@nats-io/nats-core");

The main entry point for this library is the NatsConnection.

Basics

Connecting to a nats-server

To connect to a server you use the connect() exposed by your selected transport. The connect function returns a connection which implements the NatsConnection that you can use to interact with the server.

The connect() function will accept a ConnectOptions which customizes some of the client behaviors. In general all options apply to all transports where possible. Options that are non-sensical on a particular runtime will be documented by your transport module.

By default, connect() will attempt a connection on 127.0.0.1:4222. If the connection is dropped, the client will attempt to reconnect. You can customize the server you want to connect to by specifying port (for local connections), or full hostport on the servers option. Note that the servers option can be a single hostport (a string) or an array of hostports.

The example below will attempt to connect to different servers by specifying different ConnectionOptions. At least two of them should work if your internet is working.

// import the connect function from a transport
import { connect } from "jsr:@nats-io/nats-transport-deno@3.0.0-4";

const servers = [
  {},
  { servers: ["demo.nats.io:4442", "demo.nats.io:4222"] },
  { servers: "demo.nats.io:4443" },
  { port: 4222 },
  { servers: "localhost" },
];
servers.forEach(async (v) => {
  try {
    const nc = await connect(v);
    console.log(`connected to ${nc.getServer()}`);
    // this promise indicates the client closed
    const done = nc.closed();
    // do something with the connection

    // close the connection
    await nc.close();
    // check if the close was OK
    const err = await done;
    if (err) {
      console.log(`error closing:`, err);
    }
  } catch (_err) {
    console.log(`error connecting to ${JSON.stringify(v)}`);
  }
});

To disconnect from the nats-server, call close() on the connection. A connection can also be terminated when an unexpected error happens. For example, the server returns a run-time error. In those cases, the client will re-initiate a connection, it the connection options allow it.

By default, the client will always attempt to reconnect if the connection is disrupted for a reason other than calling close(). To get notified when the connection is closed, await the resolution of the Promise returned by closed(). If closed resolves to a value, the value is a NatsError indicating why the connection closed.

Publish and Subscribe

The basic NATS client operations are publish to send messages and subscribe to receive messages.

Messages are published to a subject. A subject is like a URL with the exception that it doesn't specify an actual endpoint. Subjects can be any string, but until you learn more about NATS stick to the simple rule that subjects that are just simple ASCII printable letters and number characters. All recipients that have expressed interest in a subject will receive messages addressed to that subject (provided they have access and permissions to get it). To express interest in a subject, you create a subscription.

In JavaScript clients subscriptions work as an async iterator - clients simply loop to process messages as they happen.

NATS messages are payload agnostic. Payloads are Uint8Arrays or string. Messages also provide a string() and json() that allows you to convert the underlying Uint8Array into a string or parse by using JSON.parse() (which can fail to parse if the payload is not the expected format).

To cancel a subscription and terminate your interest, you call unsubscribe() or drain() on a subscription. Unsubscribe will typically terminate regardless of whether there are messages in flight for the client. Drain ensures that all messages that are inflight are processed before canceling the subscription. Connections can also be drained as well. Draining a connection closes it, after all subscriptions have been drained and all outbound messages have been sent to the server.

// import the connect function from a transport
import { connect } from "jsr:@nats-io/nats-transport-deno@3.0.0-4";

// to create a connection to a nats-server:
const nc = await connect({ servers: "demo.nats.io:4222" });

// create a simple subscriber and iterate over messages
// matching the subscription
const sub = nc.subscribe("hello");
(async () => {
  for await (const m of sub) {
    console.log(`[${sub.getProcessed()}]: ${m.string()}`);
  }
  console.log("subscription closed");
})();

nc.publish("hello", "world");
nc.publish("hello", "again");

// we want to ensure that messages that are in flight
// get processed, so we are going to drain the
// connection. Drain is the same as close, but makes
// sure that all messages in flight get seen
// by the iterator. After calling drain,
// the connection closes.
await nc.drain();

interface Person {
  name: string;
}

// create a simple subscriber and iterate over messages
// matching the subscription
const sub = nc.subscribe("people");
(async () => {
  for await (const m of sub) {
    // typescript will see this as a Person
    const p = m.json<Person>();
    console.log(`[${sub.getProcessed()}]: ${p.name}`);
  }
})();

const p = { name: "Memo" } as Person;
nc.publish("people", JSON.stringify(p));

Wildcard Subscriptions

Subjects can be used to organize messages into hierarchies. For example, a subject may contain additional information that can be useful in providing a context to the message, such as the ID of the client that sent the message, or the region where a message originated.

Instead of subscribing to each specific subject, you can create subscriptions that have subjects with wildcards. Wildcards match one or more tokens in a subject. A token is a string following a period (.).

All subscriptions are independent. If two different subscriptions match a subject, both will get to process the message:

import { connect } from "jsr:@nats-io/nats-transport-deno@3.0.0-4";
import type { Subscription } from "jsr:@nats-io/nats-transport-deno@3.0.0-4";
const nc = await connect({ servers: "demo.nats.io:4222" });

// subscriptions can have wildcard subjects
// the '*' matches any string in the specified token position
const s1 = nc.subscribe("help.*.system");
const s2 = nc.subscribe("help.me.*");
// the '>' matches any tokens in that position or following
// '>' can only be specified at the end
const s3 = nc.subscribe("help.>");

async function printMsgs(s: Subscription) {
  const subj = s.getSubject();
  console.log(`listening for ${subj}`);
  const c = 13 - subj.length;
  const pad = "".padEnd(c);
  for await (const m of s) {
    console.log(
      `[${subj}]${pad} #${s.getProcessed()} - ${m.subject} ${
        m.data ? " " + m.string() : ""
      }`,
    );
  }
}

printMsgs(s1);
printMsgs(s2);
printMsgs(s3);

// don't exit until the client closes
await nc.closed();

Services: Request/Reply

Request/Reply is NATS equivalent to an HTTP request. To make requests you publish messages as you did before, but also specify a reply subject. The reply subject is where a service will publish (send) your response.

NATS provides syntactic sugar, for publishing requests. The request() API will generate a reply subject and manage the creation of a subscription under the covers to receive the reply. It will also start a timer to ensure that if a response is not received within your specified time, the request fails. The example also illustrates a graceful shutdown.

Services

Here's an example of a service. It is a bit more complicated than expected simply to illustrate not only how to create responses, but how the subject itself is used to dispatch different behaviors.

import { connect, Subscription } from "@nats-io/nats-deno";

// create a connection
const nc = await connect({ servers: "demo.nats.io" });

// this subscription listens for `time` requests and returns the current time
const sub = nc.subscribe("time");
(async (sub: Subscription) => {
  console.log(`listening for ${sub.getSubject()} requests...`);
  for await (const m of sub) {
    if (m.respond(sc.encode(new Date().toISOString()))) {
      console.info(`[time] handled #${sub.getProcessed()}`);
    } else {
      console.log(`[time] #${sub.getProcessed()} ignored - no reply subject`);
    }
  }
  console.log(`subscription ${sub.getSubject()} drained.`);
})(sub);

// this subscription listens for admin.uptime and admin.stop
// requests to admin.uptime returns how long the service has been running
// requests to admin.stop gracefully stop the client by draining
// the connection
const started = Date.now();
const msub = nc.subscribe("admin.*");
(async (sub: Subscription) => {
  console.log(`listening for ${sub.getSubject()} requests [uptime | stop]`);
  // it would be very good to verify the origin of the request
  // before implementing something that allows your service to be managed.
  // NATS can limit which client can send or receive on what subjects.
  for await (const m of sub) {
    const chunks = m.subject.split(".");
    console.info(`[admin] #${sub.getProcessed()} handling ${chunks[1]}`);
    switch (chunks[1]) {
      case "uptime":
        // send the number of millis since up
        m.respond(sc.encode(`${Date.now() - started}`));
        break;
      case "stop": {
        m.respond(sc.encode(`[admin] #${sub.getProcessed()} stopping....`));
        // gracefully shutdown
        nc.drain()
          .catch((err) => {
            console.log("error draining", err);
          });
        break;
      }
      default:
        console.log(
          `[admin] #${sub.getProcessed()} ignoring request for ${m.subject}`,
        );
    }
  }
  console.log(`subscription ${sub.getSubject()} drained.`);
})(msub);

// wait for the client to close here.
await nc.closed().then((err?: void | Error) => {
  let m = `connection to ${nc.getServer()} closed`;
  if (err) {
    m = `${m} with an error: ${err.message}`;
  }
  console.log(m);
});

Making Requests

Here's a simple example of a client making a simple request from the service above:

import { connect, Empty, StringCodec } from "../../src/types.ts";

// create a connection
const nc = await connect({ servers: "demo.nats.io:4222" });

// create an encoder
const sc = StringCodec();

// the client makes a request and receives a promise for a message
// by default the request times out after 1s (1000 millis) and has
// no payload.
await nc.request("time", Empty, { timeout: 1000 })
  .then((m) => {
    console.log(`got response: ${sc.decode(m.data)}`);
  })
  .catch((err) => {
    console.log(`problem with request: ${err.message}`);
  });

await nc.close();

Of course you can also use a tool like the nats cli:

> nats -s demo.nats.io req time ""
11:39:59 Sending request on "time"
11:39:59 Received with rtt 97.814458ms
2024-06-26T16:39:59.710Z

> nats -s demo.nats.io req admin.uptime ""
11:38:41 Sending request on "admin.uptime"
11:38:41 Received with rtt 99.065458ms
61688

>nats -s demo.nats.io req admin.stop ""
11:39:08 Sending request on "admin.stop"
11:39:08 Received with rtt 100.004959ms
[admin] #5 stopping....

Queue Groups

Queue groups allow scaling of services horizontally. Subscriptions for members of a queue group are treated as a single service. When you send a message to a queue group subscription, only a single client in a queue group will receive it.

There can be any number of queue groups. Each group is treated as its own independent unit. Note that non-queue subscriptions are also independent of subscriptions in a queue group.

import { connect } from "jsr:@nats-io/nats-transport-deno@3.0.0-5";
import type {
  NatsConnection,
  Subscription,
} from "jsr:@nats-io/nats-transport-deno@3.0.0-4";

async function createService(
  name: string,
  count = 1,
  queue = "",
): Promise<NatsConnection[]> {
  const conns: NatsConnection[] = [];
  for (let i = 1; i <= count; i++) {
    const n = queue ? `${name}-${i}` : name;
    const nc = await connect(
      { servers: "demo.nats.io:4222", name: `${n}` },
    );
    nc.closed()
      .then((err) => {
        if (err) {
          console.error(
            `service ${n} exited because of error: ${err.message}`,
          );
        }
      });
    // create a subscription - note the option for a queue, if set
    // any client with the same queue will be the queue group.
    const sub = nc.subscribe("echo", { queue: queue });
    const _ = handleRequest(n, sub);
    console.log(`${n} is listening for 'echo' requests...`);
    conns.push(nc);
  }
  return conns;
}

// simple handler for service requests
async function handleRequest(name: string, s: Subscription) {
  const p = 12 - name.length;
  const pad = "".padEnd(p);
  for await (const m of s) {
    // respond returns true if the message had a reply subject, thus it could respond
    if (m.respond(m.data)) {
      console.log(
        `[${name}]:${pad} #${s.getProcessed()} echoed ${m.string()}`,
      );
    } else {
      console.log(
        `[${name}]:${pad} #${s.getProcessed()} ignoring request - no reply subject`,
      );
    }
  }
}

// let's create two queue groups and a standalone subscriber
const conns: NatsConnection[] = [];
conns.push(...await createService("echo", 3, "echo"));
conns.push(...await createService("other-echo", 2, "other-echo"));
conns.push(...await createService("standalone"));

const a: Promise<void | Error>[] = [];
conns.forEach((c) => {
  a.push(c.closed());
});
await Promise.all(a);

Run it and publish a request to the subject echo to see what happens.

Advanced Usage

Headers

NATS headers are similar to HTTP headers. Headers are enabled automatically if the server supports them. Note that if you publish a message using headers but the server doesn't support them, an Error is thrown. Also note that even if you are publishing a message with a header, it is possible for the recipient to not support them.

import { connect, createInbox, Empty, headers } from "../../src/types.ts";
import { nuid } from "../../nats-base-client/nuid.ts";

const nc = await connect(
  {
    servers: `demo.nats.io`,
  },
);

const subj = createInbox();
const sub = nc.subscribe(subj);
(async () => {
  for await (const m of sub) {
    if (m.headers) {
      for (const [key, value] of m.headers) {
        console.log(`${key}=${value}`);
      }
      // reading a header is not case sensitive
      console.log("id", m.headers.get("id"));
    }
  }
})().then();

// header names can be any printable ASCII character with the  exception of `:`.
// header values can be any ASCII character except `\r` or `\n`.
// see https://www.ietf.org/rfc/rfc822.txt
const h = headers();
h.append("id", nuid.next());
h.append("unix_time", Date.now().toString());
nc.publish(subj, Empty, { headers: h });

await nc.flush();
await nc.close();

No Responders

Requests can fail for many reasons. A common reason for a failure is the lack of interest in the subject. Typically these surface as a timeout error. If the server is enabled to use headers, it will also enable a no responders feature. If you send a request for which there's no interest, the request will be immediately rejected:

import { connect, ErrorCode } from "jsr:@nats-io/nats-transport-deno@3.0.0-5";
import type { NatsError } from "jsr:@nats-io/nats-transport-deno@3.0.0-5";

const nc = await connect(
  {
    servers: `demo.nats.io`,
  },
);

try {
  const m = await nc.request("hello.world");
  console.log(m.data);
} catch (err) {
  const nerr = err as NatsError;
  switch (nerr.code) {
    case ErrorCode.NoResponders:
      console.log("no one is listening to 'hello.world'");
      break;
    case ErrorCode.Timeout:
      console.log("someone is listening but didn't respond");
      break;
    default:
      console.log("request failed", err);
  }
}

await nc.close();

Authentication

NATS supports many different forms of credentials:

• username/password
• token
• NKEYS
• client certificates
• JWTs

For user/password and token authentication, you can simply provide them as ConnectionOptions - see user, pass, token. Internally these mechanisms are implemented as an Authenticator. An Authenticator is simply a function that handles the type of authentication specified.

Setting the user/pass or token options, simply initializes an Authenticator and sets the username/password.

// if the connection requires authentication, provide `user` and `pass` or
// `token` options in the NatsConnectionOptions
import { connect } from "jsr:@nats-io/nats-transport-deno@3.0.0-5";

const nc1 = await connect({
  servers: "127.0.0.1:4222",
  user: "jenny",
  pass: "867-5309",
});
const nc2 = await connect({ port: 4222, token: "t0pS3cret!" });

Authenticators

NKEYs and JWT authentication are more complex, as they cryptographically respond to a server challenge.

Because NKEY and JWT authentication may require reading data from a file or an HTTP cookie, these forms of authentication will require a bit more from the developer to activate them. The work is related to accessing these resources varies depending on the platform.

After the credential artifacts are read, you can use one of these functions to create the authenticator. You then simply assign it to the authenticator property of the ConnectionOptions:

• nkeyAuthenticator(seed?: Uint8Array | (() => Uint8Array)): Authenticator
• jwtAuthenticator(jwt: string | (() => string), seed?: Uint8Array | (()=> Uint8Array)): Authenticator
• credsAuthenticator(creds: Uint8Array | (() => Uint8Array)): Authenticator

Note that the authenticators provide the ability to specify functions that return the desired value. This enables dynamic environments such as a browser where values accessed by fetching a value from a cookie.

Here's an example:

// read the creds file as necessary, in the case it
// is part of the code for illustration purposes (this a partial creds)
const creds = `-----BEGIN NATS USER JWT-----
    eyJ0eXAiOiJqdSDJB....
  ------END NATS USER JWT------

************************* IMPORTANT *************************
  NKEY Seed printed below can be used sign and prove identity.
  NKEYs are sensitive and should be treated as secrets.

  -----BEGIN USER NKEY SEED-----
    SUAIBDPBAUTW....
  ------END USER NKEY SEED------
`;

const nc = await connect(
  {
    port: 4222,
    authenticator: credsAuthenticator(new TextEncoder().encode(creds)),
  },
);

Flush

Flush sends a PING protocol message to the server. When the server responds with PONG you are guaranteed that all pending data was sent and received by the server. Note ping() effectively adds a server round-trip. All NATS clients handle their buffering optimally, so ping(): Promise<void> shouldn't be used except in cases where you are writing some sort of test, as you may be degrading the performance of the client.

nc.publish("foo");
nc.publish("bar");
await nc.flush();

PublishOptions

When you publish a message you can specify some options:

• reply - this is a subject to receive a reply (you must set up a subscription on the reply subject) before you publish.
• headers - a set of headers to decorate the message.

SubscriptionOptions

You can specify several options when creating a subscription:

• max: maximum number of messages to receive - auto unsubscribe
• timeout: how long to wait for the first message
• queue: the queue group name the subscriber belongs to
• callback: a function with the signature (err: NatsError|null, msg: Msg) => void; that should be used for handling the message. Subscriptions with callbacks are NOT iterators.

Auto Unsubscribe

// subscriptions can auto unsubscribe after a certain number of messages
nc.subscribe("foo", { max: 10 });

Timeout Subscriptions

// create subscription with a timeout, if no message arrives
// within the timeout, the subscription throws a timeout error
const sub = nc.subscribe("hello", { timeout: 1000 });
(async () => {
  for await (const _m of sub) {
    // handle the messages
  }
})().catch((err) => {
  if (err.code === ErrorCode.Timeout) {
    console.log(`sub timed out!`);
  } else {
    console.log(`sub iterator got an error!`);
  }
  nc.close();
});

RequestOptions

When making a request, there are several options you can pass:

• timeout: how long to wait for the response
• headers: optional headers to include with the message
• noMux: create a new subscription to handle the request. Normally a shared subscription is used to receive response messages.
• reply: optional subject where the reply should be sent.

noMux and reply

Under the hood, the request API simply uses a wildcard subscription to handle all requests you send.

In some cases, the default subscription strategy doesn't work correctly. For example, a client may be constrained by the subjects where it can receive replies.

When noMux is set to true, the client will create a normal subscription for receiving the response to a generated inbox subject before the request is published. The reply option can be used to override the generated inbox subject with an application provided one. Note that setting reply requires noMux to be true:

const m = await nc.request(
  "q",
  Empty,
  { reply: "bar", noMux: true, timeout: 1000 },
);

Draining Connections and Subscriptions

Draining provides for a graceful way to unsubscribe or close a connection without losing messages that have already been dispatched to the client.

You can drain a subscription or all subscriptions in a connection.

When you drain a subscription, the client sends an unsubscribe protocol message to the server followed by a flush. The subscription handler is only removed after the server responds. Thus all pending messages for the subscription have been processed.

Draining a connection, drains all subscriptions. However when you drain the connection it becomes impossible to make new subscriptions or send new requests. After the last subscription is drained it also becomes impossible to publish a message. These restrictions do not exist when just draining a subscription.

Lifecycle/Informational Events

Clients can get notification on various event types:

• Events.DISCONNECT
• Events.RECONNECT
• Events.UPDATE
• Events.LDM
• Events.ERROR

The first two fire when a client disconnects and reconnects respectively. The payload will be the server where the event took place.

The UPDATE event notifies whenever the client receives a cluster configuration update. The ServersChanged interface provides two arrays: added and deleted listing the servers that were added or removed.

The LDM event notifies that the current server has signaled that it is running in Lame Duck Mode and will evict clients. Depending on the server configuration policy, the client may want to initiate an ordered shutdown, and initiate a new connection to a different server in the cluster.

The ERROR event notifies you of async errors that couldn't be routed in a more precise way to your client. For example, permission errors for a subscription or request, will properly be reported by the subscription or request. However, permission errors on publish will be reported via the status mechanism.

const nc = await connect(opts);
(async () => {
  console.info(`connected ${nc.getServer()}`);
  for await (const s of nc.status()) {
    console.info(`${s.type}: ${s.data}`);
  }
})().then();

nc.closed()
  .then((err) => {
    console.log(
      `connection closed ${err ? " with error: " + err.message : ""}`,
    );
  });

Be aware that when a client closes, you will need to wait for the closed() promise to resolve. When it resolves, the client is done and will not reconnect.

Async vs. Callbacks

Previous versions of the JavaScript NATS clients specified callbacks for message processing. This required complex handling logic when a service required coordination of operations. Callbacks are an inversion of control anti-pattern.

The async APIs trivialize complex coordination and makes your code easier to maintain. With that said, there are some implications:

• Async subscriptions buffer inbound messages.
• Subscription processing delays until the runtime executes the promise related microtasks at the end of an event loop.

In a traditional callback-based library, I/O happens after all data yielded by a read in the current event loop completes processing. This means that callbacks are invoked as part of processing. With async, the processing is queued in a microtask queue. At the end of the event loop, the runtime processes the microtasks, which in turn resumes your functions. As expected, this increases latency, but also provides additional liveliness.

To reduce async latency, the NATS client allows processing a subscription in the same event loop that dispatched the message. Simply specify a callback in the subscription options. The signature for a callback is (err: (NatsError|null), msg: Msg) => void. When specified, the subscription iterator will never yield a message, as the callback will intercept all messages.

Note that callback likely shouldn't even be documented, as likely it is a workaround to an underlying application problem where you should be considering a different strategy to horizontally scale your application, or reduce pressure on the clients, such as using queue workers, or more explicitly targeting messages. With that said, there are many situations where using callbacks can be more performant or appropriate.

Connection Options

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

Option	Default	Description
authenticator	none	Specifies the authenticator function that sets the client credentials.
debug	false	If true, the client prints protocol interactions to the console. Useful for debugging.
ignoreClusterUpdates	false	If true the client will ignore any cluster updates provided by the server.
ignoreAuthErrorAbort	false	Prevents client connection aborts if the client fails more than twice in a row with an authentication error
inboxPrefix	"_INBOX"	Sets de prefix for automatically created inboxes - createInbox(prefix)
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 - recommended to be set to a unique client name.
noAsyncTraces	false	When true the client will not add additional context to errors associated with request operations. Setting this option to true will greatly improve performance of request/reply and JetStream publishers.
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.
noResolve	none	If true, client will not resolve host names.
pass		Sets the password for a connection.
pedantic	false	Turns on strict subject format checks.
pingInterval	120000	Number of milliseconds between client-sent pings.
port	4222	Port to connect to (only used if servers is not specified).
reconnect	true	If false, client will not attempt reconnecting.
reconnectDelayHandler	Generated function	A function that returns the number of millis to wait before the next connection to a server it connected to ()=>number.
reconnectJitter	100	Number of millis to randomize after reconnectTimeWait.
reconnectJitterTLS	1000	Number of millis to randomize after reconnectTimeWait when TLS options are specified.
reconnectTimeWait	2000	If disconnected, the client will wait the specified number of milliseconds between reconnect attempts.
servers	"localhost:4222"	String or Array of hostport for servers.
timeout	20000	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	TlsOptions	A configuration object for requiring a TLS connection (not applicable to nats.ws).
token		Sets a authorization token for a connection.
user		Sets the username for a connection.
verbose	false	Turns on +OK protocol acknowledgements.
waitOnFirstConnect	false	If true the client will fall back to a reconnect mode if it fails its first connection attempt.

TlsOptions

Option	Default	Description
ca	N/A	CA certificate
caFile		CA certificate filepath
cert	N/A	Client certificate
certFile	N/A	Client certificate file path
key	N/A	Client key
keyFile	N/A	Client key file path
handshakeFirst	false	Connects to the server directly as TLS rather than upgrade the connection. Note that the server must be configured accordingly.

In some Node and Deno clients, having the option set to an empty option, requires the client have a secured connection.

Jitter

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.