PubSub

Javascript implementation of the Publish/Subscribe pattern.

Install

npm

$ npm install PubSub

Usage

The library is exported in UMD, CommonJS, and ESM formats. You can import it the following ways:

Using ESM import statement

import PubSub from 'PubSub';

Using CommonJS require statement

const PubSub = require('PubSub');

// If you use a bundler like Webpack, you may need to import it the following way 
// as it might try to use the ESM module instead of the CommonJS.
const PubSub = require('PubSub').default; 

Old school browser global

<script src="https://unpkg.com/PubSub"></script>

API

• PubSub
  • new PubSub([options])
  • instance
    • .subscribe(topic, callback, [once]) ⇒ number
    • .subscribeOnce(topic, callback) ⇒ number
    • .publish(topic, [...data]) ⇒ boolean
    • .publishSync(topic, [...data]) ⇒ boolean
    • .unsubscribe(topic) ⇒ boolean | string
    • .unsubscribeAll() ⇒ PubSub
    • .hasSubscribers([topic]) ⇒ boolean
    • .subscribers() ⇒ object
    • .subscribersByTopic(topic) ⇒ array
    • .alias(aliasMap) ⇒ PubSub
  • static
    • .createInstance([options]) ⇒ PubSub

new PubSub([options])

Creates a PubSub instance.

Available options

Param	Type	Default	Description
immediateExceptions1	boolean	false	Force immediate exceptions (instead of delayed exceptions).

¹ Before version 3.6.0 PubSub would fail to deliver your topics to all subscribers if one or more failed (see issue #4). As of version 3.6.0 PubSub handles this by delaying thrown exceptions by default. You can set immediateExceptions to true or any truthy value in order to maintain the stack trace for development reasons but this is not recommended for production.

Public Methods

subscribe(topic, callback, [once]) ⇒ number

Subscribe to events of interest with a specific topic name and a callback function, to be executed when the topic/event is observed.

Kind: instance method of PubSub
Returns: number - The topic's token

Param	Type	Default	Description
topic	string		The topic's name
callback	function		Callback function to execute on event, taking two arguments: - {*} data The data passed when publishing an event - {object} The topic's info (name & token)
[once]	boolean	false	Checks if event will be triggered only one time

Example

const pubsub = new PubSub();

const onUserAdd = pubsub.subscribe('user_add', (data, topic) => {
  console.log('User added');
  console.log('user data:', data);
});

subscribeOnce(topic, callback) ⇒ number

Subscribe to events of interest setting a flag indicating the event will be published only one time.

Kind: instance method of PubSub
Returns: number - The topic's token

Param	Type	Description
topic	string	The topic's name
callback	function	Callback function to execute on event, taking two arguments: - {*} data The data passed when publishing an event - {object} The topic's info (name & token)

Example

const pubsub = new PubSub();

const onUserAdd = pubsub.subscribeOnce('user_add', (data, topic) => {
  console.log('User added');
  console.log('user data:', data);
});

publish(topic, [data]) ⇒ boolean

Publishes a topic asynchronously, passing the data to its subscribers.
Asynchronous publication helps in that the originator of the topics will not be blocked while consumers process them.
For synchronous topic publication check publishSync.

Kind: instance method of PubSub
Returns: boolean - Returns true if topic exists and event is published; otheriwse false

Param	Type	Description
topic	string	The topic's name
[data]	...*	The data to be passed to its subscribers

Example

const pubsub = new PubSub();

pubsub.publish('user_add', {
  firstName: 'John',
  lastName: 'Doe',
  email: 'johndoe@gmail.com'
});

publishSync(topic, [data]) ⇒ boolean

Publishes a topic synchronously, passing the data to its subscribers.

Kind: instance method of PubSub
Returns: boolean - Returns true if topic exists and event is published; otheriwse false

Param	Type	Description
topic	string	The topic's name
[data]	...*	The data to be passed to its subscribers

Example

const pubsub = new PubSub();

pubsub.publishSync('user_add', {
  firstName: 'John',
  lastName: 'Doe',
  email: 'johndoe@gmail.com'
});

unsubscribe(topic) ⇒ boolean | string

Unsubscribes from a specific topic, based on the topic name, or based on a tokenized reference to the subscription.

Kind: instance method of PubSub
Returns: boolean | string - Returns false if topic does not match a subscribed event; otherwise the topic's name

Param	Type	Description
topic	string | number	Topic's name or subscription reference

Example

const pubsub = new PubSub();

// Unsubscribe using the topic's name.
pubsub.unsubscribe('user_add');

// Unsubscribe using a tokenized reference to the subscription.
pubsub.unsubscribe(onUserAdd);

unsubscribeAll() ⇒ PubSub

Clears all subscriptions whatsoever.

Kind: instance method of PubSub
Returns: PubSub - The PubSub instance.
Example

const pubsub = new PubSub();

pubsub.subscribe('message1', () => {});
pubsub.subscribe('message2', () => {});
pubsub.subscribe('message3', () => {});
pubsub.unsubscribeAll();
pubsub.hasSubscribers(); // -> false

hasSubscribers([topic]) ⇒ boolean

Checks if there are subscribers for a specific topic. If topic is not provided, checks if there is at least one subscriber.

Kind: instance method of PubSub
Returns: boolean - Returns true there are subscribers; otherwise false

Param	Type	Description
[topic]	string	The topic's name to check

Example

const pubsub = new PubSub();

pubsub.on('message', data => console.log(data));

pubsub.hasSubscribers('message');
// -> true

subscribers() ⇒ object

Gets all the subscribers as a set of key value pairs that represent the topic's name and the event listener(s) bound.

Kind: instance method of PubSub
Returns: object - A readonly object with all subscribers.
Note: Mutating the result of this method does not affect the real subscribers. This is for reference only.
Example

const pubsub = new PubSub();

pubsub.subscribe('message', listener);
pubsub.subscribe('message', listener);
pubsub.subscribe('another_message', listener);

pubsub.subscribers();
// -> Object { message: Array[2], another_message: Array[1] }

subscribersByTopic(topic) ⇒ array

Gets subscribers for a specific topic.

Kind: instance method of PubSub
Returns: array - A copy array of all subscribers for a topic if exist; otherwise an empty array
Note: Mutating the result of this method does not affect the real subscribers. This is for reference only.

Param	Type	Description
topic	String	The topic's name to check for subscribers

Example

const pubsub = new PubSub();

pubsub.subscribe('message', listener1);
pubsub.subscribeOnce('message', listener2);
pubsub.subscribe('another_message', listener1);

pubsub.subscribersByTopic('message');
// -> Array [{token: 0, once: false, callback: listener1()}, {token: 1, once: true, callback: listener2()}]

pubsub.subscribersByTopic('another_message');
// -> Array [{token: 2, once: false, callback: listener1()}]

pubsub.subscribersByTopic('some_message_not_existing');
// -> Array []

alias(aliasMap) ⇒ PubSub

Creates aliases for public methods.

Kind: instance method of PubSub
Returns: PubSub - The PubSub instance.

Param	Type	Description
aliasMap	Object	A plain object that maps the public methods to their aliases.

Example

const pubsub = new PubSub().alias({
  subscribe: 'on',
  subscribeOnce: 'once',
  publish: 'trigger',
  publishSync: 'triggerSync',
  unsubscribe: 'off',
  hasSubscribers: 'has'
});

Static methods

PubSub.createInstance([options]) ⇒ PubSub

Creates a PubSub instance. This is an alternative way to create a new instance if you don't prefer using the new keyword.

Kind: static method of PubSub
Returns: PubSub - The PubSub constructor.
Example

const pubsub = PubSub.createInstance();

Changelog

For API updates and breaking changes, check the CHANGELOG.

More about Publish/Subscribe pattern

• The Observer Pattern - Addy Osmani
• Publish–Subscribe pattern - Wikipedia

License

The MIT License (MIT)