PubSub

Javascript implementation of the Publish/Subscribe pattern.

Install

$3

``sh
$ npm install PubSub
`

Usage

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

$3

`js
import PubSub from 'PubSub';
`

$3

`js
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;
`

$3

`html

`

API

* PubSub
* [new PubSub([options])](#new_PubSub_new)
* _instance_
* [.subscribe(topic, callback, [once])](#PubSub+subscribe) ⇒ number
* .subscribeOnce(topic, callback) ⇒ number
* [.publish(topic, [...data])](#PubSub+publish) ⇒ boolean
* [.publishSync(topic, [...data])](#PubSub+publishSync) ⇒ boolean
* .unsubscribe(topic) ⇒ boolean \| string
* .unsubscribeAll() ⇒ PubSub
* [.hasSubscribers([topic])](#PubSub+hasSubscribers) ⇒ boolean
* .subscribers() ⇒ object
* .subscribersByTopic(topic) ⇒ array
* .alias(aliasMap) ⇒ PubSub
* _static_
* [.createInstance([options])](#PubSub.createInstance) ⇒ PubSub

$3

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

$3

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
`js
const pubsub = new PubSub();

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


$3

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
`js
const pubsub = new PubSub();

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


$3


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
`js
const pubsub = new PubSub();

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


$3

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
`js
const pubsub = new PubSub();

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


$3

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
`js
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);
`


$3

Clears all subscriptions whatsoever.

Kind: instance method of PubSub
Returns: PubSub - The PubSub instance.
Example
`js
const pubsub = new PubSub();

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


$3


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
`js
const pubsub = new PubSub();

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

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


$3

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
`js
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] }
`


$3

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
`js
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 []
`


$3

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

Static methods

$3


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
`js
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)