https://github.com/martinheidegger/has-listener

Change behavior of a Node.js EventEmitter depending on the existence of a listener.
https://github.com/martinheidegger/has-listener

event-emitter events nodejs observer-effect

Last synced: about 4 hours ago
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Change behavior of a Node.js EventEmitter depending on the existence of a listener.

• Host: GitHub
• URL: https://github.com/martinheidegger/has-listener
• Owner: martinheidegger
• License: mit
• Created: 2019-03-29T11:01:30.000Z (over 5 years ago)
• Default Branch: master
• Last Pushed: 2019-03-31T17:29:06.000Z (over 5 years ago)
• Last Synced: 2024-08-18T09:11:43.542Z (about 2 months ago)
• Topics: event-emitter, events, nodejs, observer-effect
• Language: JavaScript
• Homepage:
• Size: 9.77 KB
• Stars: 1
• Watchers: 3
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md
  • Changelog: CHANGELOG.md
  • Contributing: CONTRIBUTING.md
  • License: LICENSE

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README

        
# has-listener



[![JavaScript Style Guide](https://img.shields.io/badge/code_style-standard-brightgreen.svg)](https://standardjs.com)

[![Maintainability](https://api.codeclimate.com/v1/badges/fe7f58777d60b93f2e42/maintainability)](https://codeclimate.com/github/martinheidegger/has-listener/maintainability)

[![Test Coverage](https://api.codeclimate.com/v1/badges/fe7f58777d60b93f2e42/test_coverage)](https://codeclimate.com/github/martinheidegger/has-listener/test_coverage)

`has-listener` is a little util, useful in combination with [`EventEmitter`][events].

It allows to do some operation only if an event listener is added to an emitter.

[events]: https://nodejs.org/api/events.html#events_class_eventemitter

`npm i has-listener --save`

### Why

Let's embrace the [Observer Effect][observer-effect]! 👀 With this little tool you can 

let your classes do different things depending on whether there is an observer or not.

[observer-effect]: https://en.wikipedia.org/wiki/Observer_effect_(physics)

### Usage

```js

const { hasListener } = require('has-listener')

const { EventEmitter } = require('events')

const emitter = new EventEmitter()

function onHasListener (hasListener) {

  console.log(`hasListener: ${hasListener}`)

}

const stop = hasListener(emitter, 'loop', onHasListener) // hasListener: false

emitter.on('loop', () => {}) // hasListener: true

emitter.removeAllListeners() // hasListener: false

stop() // stops updating

emitter.on('loop', () => {}) // nothing, because stop was called.

```

### License

[MIT](./LICENSE)