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# event-emitter
## Environment agnostic event emitter
### Installation
$ npm install event-emitter
To port it to Browser or any other (non CJS) environment, use your favorite CJS bundler. No favorite yet? Try: [Browserify](http://browserify.org/), [Webmake](https://github.com/medikoo/modules-webmake) or [Webpack](http://webpack.github.io/)
### Usage
```javascript
var ee = require('event-emitter');
var MyClass = function () { /* .. */ };
ee(MyClass.prototype); // All instances of MyClass will expose event-emitter interface
var emitter = new MyClass(), listener;
emitter.on('test', listener = function (args) {
// … react to 'test' event
});
emitter.once('test', function (args) {
// … react to first 'test' event (invoked only once!)
});
emitter.emit('test', arg1, arg2/*…args*/); // Two above listeners invoked
emitter.emit('test', arg1, arg2/*…args*/); // Only first listener invoked
emitter.off('test', listener); // Removed first listener
emitter.emit('test', arg1, arg2/*…args*/); // No listeners invoked
```
### Additional utilities
#### allOff(obj) _(event-emitter/all-off)_
Removes all listeners from given event emitter object
#### hasListeners(obj[, name]) _(event-emitter/has-listeners)_
Whether object has some listeners attached to the object.
When `name` is provided, it checks listeners for specific event name
```javascript
var emitter = ee();
var hasListeners = require('event-emitter/has-listeners');
var listener = function () {};
hasListeners(emitter); // false
emitter.on('foo', listener);
hasListeners(emitter); // true
hasListeners(emitter, 'foo'); // true
hasListeners(emitter, 'bar'); // false
emitter.off('foo', listener);
hasListeners(emitter, 'foo'); // false
```
#### pipe(source, target[, emitMethodName]) _(event-emitter/pipe)_
Pipes all events from _source_ emitter onto _target_ emitter (all events from _source_ emitter will be emitted also on _target_ emitter, but not other way).
Returns _pipe_ object which exposes `pipe.close` function. Invoke it to close configured _pipe_.
It works internally by redefinition of `emit` method, if in your interface this method is referenced differently, provide its name (or symbol) with third argument.
#### unify(emitter1, emitter2) _(event-emitter/unify)_
Unifies event handling for two objects. Events emitted on _emitter1_ would be also emitted on _emitter2_, and other way back.
Non reversible.
```javascript
var eeUnify = require('event-emitter/unify');
var emitter1 = ee(), listener1, listener3;
var emitter2 = ee(), listener2, listener4;
emitter1.on('test', listener1 = function () { });
emitter2.on('test', listener2 = function () { });
emitter1.emit('test'); // Invoked listener1
emitter2.emit('test'); // Invoked listener2
var unify = eeUnify(emitter1, emitter2);
emitter1.emit('test'); // Invoked listener1 and listener2
emitter2.emit('test'); // Invoked listener1 and listener2
emitter1.on('test', listener3 = function () { });
emitter2.on('test', listener4 = function () { });
emitter1.emit('test'); // Invoked listener1, listener2, listener3 and listener4
emitter2.emit('test'); // Invoked listener1, listener2, listener3 and listener4
```
### Tests [](https://travis-ci.org/medikoo/event-emitter)
$ npm test
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