# Emitter

  Event emitter component.

## Installation

```
$ component install component/emitter
```

## API

### Emitter(obj)

  The `Emitter` may also be used as a mixin. For example
  a "plain" object may become an emitter, or you may
  extend an existing prototype:

```js
var obj = {};
Emitter(obj);
Emitter(User.prototype);
```

  __Warning__: if you use `Emitter(Some.prototype)` on a prototype
  you __must__ invoke `Emitter.call(this)` in the constructor to
  clear the callbacks object, otherwise events will be shared
  between multiple objects.
  
### Emitter#on(event, fn)

  Register an `event` handler `fn`.

### Emitter#once(event, fn)

  Register a single-shot `event` handler `fn`,
  removed immediately after it is invoked the
  first time.

### Emitter#off(event, fn)

  Remove `event` handler `fn`, or pass only the `event`
  name to remove all handlers for `event`.

### Emitter#emit(event, ...)

  Emit an `event` with variable option args.

### Emitter#listeners(event)

  Return an array of callbacks, or an empty array.

### Emitter#hasListeners(event)

  Check if this emitter has `event` handlers.