npm package: @libit/emittery
Description: Simple and modern async event emitter
Latest version: 0.10.1

# <img src="media/header.png" width="1000">

> Simple and modern async event emitter

It works in Node.js and the browser (using a bundler).

Emitting events asynchronously is important for production code where you want the least amount of synchronous
operations. Since JavaScript is single-threaded, no other code can run while doing synchronous operations. For Node.js,
that means it will block other requests, defeating the strength of the platform, which is scalability through async. In
the browser, a synchronous operation could potentially cause lags and block user interaction.

## Install

```
$ npm install @mindary/emittery
```

## Usage

```js
const {Emittery} = require('@mindary/emittery');

const emitter = new Emittery();

emitter.on('🦄', data => {
  console.log(data);
});

const myUnicorn = Symbol('🦄');

emitter.on(myUnicorn, data => {
  console.log(`Unicorns love ${data}`);
});

emitter.emit('🦄', '🌈'); // Will trigger printing '🌈'
emitter.emit(myUnicorn, '🦋'); // Will trigger printing 'Unicorns love 🦋'
```

## API

### eventName

Emittery accepts strings and symbols as event names.

Symbol event names can be used to avoid name collisions when your classes are extended, especially for internal events.

### isDebugEnabled

Toggle debug mode for all instances.

Default: `true` if the `DEBUG` environment variable is set to `emittery` or `*`, otherwise `false`.

Example:

```js
const {Emittery} = require('@mindary/emittery');

Emittery.isDebugEnabled = true;

const emitter1 = new Emittery({debug: {name: 'myEmitter1'}});
const emitter2 = new Emittery({debug: {name: 'myEmitter2'}});

emitter1.on('test', data => {
  // …
});

emitter2.on('otherTest', data => {
  // …
});

emitter1.emit('test');
//=> [16:43:20.417][emittery:subscribe][myEmitter1] Event Name: test
//	data: undefined

emitter2.emit('otherTest');
//=> [16:43:20.417][emittery:subscribe][myEmitter2] Event Name: otherTest
//	data: undefined
```

### emitter = new Emittery(options?)

Create a new instance of Emittery.

#### options?

Type: `object`

Configure the new instance of Emittery.

##### debug?

Type: `objcect`

Configure the debugging options for this instance.

###### name

Type: `string`\
Default: `undefined`

Define a name for the instance of Emittery to use when outputting debug data.

Example:

```js
const {Emittery} = require('@mindary/emittery');

Emittery.isDebugEnabled = true;

const emitter = new Emittery({debug: {name: 'myEmitter'}});

emitter.on('test', data => {
  // …
});

emitter.emit('test');
//=> [16:43:20.417][emittery:subscribe][myEmitter] Event Name: test
//	data: undefined
```

###### enabled?

Type: `boolean`\
Default: `false`

Toggle debug logging just for this instance.

Example:

```js
const {Emittery} = require('@mindary/emittery');

const emitter1 = new Emittery({debug: {name: 'emitter1', enabled: true}});
const emitter2 = new Emittery({debug: {name: 'emitter2'}});

emitter1.on('test', data => {
  // …
});

emitter2.on('test', data => {
  // …
});

emitter1.emit('test');
//=> [16:43:20.417][emittery:subscribe][emitter1] Event Name: test
//	data: undefined

emitter2.emit('test');
```

###### logger?

Type: `Function(string, string, EventName?, Record<string, any>?) => void`

Default:

```js
(type, debugName, eventName, eventData) => {
  if (typeof eventData === 'object') {
    eventData = JSON.stringify(eventData);
  }

  if (typeof eventName === 'symbol') {
    eventName = eventName.toString();
  }

  const currentTime = new Date();
  const logTime = `${currentTime.getHours()}:${currentTime.getMinutes()}:${currentTime.getSeconds()}.${currentTime.getMilliseconds()}`;
  console.log(`[${logTime}][emittery:${type}][${debugName}] Event Name: ${eventName}\n\tdata: ${eventData}`);
};
```

Function that handles debug data.

Example:

```js
const {Emittery} = require('@mindary/emittery');

const myLogger = (type, debugName, eventName, eventData) => console.log(`[${type}]: ${eventName}`);

const emitter = new Emittery({
  debug: {
    name: 'myEmitter',
    enabled: true,
    logger: myLogger,
  },
});

emitter.on('test', data => {
  // …
});

emitter.emit('test');
//=> [subscribe]: test
```

#### on(eventName | eventName[], listener)

Subscribe to one or more events.

Returns an unsubscribe method.

Using the same listener multiple times for the same event will result in only one method call per emitted event.

```js
const {Emittery} = require('@mindary/emittery');

const emitter = new Emittery();

emitter.on('🦄', data => {
  console.log(data);
});

emitter.on(['🦄', '🐶'], data => {
  console.log(data);
});

emitter.emit('🦄', '🌈'); // log => '🌈' x2
emitter.emit('🐶', '🍖'); // log => '🍖'
```

##### Custom subscribable events

Emittery exports some symbols which represent custom events that can be passed to `Emitter.on` and similar methods.

- `Emittery.listenerAdded` - Fires when an event listener was added.
- `Emittery.listenerRemoved` - Fires when an event listener was removed.

```js
const {Emittery} = require('@mindary/emittery');

const emitter = new Emittery();

emitter.on(Emittery.listenerAdded, ({listener, eventName}) => {
  console.log(listener);
  //=> data => {}

  console.log(eventName);
  //=> '🦄'
});

emitter.on('🦄', data => {
  // Handle data
});
```

###### Listener data

- `listener` - The listener that was added.
- `eventName` - The name of the event that was added or removed if `.on()` or `.off()` was used, or `undefined` if
  `.onAny()` or `.offAny()` was used.

Only events that are not of this type are able to trigger these events.

##### listener(data)

#### off(eventName | eventName[], listener)

Remove one or more event subscriptions.

```js
const {Emittery} = require('@mindary/emittery');

const emitter = new Emittery();

const listener = data => console.log(data);

(async () => {
  emitter.on(['🦄', '🐶', '🦊'], listener);
  await emitter.emit('🦄', 'a');
  await emitter.emit('🐶', 'b');
  await emitter.emit('🦊', 'c');
  emitter.off('🦄', listener);
  emitter.off(['🐶', '🦊'], listener);
  await emitter.emit('🦄', 'a'); // Nothing happens
  await emitter.emit('🐶', 'b'); // Nothing happens
  await emitter.emit('🦊', 'c'); // Nothing happens
})();
```

##### listener(data)

#### once(eventName | eventName[])

Subscribe to one or more events only once. It will be unsubscribed after the first event.

Returns a promise for the event data when `eventName` is emitted.

```js
const {Emittery} = require('@mindary/emittery');

const emitter = new Emittery();

emitter.once('🦄').then(data => {
  console.log(data);
  //=> '🌈'
});

emitter.once(['🦄', '🐶']).then(data => {
  console.log(data);
});

emitter.emit('🦄', '🌈'); // Log => '🌈' x2
emitter.emit('🐶', '🍖'); // Nothing happens
```

#### events(eventName)

Get an async iterator which buffers data each time an event is emitted.

Call `return()` on the iterator to remove the subscription.

```js
const {Emittery} = require('@mindary/emittery');

const emitter = new Emittery();
const iterator = emitter.events('🦄');

emitter.emit('🦄', '🌈1'); // Buffered
emitter.emit('🦄', '🌈2'); // Buffered

iterator
  .next()
  .then(({value, done}) => {
    // done === false
    // value === '🌈1'
    return iterator.next();
  })
  .then(({value, done}) => {
    // done === false
    // value === '🌈2'
    // Revoke subscription
    return iterator.return();
  })
  .then(({done}) => {
    // done === true
  });
```

In practice, you would usually consume the events using the
[for await](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for-await...of) statement. In
that case, to revoke the subscription simply break the loop.

```js
const {Emittery} = require('@mindary/emittery');

const emitter = new Emittery();
const iterator = emitter.events('🦄');

emitter.emit('🦄', '🌈1'); // Buffered
emitter.emit('🦄', '🌈2'); // Buffered

// In an async context.
for await (const data of iterator) {
  if (data === '🌈2') {
    break; // Revoke the subscription when we see the value '🌈2'.
  }
}
```

It accepts multiple event names.

```js
const {Emittery} = require('@mindary/emittery');

const emitter = new Emittery();
const iterator = emitter.events(['🦄', '🦊']);

emitter.emit('🦄', '🌈1'); // Buffered
emitter.emit('🦊', '🌈2'); // Buffered

iterator
  .next()
  .then(({value, done}) => {
    // done === false
    // value === '🌈1'
    return iterator.next();
  })
  .then(({value, done}) => {
    // done === false
    // value === '🌈2'
    // Revoke subscription
    return iterator.return();
  })
  .then(({done}) => {
    // done === true
  });
```

#### emit(eventName, data?)

Trigger an event asynchronously, optionally with some data. Listeners are called in the order they were added, but
executed concurrently.

Returns a promise that resolves when all the event listeners are done. _Done_ meaning executed if synchronous or
resolved when an async/promise-returning function. You usually wouldn't want to wait for this, but you could for example
catch possible errors. If any of the listeners throw/reject, the returned promise will be rejected with the error, but
the other listeners will not be affected.

#### emitSerial(eventName, data?)

Same as above, but it waits for each listener to resolve before triggering the next one. This can be useful if your
events depend on each other. Although ideally they should not. Prefer `emit()` whenever possible.

If any of the listeners throw/reject, the returned promise will be rejected with the error and the remaining listeners
will _not_ be called.

#### onAny(listener)

Subscribe to be notified about any event.

Returns a method to unsubscribe.

##### listener(eventName, data)

#### offAny(listener)

Remove an `onAny` subscription.

#### anyEvent()

Get an async iterator which buffers a tuple of an event name and data each time an event is emitted.

Call `return()` on the iterator to remove the subscription.

```js
const {Emittery} = require('@mindary/emittery');

const emitter = new Emittery();
const iterator = emitter.anyEvent();

emitter.emit('🦄', '🌈1'); // Buffered
emitter.emit('🌟', '🌈2'); // Buffered

iterator
  .next()
  .then(({value, done}) => {
    // done === false
    // value is ['🦄', '🌈1']
    return iterator.next();
  })
  .then(({value, done}) => {
    // done === false
    // value is ['🌟', '🌈2']
    // Revoke subscription
    return iterator.return();
  })
  .then(({done}) => {
    // done === true
  });
```

In the same way as for `events`, you can subscribe by using the `for await` statement

#### clearListeners(eventNames?)

Clear all event listeners on the instance.

If `eventNames` is given, only the listeners for that events are cleared.

#### listenerCount(eventNames?)

The number of listeners for the `eventNames` or all events if not specified.

#### bindMethods(target, methodNames?)

Bind the given `methodNames`, or all `Emittery` methods if `methodNames` is not defined, into the `target` object.

```js
import Emittery = require('@mindary/emittery');

const object = {};

new Emittery().bindMethods(object);

object.emit('event');
```

## TypeScript

The default `Emittery` class has generic types that can be provided by TypeScript users to strongly type the list of
events and the data passed to their event listeners.

```ts
import Emittery = require('@mindary/emittery');

const emitter =
  new Emittery<// Pass `{[eventName]: undefined | <eventArg>}` as the first type argument for events that pass data to their listeners.
  // A value of `undefined` in this map means the event listeners should expect no data, and a type other than `undefined` means the listeners will receive one argument of that type.
  {
    open: string;
    close: undefined;
  }>();

// Typechecks just fine because the data type for the `open` event is `string`.
emitter.emit('open', 'foo\n');

// Typechecks just fine because `close` is present but points to undefined in the event data type map.
emitter.emit('close');

// TS compilation error because `1` isn't assignable to `string`.
emitter.emit('open', 1);

// TS compilation error because `other` isn't defined in the event data type map.
emitter.emit('other');
```

### Emittery.mixin(emitteryPropertyName, methodNames?)

A decorator which mixins `Emittery` as property `emitteryPropertyName` and `methodNames`, or all `Emittery` methods if
`methodNames` is not defined, into the target class.

```ts
import Emittery = require('@mindary/emittery');

@Emittery.mixin('emittery')
class MyClass {}

const instance = new MyClass();

instance.emit('event');
```

## Scheduling details

Listeners are not invoked for events emitted _before_ the listener was added. Removing a listener will prevent that
listener from being invoked, even if events are in the process of being (asynchronously!) emitted. This also applies to
`.clearListeners()`, which removes all listeners. Listeners will be called in the order they were added. So-called _any_
listeners are called _after_ event-specific listeners.

Note that when using `.emitSerial()`, a slow listener will delay invocation of subsequent listeners. It's possible for
newer events to overtake older ones.

## Debugging

Emittery can collect and log debug information.

To enable this feature set the DEBUG environment variable to 'emittery' or '\*'. Additionally you can set the static
`isDebugEnabled` variable to true on the Emittery class, or `myEmitter.debug.enabled` on an instance of it for debugging
a single instance.

See [API](#api) for more details on how debugging works.

## FAQ

### How is this different than the built-in `EventEmitter` in Node.js?

There are many things to not like about `EventEmitter`: its huge API surface, synchronous event emitting, magic error
event, flawed memory leak detection. Emittery has none of that.

### Isn't `EventEmitter` synchronous for a reason?

Mostly backwards compatibility reasons. The Node.js team can't break the whole ecosystem.

It also allows silly code like this:

```js
let unicorn = false;

emitter.on('🦄', () => {
  unicorn = true;
});

emitter.emit('🦄');

console.log(unicorn);
//=> true
```

But I would argue doing that shows a deeper lack of Node.js and async comprehension and is not something we should
optimize for. The benefit of async emitting is much greater.

### Can you support multiple arguments for `emit()`?

No, just use
[destructuring](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment):

```js
emitter.on('🦄', ([foo, bar]) => {
  console.log(foo, bar);
});

emitter.emit('🦄', [foo, bar]);
```

## Related

- [p-event](https://github.com/sindresorhus/p-event) - Promisify an event by waiting for it to be emitted