eenext

EventEmitter for next generation, which is well-typed in TypeScript and supports Promise and AsyncIterator

Install

NPM:

$ npm install eenext

Yarn:

$ yarn add eenext

NOTE: this package uses setImmediate, so you maybe need to use core-js or some setImmediate shim providers to run this on browsers.

Features

1. The best™ typed EventEmitter in TypeScript
2. Promise and AsyncIterator support
3. Async emit

The best™ typed EventEmitter in TypeScript

You can pass Events type parameter to EventEmitter.

import EventEmitter from 'eenext';

// `Events` type: it is mapper of event name to type.
interface FooEvents {
  foo: string;
  bar: void;
}

const ee = new EventEmitter<FooEvents>();

// It can be compiled because `'foo'` is known event name in `FooEvents`:
ee.on('foo', value => {
  console.log(`foo: ${value}`);
});

// It can be compiled too:
ee.on('bar', () => {
  console.log('bar');
});

// But it cannot be compiled because `'baz'` is not known:
// ee.on('baz', () => {
//   console.log('baz');
// });

ee.emit('foo', 'value');
ee.emit('bar'); // You can omit a value when event type is `void`.

// Also they cannot be compiled:
// ee.emit('foo'); // value is needed.
// ee.emit('foo', 42); // value type is not matched.
// ee.emit('baz', 'value'); // unknown event type.

NOTE: Currently eenext does not support two or more arguments event due to TypeScript limitation.

Promise and AsyncIterator support

on method without event listener returns AsyncIterator, and once returns Promise.

// `FooEvents` is defined in above example.

// Pass option to set `'bar'` as end event of stream returned by `on` method.
const ee = new EventEmitter<FooEvents>({end: 'bar'});

(async () => {
  // Wait `'foo'` event.
  const foo1st = await ee.once('foo');
  console.log(`foo 1st: ${foo1st}`);

  // Show `'foo'` event value until `'bar'` event emitted.
  for await (const foo of ee.on('foo')) {
    console.log(`foo: ${foo}`);
  }
})();

// Emit events.
ee.emit('foo', '1st');
ee.emit('foo', '2nd');
ee.emit('foo', '3rd');
ee.emit('bar');

Async emit

emit method is asynchronous action.

It returns a promise to wait for real emitting.

const ee = new EventEmitter<FooEvents>();

(async () => {
  // Pass async function as event listener:
  ee.once('bar', async () => {
    console.log('before bar');
    await new Promise(resolve => setTimeout(resolve, 1000));
    console.log('after bar');
  });

  // Wait 1sec to go.
  console.log('before await');
  await ee.emit('bar');
  console.log('after await');

  // Handle error in event listener.
  ee.once('bar', async () => {
    throw new Error('error');
  });

  try {
    await ee.emit('bar');
  } catch (err) {
    console.error(err); // => Error: error
  }
})();

API

import EventEmitter from 'eenext';

import {
  Options,
  StreamOptions,
  WaitOptions,
  Listener,
  EventStream,
  VoidKeys,
  NonVoidKeys,
} from 'eenext';

class EventEmitter<Events = any>

An event emitter class.

Type Parameters:

• Events: event names and types.

constructor(opts: Options<Events>)

Create a new EventEmitter instance with given options.

Arguments:

• opts: a options.

on<K extends keyof Events>(name: K, listener: Listener<Events[K]>): void

Add event listener.

Arguments:

• name: an event name.
• listener: an event listener.

on<K extends keyof Events>(name: K): EventStream<Events[K]>

Return an event stream. Is is same as this.stream(name).

Arguments:

• name: an event name.

once<K extends keyof Events>(name: K, listener: Listener<Events[K]>): void

Add event listener invoked only once,

Arguments:

• name: an event name.
• listener: an event listener.

once<K extends keyof Events>(name: K): Promise<Events[K]>

Return an event promise. It is same as this.wait(name).

Arguments:

• name: an event name.

off<K extends keyof Events>(name: K, listener: Listener<Events[K]>): void

Remove event listener.

Arguments:

• name: an event name.
• listener: an event listener.

stream<K extends Events>(name: K, opts: StreamOptions<Events>): EventStream<Events[K]>

Create an event stream.

Arguments:

• next: an event name.
• opts: options.

wait<K extends Events>(name: K, opts: WaitOptions<Events>): EventStream<Events[K]>

Create an event promise.

Arguments:

• name: an event name.
• opts: options.

emit<K extends VoidKeys<Events>>(name: K, value?: Events[K]): Promise<void>

Emit an event with or without the given value.

Arguments:

• name: an event name.
• value: a value to emit.

emit<K extends NonVoidKeys<Events>>(name: K, value: Events[K]): Promise<void>

Emit an event with the given value.

Arguments:

• name: an event name.
• value: a value to emit.

interface Options<Events>

A type of EventEmitter options.

Type Parameters:

• Events: event names and types.

Properties:

• end: an event name to detect ending (optional).
• error: an event name to detect an error (optional).
• maxBufferSize: maximum buffer size of stream (optional).

type StreamOptions<Events>

A type of EventEmitter#stream options.

Type Parameters:

• Events: event names and types.

type StreamOptions<Events> = Partial<Pick<Options<Events>, 'end' | 'error' | 'maxBufferSize'>>;

type WaitOptions<Events>

A type of EventEmitter#wait options.

Type Parameters:

• Events: event names and types.

Definition:

type WaitOptions<Events> = Partial<Pick<Options<Events>, 'error'>>;

type Lisrener<T>

A type of event listener function.

Type Parameters:

• T: an event type.

Definition:

type Listener<T> = (value: T) => void | Promise<void>;

interface EventStream<T>

A type of event stream. It implements AsyncIterator<T> and AsyncIterable<T>.

Type Parameters:

• T: a value type of this stream.

Methods:

• next(): Promise<IteratorResult<T>>
• return(value?: any): Promise<IteratorResult<T>>
• throw(reason?: any): Promise<IteratorResult<T>>
• [Symbol.asyncIterator](): AsyncIterator<T>

type VoidKeys<T>

Like keyof T but it only includes void typed keys.

Type Parameters:

• T: a type.

type NonVoidKeys<T>

Like keyof T but it excludes void typed keys.

Type Parameters:

• T: a type.

License

MIT License.