strict-event-emitter-types

What is strict-event-emitter-types?

The 'strict-event-emitter-types' npm package provides TypeScript types for strongly-typed event emitters. It allows developers to define event types and their corresponding payloads, ensuring type safety when emitting and listening to events.

What are strict-event-emitter-types's main functionalities?

Defining Typed Event Emitters

This feature allows you to define a typed event emitter with specific event names and their corresponding payload types. The code sample demonstrates how to create a custom event emitter class with typed events 'foo' and 'bar'.

const { EventEmitter } = require('events');
const { StrictEventEmitter } = require('strict-event-emitter-types');

interface Events {
  foo: (arg: string) => void;
  bar: (arg: number) => void;
}

class MyEmitter extends (EventEmitter as new () => StrictEventEmitter<EventEmitter, Events>) {}

const emitter = new MyEmitter();
emitter.on('foo', (arg) => console.log(arg));
emitter.emit('foo', 'Hello, world!');

Ensuring Type Safety

This feature ensures type safety by enforcing the correct argument types for each event. The code sample shows how TypeScript will throw an error if the argument type does not match the expected type for the event.

const { EventEmitter } = require('events');
const { StrictEventEmitter } = require('strict-event-emitter-types');

interface Events {
  foo: (arg: string) => void;
  bar: (arg: number) => void;
}

class MyEmitter extends (EventEmitter as new () => StrictEventEmitter<EventEmitter, Events>) {}

const emitter = new MyEmitter();

// TypeScript will throw an error if the argument type does not match
// emitter.emit('foo', 123); // Error: Argument of type 'number' is not assignable to parameter of type 'string'.
emitter.emit('foo', 'Hello, world!');

Typed Event Listeners

This feature allows you to define typed event listeners, ensuring that the callback functions match the expected argument types for each event. The code sample demonstrates how TypeScript will throw an error if the listener callback does not match the expected type.

const { EventEmitter } = require('events');
const { StrictEventEmitter } = require('strict-event-emitter-types');

interface Events {
  foo: (arg: string) => void;
  bar: (arg: number) => void;
}

class MyEmitter extends (EventEmitter as new () => StrictEventEmitter<EventEmitter, Events>) {}

const emitter = new MyEmitter();

// TypeScript will throw an error if the listener callback does not match the expected type
// emitter.on('foo', (arg: number) => console.log(arg)); // Error: Argument of type '(arg: number) => void' is not assignable to parameter of type '(arg: string) => void'.
emitter.on('foo', (arg: string) => console.log(arg));

Other packages similar to strict-event-emitter-types

typed-emitter

The 'typed-emitter' package provides a similar functionality by allowing you to create strongly-typed event emitters in TypeScript. It offers a more modern API and better integration with TypeScript's type system compared to 'strict-event-emitter-types'.

eventemitter3

The 'eventemitter3' package is a lightweight and fast event emitter implementation. While it does not provide built-in TypeScript type safety, it can be extended with custom TypeScript types to achieve similar functionality as 'strict-event-emitter-types'.

mitt

The 'mitt' package is a tiny functional event emitter. It is not strictly typed out of the box, but you can use TypeScript's type system to create type-safe event emitters similar to 'strict-event-emitter-types'.

README

Typed Event Emitter

NOTE: REQUIRES TYPESCRIPT 3.0

A TypeScript library for strongly typed event emitters in 0 kB. Declare events using a simple interface mapping event names to their payloads to get stricter versions of emit, on, and other common EventEmitter APIs. Works with any kind of EventEmitter.

Installation

npm i strict-event-emitter-types

Example

import StrictEventEmitter from 'strict-event-emitter-types';

// define your events
interface Events {
  request: (request: Request, response: Response) => void;
  done: void;
}

// grab an event emitter
import { EventEmitter } from 'events';

// Create strict event emitter types
let ee: StrictEventEmitter<EventEmitter, Events> = new EventEmitter;

// now enjoy your strongly typed EventEmitter API!🎉

ee.on('request', (r, resp) => ... );
// r and resp are contextually typed to Request and Response

ee.on('somethingElse');
// Error: unknown event

ee.on('done', x => x);
// Error: The 'done' event does not have a payload

ee.emit('request', new Request());
// Error: missing event payload (the response)

ee.emit('request', new Request(), false);
// Error: incorrect payload type

Usage

Event Records

Event records are interfaces or object types that map event names to the event's payload types. In the following example, three events are declared:

interface Events {
  req: (request: Request, response: Response) => void;
  done: void;
  conn: Connection;
}

Each event shows one of three ways to type the event payloads:

1. Function type: Parameters are the event payload. The return type is ignored.
2. void: A shortcut for an event with no payload, i.e. () => void
3. Anything else: A shortcut for an event with one payload, for example (p: number) => void can be written as just number.

StrictEventEmitter<TEmitterType, TEventRecord, TEmitRecord>

The default export. A generic type that takes three type parameters:

1. TEmitterType: Your EventEmitter type (e.g. node's EventEmitter or socket.io socket)
2. TEventRecord: A type mapping event names to event payloads
3. TEmitRecord: Optionally, a similar type mapping things you can emit.

The third parameter is handy when typing web sockets where client and server can listen to and emit different events. For example, if you are using socket.io:

// create types representing the server side and client
// side sockets
export type ServerSocket =
  StrictEventEmitter<SocketIO.Socket, EventsFromServer, EventsFromClient>;
export type ClientSocket =
  StrictEventEmitter<SocketIOClient.Socket, EventsFromClient, EventsFromServer>;

// elsewhere on server
let serverSocket: ServerSocket = new SocketIO.Socket();
serverSocket.on(/* only events that are sent from the client are allowed */, ...)
serverSocket.emit(/* only events that are emitted from the server are allowed */, ...)

// elsewhere on client
let clientSocket: ClientSocket = new SocketIOClient.Socket();
clientSocket.on(/* only events that are sent from the server are allowed */, ...)
clientSocket.emit(/* only events that are emitted from the client are allowed */, ...)

Usage with Subclasses

To subclass an EventEmitter you need to cast the base EventEmitter to the strict EventEmitter before extending:

type MyEmitter = StrictEventEmitter<EventEmitter, Events>;

class MyEventEmitter extends (EventEmitter as { new(): MyEmitter }) {
  doEmit() {
    this.emit(...); // strict
  }
}

StrictBroadcast<TStrictEventEmitter>

A type for a function which takes (and strictly checks) an emit event and a payload. TStrictEventEmitter is the event emitter type instantiated from StrictEventEmitter.

Useful for broadcast abstractions. It is not possible to contextually type assigments to this type, so your declarations will look something like this:

import { StrictBroadcast } from 'strict-event-emitter-types';

const broadcast: StrictBroadcast<ServerSocket> = function(
  event: string,
  payload?: any
) {
  // ...
};

Note that the loose types for event and payload only apply inside the broadcast function (consumers will see a much stricter signature). Declaring more precise parameter types or narrowing using type guards would allow strongly-typed dispatching to emitters.