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1.1.1 • Public • Published

Nest Emitter

Strongly 💪🏼 Typed Eventemitter Module For Nestjs Framework 🦁

Quick Overview

Ever wondered if there is a way to have a strongly typed way to use event emitter names ?

Ever wondered why your event emitter is not working as intended and then realized that there was a typo on your events name? if so, then this ones for you 😄 .


By Declaring events using a simple interface mapping event names to their payloads to get stricter versions of emit, on, and other common EventEmitter APIs.

and not only that, it will work with any kind of EventEmitter that implements NodeJS.Events.


IMPORTANT: you will need typescript 3.0+

npm install nest-emitter


yarn add nest-emitter


As Normal Import NestEmitterModule into your root module (aka AppModule)

The NestEmitterModule#forRoot(emitter: NodeJS.Events) takes any event emitter that implements NodeJS.Events.

For simplicity I will use nodejs built-in eventemitter, but of course you can use whatever you need.

// app.module.ts
import { Module } from '@nestjs/common';
import { AppController } from './app.controller';
import { AppService } from './app.service';
import { NestEmitterModule } from 'nest-emitter';
import { EventEmitter } from 'events';
  imports: [NestEmitterModule.forRoot(new EventEmitter())],
  controllers: [AppController],
  providers: [AppService],
export class AppModule {}

Now it's time to define our events, let's add two events one called notification and it's payload will be a string. and another one is newRequest and it's payload will be function that has one arg of type Request.

interface AppEvents {
  notification: string;
  // as a side note: that is equivalent to
  // newRequest: Express.Request;
  newRequest: (req: Express.Request) => void;

After that let's bring up our secret weapon; the StrictEventEmitter!

import { EventEmitter } from 'events';
import { StrictEventEmitter } from 'nest-emitter';
interface AppEvents {
  notification: string;
  newRequest: (req: Express.Request) => void;
export type MyEventEmitter = StrictEventEmitter<EventEmitter, AppEvents>;

good good, now let's use it.

👍 TIP: Keep all of your events in a separate file like {prefix}.events.ts.

I will use it to send a notification when we receive a request

// app.controller.ts
import { Get, Controller, Req } from '@nestjs/common';
import { AppService } from './app.service';
import { InjectEventEmitter } from 'nest-emitter';
import { MyEventEmitter } from '';
export class AppController {
    private readonly appService: AppService,
    @InjectEventEmitter() private readonly emitter: MyEventEmitter,
  ) {}
  root(@Req() req: Express.Request): string {
    this.emitter.emit('notification', 'new req');
    // this will throw an error at compile-time
    // as `notification` event only accepts `string`
    // this.emitter.emit('notification', 1234);
    this.emitter.emit('newRequest', req);
    return this.appService.root();

Did you notice @InjectEventEmitter()? you guessed it, it's a helper decorator to get the instance of the underlying eventemitter.

now on the other side

import { Injectable, OnModuleInit } from '@nestjs/common';
import { InjectEventEmitter } from 'nest-emitter';
import { MyEventEmitter } from '';
export class AppService implements OnModuleInit {
  constructor(@InjectEventEmitter() private readonly emitter: MyEventEmitter) {}
  onModuleInit() {
    this.emitter.on('notification', async msg => await this.onNotification(msg));
    this.emitter.on('newRequest', async req => await this.onRequest(req));
  root(): string {
    return 'Hello World!';
  private async onNotification(msg: string) {
    console.log(`OnNotification: ${msg}`);
  private async onRequest(req: Express.Request) {
    console.log(`OnRequest from: ${req['ip']}`);

And that's it! Easy? now let's dive in.

In Depth

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 AppEvents {
  req: (request: Express.Request, response: Express.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 = TEventRecord>

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

  1. TEmitterType: Your EventEmitter type (e.g. node's EventEmitter or 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

// 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 */, ...)

For more information about StrictEventEmitter see @bterlson 's library


See CHANGELOG for more information.


You are welcome to contribute to this project, just open a PR.


  • Shady Khalifa (@shekohex) - Initial work
  • Brian Terlson (@bterlson) - strict event emitter types

See also the list of contributors who participated in this project.


This project is licensed under the MIT License - see the file for details.

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