Declaring events in a TypeScript class which extends EventEmitter

Question

I have a class extends EventEmitter that can emit event hello. How can I declare the on method with specific event name and listener signature?

class MyClass extends events.EventEmitter {    emitHello(name: string): void {     this.emit('hello', name);   }    // compile error on below line   on(event: 'hello', listener: (name: string) => void): this; } 

Answer 1

Most usable way of doing this, is to use declare:

declare interface MyClass {     on(event: 'hello', listener: (name: string) => void): this;     on(event: string, listener: Function): this; }  class MyClass extends events.EventEmitter {     emitHello(name: string): void {         this.emit('hello', name);     } } 

Note that if you are exporting your class, both the interface and class have to be declared with the export keyword.

Answer 2

to extend @SergeyK's answer, with this you can get type-checking and completion on both emit and on functions without repeating event types.

1. Define event listener signatures for each event type:

interface MyClassEvents {   'add': (el: string, wasNew: boolean) => void;   'delete': (changedCount: number) => void; } 

2. Declare interface which constructs types for MyClass, based on EventListeners (MyClassEvents) function signature:

declare interface MyClass {   on<U extends keyof MyClassEvents>(     event: U, listener: MyClassEvents[U]   ): this;    emit<U extends keyof MyClassEvents>(     event: U, ...args: Parameters<MyClassEvents[U]>   ): boolean; } 

3. Simply define you class extending EventEmitter:

class MyClass extends EventEmitter {   constructor() {     super();   } } 

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Now you will get type checking for on and emit functions:

Unfortunately you will get completion and type-checking only on those two functions (unless you define more functions inside MyClass interface).

To get more generic solution, you can use this package. note: it adds no runtime overhead.

import { TypedEmitter } from 'tiny-typed-emitter';  interface MyClassEvents {   'add': (el: string, wasNew: boolean) => void;   'delete': (changedCount: number) => void; }  class MyClass extends TypedEmitter<MyClassEvents> {   constructor() {     super();   } }