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 s

Answer 1

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(
    event: U, listener: MyClassEvents[U]
  ): this;

  emit(
    event: U, ...args: Parameters
  ): boolean;
}

3. Simply define you class extending EventEmitter:

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

---

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 {
  constructor() {
    super();
  }
}