Type safe event handlers

upload.ts

type Events = {
  start: [];
  progress: [number];
  complete: [string, boolean];
};

class Uploader {
  listeners: {
    [key in keyof Events]: ((...params: Events[key]) => void)[];
  };

  constructor() {
    this.listeners = {
      start: [],
      complete: [],
      progress: [],
    };
  }

  addEventListener<Type extends keyof Events>(
    type: Type,
    listener: (...params: Events[Type]) => void
  ) {
    this.listeners[type].push(listener);
  }

  onStart() {
    this.listeners.start.forEach((listener) => {
      listener();
    });
  }

  onProgress() {
    this.listeners.progress.forEach((listener) => {
      listener(1);
    });
  }

  onComplete() {
    this.listeners.complete.forEach((listener) => {
      listener("ok", true);
    });
  }
}