A Map class with built-in event emitter that emits a `update` event whenever something changes

ObservableMap.ts

class ObservableMap<K, V> extends Map<K, V> {
  readonly events = new EventEmitter();
  set(key: K, value: V) {
    let previousValue = this.get(key);
    super.set(key, value);
    this.events.emit("update", {
      key,
      action: "set",
      previousValue,
      currentValue: this.get(key)
    });
    return this;
  }
  delete(key: K) {
    let previousValue = this.get(key);
    let ret = super.delete(key);
    this.events.emit("update", {
      key,
      action: "delete",
      previousValue
    });
    return ret;
  }
  clear() {
    this.events.emit("update", {
      action: "clear"
    });
    super.clear();
    return this;
  }
}

// test it out ....
let map = new ObservableMap<string, number>();
map.events.on("update", console.log);
map.set("a", 1);
map.set("a", 2);