KeyedCollection.js

class KeyedCollection extends Map {
  map(fn) {
    const mappedEntries = Array.from(this, ([k, v]) => [k, fn(v, k, this)]);
    return new this.constructor(mappedEntries);
  }
  
  mapKeys(fn) {
    const mappedEntries = Array.from(this, ([k, v]) => [fn(k, v, this), v]);
    return new this.constructor(mappedEntries);
  }
}