Map with Fallback: https://twitter.com/andrewingram/status/1715809443097563634

map-with-fallback.ts

const undefinedValue = Symbol();

export class MapWithFallback<Key, Value> {
  #map: Map<Key, Value>;
  #fallback: Value;
  
  constructor(fallback: Value) {
    this.#map = new Map();
    this.#fallback = fallback;
  }
  
  get(key: Key): Value {
    const value = this.#map.get(key);
    if (value === undefined) return this.#fallback;
    if (value === undefinedValue) return undefined;
    return value;
  }
  
  set(key: Key, value: Value) {
    if (value === undefined) {
      value = undefinedValue;
    }
    this.#map.set(key, value);
  }
}