lru-map.js

// Least Recently Used
class LRUMap extends Map {
  constructor(length) {
    super().length = length;
  }
  _(key) {
    const value = super.get(key);
    super.delete(key);
    super.set(key, value);
    return value;
  }
  has(key) {
    const result = super.has(key);
    if (result)
      this._(key);
    return result;
  }
  get(key) {
    return super.has(key) ? this._(key) : void 0;
  }
  set(key, value) {
    super.delete(this.size < this.length ? key : super.keys().next().value);
    return super.set(key, value);
  }
}

Simplified version that position only on set:

// Least Recently Updated
class LRUMap extends Map {
  constructor(length) {
    super().length = length;
  }
  set(key, value) {
    super.delete(this.size < this.length ? key : super.keys().next().value);
    return super.set(key, value);
  }
}

My thinking with the simplified version, is that you have length (capacity) .set(...) operations before a key goes away, without penalizing every .has(...) or .get(...) operation each time, so that the big class is always slow on .has and .get while the simplified one has slow path only on set, which is usually for new entries in the cache, while old entries could be either manually re-set, if needed, but won't impact performance on get and has.