SlotMap from private fields

slot-map.js

/*

A map class, implemented with private slots, which has the following properties:

- The presence of a mapping k => v in a SlotMap does not by itself make k or v reachable
- For each mapping k => v in a SlotMap m (whether m is reachable or not), if k is reachable then v is reachable

*/
const DELETED = {};

function createCtor() {
  return class extends (function(x) { return x }) {
    #slot;

    constructor(key, value) {
      super(key);
      this.#slot = value;
    }

    static get(key) {
      try {
        const value = key.#slot;
        if (value !== DELETED)
          return value;
      } catch (x) {}
      return undefined;
    }

    static set(key, value) {
      try { key.#slot = value }
      catch (x) { new this(key, value) }
      return this;
    }

    static has(key) {
      try { return key.#slot !== DELETED }
      catch (x) { return false }
    }

    static delete(key) {
      try {
        if (key.#slot !== DELETED) {
          key.#slot = DELETED;
          return true;
        }
      } catch (x) {}
      return false;
    }
  };
}

class SlotMap {
  #ctor;

  constructor() { this.#ctor = createCtor() }
  get(key) { return this.#ctor.get(key) }
  set(key, value) { return this.#ctor.set(key, value) }
  delete(key) { return this.#ctor.delete(key) }
  has(key) { return this.#ctor.has(key) }
}

function time(fn, msg) {
  return (...args) => {
    let start = Date.now();
    let result = fn(...args);
    console.log(`${ msg }: ${ Date.now() - start }`);
    return result;
  };
}

function testMap(constructor) {
  let map = new constructor();
  for (let i = 0; i < 100000; ++i) {
    let obj = {};
    map.set(obj, 123);
    let value = map.get(obj);
  }
}

time(testMap, 'SlotMap')(SlotMap);
time(testMap, 'WeakMap')(WeakMap);
time(testMap, 'SlotMap')(SlotMap);
time(testMap, 'WeakMap')(WeakMap);

With node v13.5.0:

SlotMap: 1051
WeakMap: 90
SlotMap: 1375
WeakMap: 93