immutable-tuple/src/universal-weak-map.js

Immutable finite list objects with constant-time equality testing (===) and no memory leaks

import { isObjRef } from "./util.js";

// A map data structure that holds object keys weakly, yet can also hold
// non-object keys, unlike the native `WeakMap`.
export class UniversalWeakMap {
  constructor() {
    // Since a `WeakMap` cannot hold primitive values as keys, we need a
    // backup `Map` instance to hold primitive keys. Both `this._weakMap`
    // and `this._strongMap` are lazily initialized.
    this._weakMap = null;
    this._strongMap = null;
    this.data = null;
  }

  // Since `get` and `set` are the only methods used, that's all I've
  // implemented here.

  get(key) {
    const map = this._getMap(key, false);
    if (map) {
      return map.get(key);
    }
  }

  set(key, value) {
    this._getMap(key, true).set(key, value);
    // An actual `Map` or `WeakMap` would return `this` here, but
    // returning the `value` is more convenient for the `tuple`
    // implementation.
    return value;
  }

  _getMap(key, canCreate) {
    if (! canCreate) {
      return isObjRef(key) ? this._weakMap : this._strongMap;
    }
    if (isObjRef(key)) {
      return this._weakMap || (this._weakMap = new WeakMap);
    }
    return this._strongMap || (this._strongMap = new Map);
  }
}