boxes.js

"use strict";
const boxes = new WeakMap;

function Boxes() {
  if (!new.target) {
    return new Boxes;
  }
  boxes.set(Object.freeze(this), Object.seal({
    list: []
  }));
}

function add(id, amount) {
  boxes.get(this).list.push(Number(id), Number(amount));
}

function clear() {
  boxes.get(this).list = [];
}

function* entries() {
  const list = boxes.get(this).list;
  let i = -1;
  while(++i < list.length /* volatile */) yield [list[i], list[++i]];
}

function length() {
  return boxes.get(this).list.length >>> 1;
}

Boxes.prototype = Object.freeze(
  Object.defineProperty({
    add,
    clear,
    entries,
    [Symbol.iterator]: entries,
    constructor: Boxes
  }, "length", {
    get: length
  })
);

module.exports = Boxes;