Weak maps and weak sets in terms of private symbols

weak-collections.js

const _wmSymbol = Symbol.private("wmSymbol")
const _wsSymbol = Symbol.private("wsSymbol")
const call = Function.call.bind(Function.call)
const hasOwn = Function.call.bind({}.hasOwnProperty)
const getOwn = Object.getOwnPropertyDescriptor
const TE = TypeError
const checkMapThisKey = makeChecker("WeakMap", "weak map", _wmSymbol)
const checkSetThisKey = makeChecker("WeakSet", "weak set", _wsSymbol)

function makeChecker(name, human, symbol) {
  return (inst, key, method) => {
    if (!hasOwn(inst, _symbol)) {
      throw new TE(
        "`" + name + ".prototype." + method +
        "` called on incompatible receiver"
      )
    }
    if (typeof key !== "object") {
      throw new TE("Invalid value used as " + human + " key")
    }
  }
}

class WeakMap {
  constructor(iter) {
    this[_wmSymbol] = Symbol.private()
    const set = this.set
    for (const [key, value] of iter) call(set, this, key, value)
  }

  delete(key) {
    checkMapThisKey(this, key, "delete")
    return delete key[this[_wmSymbol]]
  }

  get(key) {
    checkMapThisKey(this, key, "get")
    const desc = getOwn(key, this[_wmSymbol])
    return desc != null ? desc.value : void 0
  }

  has(key) {
    checkMapThisKey(this, key, "has")
    return hasOwn(key, this[_wmSymbol])
  }

  set(key, value) {
    checkMapThisKey(this, key, "set")
    key[this[_wmSymbol]] = value
    return this
  }
}

class WeakSet {
  constructor(iter) {
    this[_wsSymbol] = Symbol.private()
    const set = this.set
    for (const item of iter) call(set, this, item)
  }

  delete(key) {
    checkSetThisKey(this, key, "delete")
    return delete key[this[_wsSymbol]]
  }

  has(key) {
    checkSetThisKey(this, key, "has")
    return hasOwn(key, this[_wsSymbol])
  }

  add(key) {
    checkSetThisKey(this, key, "add")
    key[this[_wsSymbol]] = true
    return this
  }
}