javascript - 为什么 Map 构造函数不接受可迭代元组数组？

Question

我在 Javascript 中创建了一个实现 Iterable 协议(protocol)的元组类型。现在应该很容易转换为 Map:

const Tuple = (...args) => {
  const Tuple = f => f(...args);
  Tuple[Symbol.iterator] = () => args[Symbol.iterator]();
  return Tuple;
};

new Map([Tuple(1, "a"), Tuple(2, "b")]); // Map {undefined => undefined}

相反，我必须手动进行转换:

const Tuple = (...args) => {
  const Tuple = f => f(...args);
  Tuple[Symbol.iterator] = () => args[Symbol.iterator]();
  return Tuple;
};

const toArray = tx => tx((...args) => args);

const Map_ = (...pairs) => {
  return new Map(pairs.map(pair => toArray(pair)));
};

const map = Map_(Tuple(1, "a"), Tuple(2, "b"));

console.log(map.get(1), map.get(2)); // "a" "b"

似乎 Map 只接受外部复合类型的 Iterable，而不是内部对。我错过了什么？还有其他方法可以实现转换吗？

Answer 1

Map 的构造函数被定义为接受任何可迭代的键/值对象，其中键/值对象被定义为属性 0 是键的对象，属性 1 是值。它没有提到允许 Iterables 用于键/值对象。 (如果它确实允许它们并定义它会恰好调用迭代器两次，那可能会很酷，但是......那不是他们所做的。:-)而且它会更多复杂...)

如果我们看the spec ，如果它支持属性 0(对于键)和 1(对于值)，您的 Tuple 就会工作。例如:

// (Defined this way because you seemed to want a non-constructor)
const TupleMethods = {
  get "0"() {
    return this.key;
  },
  get "1"() {
    return this.value;
  }
};
const Tuple = (key, value) => {
  const t = Object.create(TupleMethods);
  t.key = key;
  t.value = value;
  return t;
}
// Usage:
const m = new Map([
  Tuple("one", "uno"),
  Tuple("two", "due"),
  Tuple("three", "tre")
]);
console.log(m.get("two")); // "due"

---

您在评论中提到了不变性。也许:

// (Defined this way because you seemed to want a non-constructor)
const TupleMethods = {};
Object.defineProperties(TupleMethods, {
    "0": {
        get() {
            return this.key;
        }
    },
    "1": {
        get() {
            return this.value;
        }
    }
});
const Tuple = (key, value) => {
    const t = Object.create(TupleMethods);
    Object.defineProperties(t, {
        key: {
            value: key
        },
        value: {
            value: value
        }
    });
    return t;
};

// Usage:
const m = new Map([
    Tuple("one", "uno"),
    Tuple("two", "due"),
    Tuple("three", "tre")
]);
console.log(m.get("two")); // "due"