ES6 Map垫片如何工作 [英] How do the ES6 Map shims work

问题描述

基于我对文档的理解(此处和此处)，则需要引用内存地址才能使其正常工作:

Based on my understanding of the docs (here and here) one would need a reference to the memory address for it to work:

const foo = {};
const map = new Map();
map.set(foo,'123');  // Can only be done if memory address of `foo` is known. Any other shimming would require stringification of foo

这是因为JavaScript对象{}键只能是字符串(至少在ES5中是).

This is because JavaScript object {} keys can only be strings (at least in ES5).

但是我看到Map垫片可用: https://github.com/zloirock/core-js#map .我尝试阅读源代码，但其源头太整齐了(内部使用强集合，然后再导入10个文件)

Yet I see Map shim being available : https://github.com/zloirock/core-js#map. I tried reading the source but its too neatly abstracted (internally uses strong collection which then imports 10 more files)

请回答以下任何一项

• 是否有一个简单的技巧，它真的可以完成(不带字符串化)吗?
• 也许它会变异foo在其上存储一些字符串，然后将其用作键?
• 还有什么可能是我在阅读文档时出错了吗?

• Is there a simple trick to it and can it truly even be done (without stringification)?
• Perhaps it mutates foo to store some string on it and then uses that as the key?
• Something else and maybe I am reading the docs wrong?

推荐答案

有两种方法.首先，显然，您可以拥有一个键数组，并进行线性搜索:

There are two ways that come to mind. First, obviously, you can have an array of keys, and search it linearly:

Map1 = {
    keys: [],
    values: [],
};

Map1.set = function(key, val) {
    var k = this.keys.indexOf(key);
    if(k < 0)
        this.keys[k = this.keys.length] = key;
    this.values[k] = val;
};

Map1.get = function(key) {
    return this.values[this.keys.indexOf(key)];
};


foo = {};
bar = {};

Map1.set(foo, 'xxx');
Map1.set(bar, 'yyy');

document.write(Map1.get(foo) + Map1.get(bar) + "<br>")

第二个选项是向用作键的对象添加特殊的键"标记:

The second option is to add a special "key" marker to an object which is used as a key:

Map2 = {
    uid: 0,
    values: {}
};

Map2.set = function(key, val) {
    key = typeof key === 'object'
        ? (key.__uid = key.__uid || ++this.uid)
        : String(key);
    this.values[key] = val;
};

Map2.get = function(key) {
    key = typeof key === 'object'
        ? key.__uid
        : String(key);
    return this.values[key];
};


foo = {};
bar = {};

Map2.set(foo, 'xxx');
Map2.set(bar, 'yyy');

document.write(Map2.get(foo) + Map2.get(bar) + "<br>")

与第一个选项不同，第二个是O(1).通过将uid设置为不可写/可枚举，可以更准确地完成此操作.此外，每个Map应该具有自己的"uid"名称(可以在Map构造函数中轻松设置).

Unlike the 1st option, the second one is O(1). It can be done more accurately by making uid non-writable/enumerable. Also, each Map should have its own "uid" name (this can be easily set up in the Map constructor).

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