# 字典

字典就是根据键来查询值的数据结构，也就是我们常说的“键值对”。

字典数据结构在 es6 的实现中为 `Map` 。 `Map` 和 `Object` 一样是由键值对组成的，键值都不能重复。但是不同之处在于 `Object` 的键只能是字符串，且是无序的；而 `Map` 的键可以是任何类型的值，包括对象，且是有序的。

```javascript
let o = { 2: 2, 1: 1 }
Object.keys(o) // ['1', '2']  键只能是无序的字符串
let map = new Map()
map.set(o, 1)
map.set(2, 2)
for (let key of map) console.log(key) // [{…}, 1] [2, 2]  键可以是任意类型，且根据添加的顺序遍历
```

在栈和队列的实现中我们用到了 `WeakMap` 类，其实 `WeakMap WeakSet` 类几乎和 `Map Set` 类一样，只有以下少许不同：

* 只能用对象作为键
* 键是弱引用类型的
* `WeakMap WeakSet` 没有遍历器，所以不能调用 `keys()、values()、entries()`方法，也没有 `size` 属性和 `clear()` 方法，最有四个增删改查方法 `get()、set()、has()、delete()`


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://mm.ricky.moe/algorithm/algorithm-and-data-structure/shu-ju-jie-gou/zi-dian.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.