> For the complete documentation index, see [llms.txt](https://lizh.gitbook.io/knowledge/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://lizh.gitbook.io/knowledge/javascript-xi-lie/00-jin-jie-zhi-shi.md).

# 进阶知识

### Object.keys 能保证返回的对象属性的顺序？

**简单回复： 不能。ECMA 规范是，先返回正整数属性，按值增序排序；再返回字符串或 Symbol，按属性创建的顺序，可保证顺序。但浏览器实现不一定都遵循规范，所以，不能信任 Object.keys() 的顺序。**

```javascript
const symbol1 = Symbol('symbol1')
const symbol2 = Symbol('symbol2')
const obj = {
  'd': 'd',
  symbol1: 'symbol1',
  'a': 'a',
  'c': 'c',
  'b': 'd',
  '1': 1,
  '3.1': 3.1,
  '2': 2,
  symbol2: 'symbol2'
}
Object.keys(obj)
// ['1', '2', 'd', 'symbol1', 'a', 'c', 'b', '3.1', 'symbol2']
```

Object.keys()：返回的数组顺序和与 for...in 循环提供的顺序相同。——[MDN-Object.keys()](https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Object/keys)

for...in：与最新的ECMAScript 规范一样，遍历顺序是明确定义且实现一致的—— 在原型链的每个组合中，首先按值升序遍历所有非负整数键（可以作为数组索引的键），然后按属性创建的升序时间顺序遍历其他字符串键。 ——[MDN-for...in](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in)

**Object.keys(O)：**

* O 转换为 Object；
* 获取属性列表：EnumerableOwnProperties(obj, key) -> O.\[\[OwnPropertyKeys]] -> OrdinaryOwnPropertyKeys(O)；
* 将属性列表转换为 Array，并返回。

> **OrdinaryOwnPropertyKeys ( O )：**
>
> The abstract operation OrdinaryOwnPropertyKeys takes argument O (an Object) and returns a List of property keys. It performs the following steps when called:
>
> 1. Let keys be a new empty List.
> 2. For each own property key P of O such that P is an array index, in ascending numeric index order, do a. a. Append P to keys.
> 3. For each own property key P of O such that P is a String and P is not an array index, in ascending chronological order of property creation, do a. a. Append P to keys.
> 4. For each own property key P of O such that P is a Symbol, in ascending chronological order of property creation, do a. a. Append P to keys.
> 5. Return keys.
>
> ——[ECMAScript 规范-OwnPropertyKeys](https://262.ecma-international.org/14.0/#sec-ordinary-object-internal-methods-and-internal-slots-ownpropertykeys)