# JS实现千位分隔符

题目：

```javascript
// given 123456
let input = 123456
// output 123,456
```

{% tabs %}
{% tab title="First Tab" %}

{% endtab %}

{% tab title="答案" %}

```javascript
const f = (str) => {
    return str.replace(/(?=(\B\d{3}+$))/g/, ',')
}
```

{% endtab %}

{% tab title="答案2" %}

```javascript
const f = (num) => {
  if(typeof num !== 'number') throw Error('not number input')
  const str = `${num}`
  let count = 0, ret = []
  for(let i=str.length-1; i>=0 ; i--) {
    count++
    ret.unshift(str[i])
    if(count === 3 && i !=0) { 
      ret.unshift(',')
      count = 0
    }
  }
  return ret.join('')
}
```

{% endtab %}
{% endtabs %}

运用到的知识点：

* `g`是表示全局匹配的修饰符，全局匹配指查找所有匹配而非在找到第一个匹配后停止
* `$`是表示结尾的量词，如`n$`，匹配的是任何以n为结尾的字符串
* `\d`是查找数字的元字符
* `n{X}`是匹配包含 X 个 n 的序列的字符串的量词
* `n+`是匹配任何包含至少一个 n 的字符串的量词
* `?=n`正向查找位置，用于匹配任何其后紧接指定字符串 n 的字符串
* `replace()`String对象的方法，作用是替换与正则表达式匹配的子串
* `\B`是表示匹配非单词边界的元字符，与其互为补集的元字符是`\b`，表示匹配单词边界。


---

# 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/other/zheng-ze/ti-mu/js-shi-xian-qian-wei-fen-ge-fu.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.