# 1.8.1 String 里的方法

## 1. `toLowerCase()` 和 `toUpperCase()`

### 1.1 原理

1. UniCode 编码
2. 两个方法
   * `str.charCodeAt(index)` 取单个字符，得到它的编码
   * `String.fromCharCode(num)` 将编码转换成字符

<figure><img src="/files/b7g4UgRulu5xTqpsN5KE" alt=""><figcaption><p>16 bits 是 two bytes</p></figcaption></figure>

上图是基础拉丁语，范围是 `0000−007F`，共 128 个字符。

### 1.2 实现

```javascript
function to(str, min, max, dist){
    let arr = str.split('');
    for(let i in arr){
        let cur = arr[i].charCodeAt();
        if(cur >= min && cur <= max) arr[i] = String.fromCharCode(cur + dist);
    }
    return arr.join('');
}

const toLowerCase = function(str){
    const A = 'A'.charCodeAt();
    const Z = 'Z'.charCodeAt();
    return to(str, A, Z, 32);
}
const toUpperCase = function(str){
    const a = 'a'.charCodeAt();
    const z = 'z'.charCodeAt();
    return to(str, a, z, -32);
}
```

### 1.3 扩展阅读

* <https://anjia1.gitbook.io/cs-foundation/string/unicode>
* <https://unicode-table.com/en/>
* <https://unicode-table.com/en/blocks/basic-latin/>

## 2. 用下标取单字符信息

### 2.1 `charAt(i)` vs `[i]`

不同点：

1. 兼容性
   1. `charAt()` ECMAScript 3，IE7 和 IE6 也支持
   2. `[]` ECMAScript 5，IE7 和 IE6 不支持
2. 边界不同
   * 非数值类型，是否进行类型转换
   * 超过有效范围时

![](/files/aimpg0l3B9xhgQMbRBmn)

<https://stackoverflow.com/questions/5943726/string-charatx-or-stringx>

### 2.2 `charAt(i)` vs `at(i)`

都返回字符（串），由指定位置的 the single UTF-16 code unit 组成的。

不同点：下标

1. `str.charAt(i)` 下标范围 `[0, str.length-1]`，且会自动进行 Number 类型转换
2. `str.at(i)` 下标支持负数

### 2.3 `charCodeAt()` vs `codePointAt(i)`

不同点：对于值大于 `0xFFFF` 的 code point，它不能被单个 UTF-16 code unit 表示

1. `str.charCodeAt(i)`，
   * 返回整数，范围在 `[0, 65535]`，表示指定位置的 UTF-16 code unit
   * 注意：如果 Unicode code point 不，那么此方法只会返回 code point 的一部分（比如 ）如果想要整个 code point 的值，可以使用 `codePointAt()`
2. `str.codePointAt(i)` &#x20;


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