[Intermediate Algorithm Scripting: Missing letters | freeCodeCamp.org](https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/intermediate-algorithm-scripting/missing-letters)

自己写的！！！

```js function fearNotLetter(str) { const allLetters = 'abcdefghijklmnopqrstuvwxyz' const index = allLetters.indexOf(str[0]) if (str.match(a-z)) { return undefined }

for (let i = 0; i < str.length; i++) { if (str[i] !== allLetters[index + i]) { return allLetters[index + i] } } } ```

用到了 `match`、正则表达式、for 循环、if 语句。

答案

```js function fearNotLetter(str) { for (let i = 0; i < str.length; i++) { let code = str.charCodeAt(i) if (code !== str.charCodeAt(0) + i) { return String.fromCharCode(code - 1) } } return undefined } ```

```js function fearNotLetter(str) { let currCharCode = str.charCodeAt(0) let missing = undefined

str.split('').forEach((letter) > { if (letter.charCodeAt(0) == currCharCode) { currCharCode++ } else { missing = String.fromCharCode(currCharCode) } })

return missing } ```

```js function fearNotLetter(str) { for (let i = 1; i < str.length; ++i) { if (str.charCodeAt(i) - str.charCodeAt(i - 1) > 1) { return String.fromCharCode(str.charCodeAt(i - 1) + 1) } } } ```