能帮忙解答一下这段代码什么意思吗

[zhangzhaoaaa]

`const fullCharCodeAtPos = (input: string, pos: number) => { const code = input.charCodeAt(pos) // 合法字符的判断，能给个依据吗，我在网上搜到这个判断，但不知道出处在哪 if (code <= 0xd7ff || code >= 0xe000) return code

const next = input.charCodeAt(pos + 1) // 以下这个运算没看懂 return (code << 10) + next - 0x35fdc00 }`

[acbde45]

https://github.com/acornjs/acorn/issues/708