新增于: v16.8.0, v14.18.0
在用 Unicode “替换字符” U+FFFD 替换任何代理代码点（或等效地，任何未配对的代理代码单元）后返回 string。
Added in: v16.8.0, v14.18.0
Returns the string after replacing any surrogate code points
(or equivalently, any unpaired surrogate code units) with the
Unicode "replacement character" U+FFFD.