You can use any so called "format-control" character (characters in the Unicode Cf category) in strings, regular expressions and comments.
The ZWNJ (\u200C) and ZWJ (\u200D) characters may even be used as part of an IdentifierName, just not as the first character of one.
The BOM character (\uFEFF) is considered white space and may occur at any point where white space would be allowed.