For characters in Basic Multilingual Plane, we can use '\uxxxx' escape it. For example, you can use /[\u4e00-\u9fff]/ to match a common chinese character(0x4e00-0x9fff is the range of CJK Unified Ideographs).
But for characters out of Basic Multilingual Plane, their codes are bigger than 0xffff. So you can't use format '\uxxxx' to escape it, because '\u20000' means character '\u2000' and character '0', not a character which code is 0x20000.
How can I escape characters out of Basic Multilingual Plane? Use those characters directly is not a good idea, because they can't show in most fonts.
Font support is a different issue, but when working with characters, you generally want to see them at some point anyway, at least in testing. So you more or less need some font(s) that cover the characters. The Fileformat.info pages also contain links to browser support info, such as (U+20000) Font Support – a good starting point, though not quite complete. For example, U+20000