Exactly, and emoji are outside the BMP, so it's not exactly an edge case, but the norm where two code units (UTF-16 double-bytes) are used to make one code point (Unicode character) [1].

And it gets even worse, when you consider that for many purposes you're not even interested in code points but in graphemes which can be sequences of code points -- e.g. a single visible emoji might actually be a sequence of 5 code points, represented by 8 UTF-16 code units, taking up 16 bytes [2]. Similarly a single accented character will often be two code points (letter plus combining diacritic).

If you want to split a string by graphemes -- e.g. to count its visible length, or delete its last visible character -- you can either use a library for it [3], or the relatively new Intl.Segmenter [4] which is in Chrome and Safari, but hasn't made it to Firefox [5].

Kind of amazing it's 2021 and you still can't calculate the number of visible characters (graphemes) in a string using native functions across all modern browsers.

[1] https://blog.jonnew.com/posts/poo-dot-length-equals-two

[2] https://www.contentful.com/blog/2016/12/06/unicode-javascrip...

[3] https://github.com/orling/grapheme-splitter

[4] https://github.com/tc39/proposal-intl-segmenter

[5] https://bugzilla.mozilla.org/show_bug.cgi?id=1423593