It should really be using `Intl.Segmenter` instead of `string.length`: https://stackoverflow.com/questions/10287887/get-grapheme-ch...
However, a quirk in the way Unicode handles emoji has meant that some of the symbols take up many more characters than others. For example, a flag can take up as many as 14 spaces in your Tweet. Twitter has announced that it's changing the way it counts emoji so that they're all counted equally, as two characters. - https://www.theverge.com/2018/10/11/17963230/twitter-emoji-c...
So 4 character flag, which is written as 1 character, is sometimes counted as 2 characters. Honestly, I don't know, I did not expect for this to go so deep.
Almost certainly those sites use much more sophisticated counting schemes.
> Honestly, I don't know, I did not expect for this to go so deep
You're not the creator of the tool in question, so it's OK for you not to know about this stuff, but I would really expect more from someone who is setting out to create a "character counter" tool.