JavaScript's access to individual parts of a string works on code units, which are distinct from code points, which in turn are distinct from characters. A sing

JavaScript's access to individual parts of a string works on code units, which are distinct from code points, which in turn are distinct from characters. A single (visible) character can be composed of multiple code points, such as a character plus an accent. However, a single code point can also be represented by multiple code units, where a code unit is a fixed number of bits. UTF-16 is a variable-width encoding, so a given code point can be either one code unit or two; in this case, the cat face is a single character, composed of a single code point, composed of two code units. (UTF-8 is another variable-width encoding where a code point can be anywhere from one to four 8-bit code units, and UTF-32 is a fixed-width encoding where every code point is exactly 32-bit one code unit. Because code points can be accents or modifiers, UTF-32 is still variable-width with respect to characters, because there's no guarantee that a given character is composed of a single code unit.) Because UTF-16 is a variable-width encoding, and because JavaScript exposes UTF-16 code units (instead of code points or characters), it is possible to delete half of a code point and even end up with an invalid UTF-16 string in some cases (IIRC). As another comment mentions, some languages (e.g. Python 3) expose code points instead, which still isn't the same as characters.