A JavaScript string is conceptually an array of 16-bit unsigned integers (https://tc39.es/ecma262/multipage/ecmascript-data-types-and-...). String indexing and substring operations treat strings this way, and they couldn't change the model easily without breaking code. If you index a string containing an emoji with a code point greater than U+FFFF, you get string containing an unpaired surrogate code unit, which makes the string UCS-2, not UTF-16. I don't know how it's implemented memory-wise in the various JavaScript engines and am not familiar with typed arrays.

> It's definitely not under the hood.

Really? I'd love to see some references to that.

I know there's some operations to convert a javascript string to UTF8, but everything which measures a string counts using 2-byte UCS2 items. For example, array-style indexing, string.length, str.charCodeAt(), split(), slice(), indexOf(), etc.

Also lots of DOM APIs use those weird string lengths too. For example, input element selectionStart / selectionEnd fields measure the selection range by counting surrogate pairs, not characters.

JS strings are exposed as UTF-16 in the language — surrogate pairs have length 2 and you can index into half of them.