AS as well as JS interpret code strings as UTF16-LE during follow methods: String.p.codePointAt, String.p.toUpperCase/toLowerCase, String.p.localeCompare, String.p.normalize, String.fromCodePoint, Array.from(str). In rest cases strings interprets as UCS-2.

The do in all the observable JS APIs, but behind the scenes there are a number of optimizations in each JS engine to deal with the fact that most JS and JSON source comes off the wire in UTF-8 or ASCII.