USVString is a sequence of Unicode scalar values. This definition differs from that of
String type in that it always represents a valid sequence suitable for text processing, while the latter can contain surrogate code points. The
USVString type is generally found in APIs that perform text processing, while
DOMString is used by most other APIs.
String with the UTF-16 encoding of its sequence of Unicode scalar values.
When a Web API accepts a
DOMString. The resulting string is then further converted to
USVString by replacing any surrogate code points (or equivalently, any unpaired surrogate code units) with the Unicode "replacement character" U+FFFD (�).