The character encoding support described in the third edition of the standard is somewhat more advanced than that in the second edition:
"A conforming implementation of this International standard shall interpret characters in conformance with the Unicode Standard, Version 2.1 or later, and ISO/IEC 10646-1 with either UCS-2 or UTF-16 as the adopted encoding form, implementation level 3. If the adopted ISO/IEC 10646-1 subset is not otherwise specified, it is presumed to be the BMP subset, collection 300. If the adopted encoding form is not otherwise specified, it is presumed to be the UTF-16 encoding form."
The third edition adds to the earlier versions in the following areas:
Richer control statements
Better string and array handling
Exception handling improvements with try/catch
Error objects for managing exceptions
ECMAScript edition 3 support is a feature of the MSIE/Jscript 5.5. upgrade and the new Netscape 6.0 release.
ISO/IEC 10646-1 Information Technology -- Universal Multiple-Octet Coded Character Set (UCS), plus its amendments and technical corrigenda.
Unicode Technical Report #8, The Unicode Standard, Version 2.1.