2.1.1 [ECMA-262/5] Section 7.6, Identifier Names and Identifiers
The specification states:
ECMAScript implementations may recognise identifier characters defined in later editions of the Unicode Standard. If portability is a concern, programmers should only employ identifier characters defined in Unicode 3.0.
IE9 Mode, IE10 Mode, and IE11 Mode (All Versions)
Unicode 2.1 is supported. Later Unicode standards are not supported.