The dollar sign is allowed character of identifier. [see JS: Allowed Characters in Identifier]
The dollar sign ($) and the underscore (_) are permitted anywhere in an identifier. The dollar sign is intended for use only in mechanically generated code.
However, later edition, ECMA-262 5.1 no longer says that.
This standard specifies specific character additions: The dollar sign ($) and the underscore (_) are permitted anywhere in an IdentifierName.
ECMAScript 5.1 §7#sec-7.6
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