JS: Allowed Characters in Identifier

What is allowed characters in JavaScript identifiers?

JavaScript identifiers (variable/function names) must:

• Begin with a Unicode Letter, or low line _, dollar sign $.
• Following characters must be Unicode Letter or Unicode digit or low line or dollar sign.

ECMAScript 2015 §ECMAScript Language: Lexical Grammar#sec-names-and-keywords

Here's a example you can test:

// α GREEK SMALL LETTER ALPHA codepoint 945, x3b1
const α = 4;
console.log(α);
// ok

// ♥ BLACK HEART SUIT codepoint 9829, hex 2665
// is not a letter
var ♥ = 3;
// SyntaxError: Invalid or unexpected token

(To search for Unicode, or find a character's code point, see: Unicode Characters ∑ ♥ 😄.)

What characters are Unicode Letter?

A “Unicode Letter” is a character whose Unicode property “general category”'s major value is “L”. In general, language's alphabets are Unicode letters, and Chinese characters are also Unicode Letter.

Examples of characters that's not Unicode letter: digits, punctuations, math symbols, emoji.

How to determine if a character is a Unicode Letter?

You can find out whether a Unicode char is a letter using Python. See: Python: Process Unicode, unicodedata Module

Or in emacs, place cursor on (or before) the char, then M-x “describe-char”, then read the line “general-category”. [see Emacs: Unicode Tutorial]