JavaScript: Allowed Characters in Identifier

What is allowed characters in JavaScript identifiers (variable/function names)?

JavaScript identifiers must:

• Begin with a Unicode Letter, or underscore _, dollar sign $.
• Following characters must be Unicode Letter or Unicode digit or underscore or dollar sign.
• The character must be a character in the Basic Multi-Lingual Plane. That is, its Unicode codepoint can be represented as 4 hexadecimal digits.

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

Here's a example you can test:

// -*- coding: utf-8 -*-
// α GREEK SMALL LETTER ALPHA codepoint 945, x3b1
var α = 4;                      // ok
console.log(α);

// -*- coding: utf-8 -*-
// ♥ BLACK HEART SUIT codepoint 9829, #x2665
// is not a letter
var ♥ = 3; // SyntaxError: Unexpected token ILLEGAL

// -*- coding: utf-8 -*-
// 𓀀, EGYPTIAN HIEROGLYPH A001, x13000, (is a letter, but Not in Basic Multi-Lingual Plane)
var x𓀀 = 3; // SyntaxError: Unexpected token ILLEGAL

More example to play with:

// -*- coding: utf-8 -*-
// test if unicode is supported in var name or function name.

// Chinese char “愛” (x611b) in BMP, and is a Unicode letter
var 愛 = "♥";

// lambda λ (x3bb) in BMP, and is a Unicode letter
function λ(x) {return x;}

console.log(λ(愛));  // prints ♥

(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, emoticons.

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〕

What are examples of Unicode characters outside Basic Multilingual Plane?

Typically, new emoticons, or rarely used Chinese characters, or rarely used math symbols.

Examples of Unicode characters outside Basic Multilingual Plane:

Unicode Emoji 😄 😱 👽

See also: JavaScript: Convert Character To/From Codepoint