JavaScript: 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.

Here is a example you can test:

// α GREEK SMALL LETTER ALPHA codepoint 945, hexadecimal 3b1
const α = 4;
console.log(α);
// ok

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

(To search for Unicode, or find a character's codepoint, see: Unicode Search 😄.)

What characters are Unicode Letter?

What Characters Are Unicode Letter