function isIdentifier(str) { if (str === '') { return false; } const first = str.codePointAt(0); if (!isIdentifierStart(first)) { return false; } const firstLen = first > 0xffff ? 2 : 1; for (let i = firstLen; i < str.length; i += 1) { const cp = str.codePointAt(i); if (!isIdentifierChar(cp)) { return false; } if (cp > 0xffff) { i += 1; } } return true; }