Parses JavaScript with acorn (ECMAScript latest) inside the Worker — the code is never executed — and reports the first syntax error with line:column and a caret excerpt, detecting whether the file is an ES module or a classic script. Valid code gets a summary (functions, classes, imports/exports, var/let/const, console calls, top-level await) and hygiene warnings: debugger, with, eval and new Function, loose == / !=, empty catch blocks and stray semicolons. JSX and TypeScript are not supported and are pointed out when they cause the error.