Code Checker
Paste Python, JavaScript, TypeScript, Java, HTML, CSS or JSON and get every syntax error and common mistake with its line number. Free, instant, and your code is never run or stored.
Forged in Faith, Built to Last
What it checks
- Python: syntax (the real Python parser) plus pyflakes: undefined names, unused imports, and common traps like mutable default arguments and bare
except:. - JavaScript / TypeScript / Java / CSS: full syntax parsing with exact positions, plus common mistakes (
==vs===, empty catch blocks, string comparison with==in Java, duplicate CSS properties, bad colours). - HTML: unclosed and stray tags, duplicate ids and attributes, missing alt text, obsolete elements, and document basics (doctype, lang, title).
- JSON: exact parse errors and duplicate keys.
Pro adds the AI tutor (explanations and a suggested fix), PDF/CSV reports and the JSON API. Java is checked by a syntax parser, not compiled, so type errors are not reported.
Free vs Pro
Free
- 7 languages: Python, JavaScript, TypeScript, Java, HTML, CSS, JSON
- Exact syntax errors with line and column
- Common-mistake lint checks
- Code is never run or stored
No account needed. Nothing you type is saved. With a free account (no card): 2 full PDF/CSV exports a month, shared across all tools.
Pro adds
- AI tutor: plain-English explanation of each problem
- Suggested corrected code, re-checked
- PDF and CSV reports
- JSON API with your licence key
Get Pro: CA$5.00 CAD/month or CA$49.00 CAD/year · one plan for all our tools · cancel any time · secure Stripe checkout
About this tool
Code Checker: Find Errors in Python, JavaScript, Java, HTML & CSS is a free online tool from Crusader Empire: Paste your code and see every syntax error and common mistake, with line numbers. Free. Paste Python, JavaScript, TypeScript, Java, HTML, CSS or JSON. Real parsers and linters find syntax errors, undefined names, unclosed tags, duplicate ids and other common mistakes with exact line…
Free: 7 languages: Python, JavaScript, TypeScript, Java, HTML, CSS, JSON, Exact syntax errors with line and column, Common-mistake lint checks. Pro adds: AI tutor: plain-English explanation of each problem, Suggested corrected code, re-checked, PDF and CSV reports.
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What would make Code Checker: Find Errors in Python, JavaScript, Java, HTML & CSS more useful? Bugs, missing features, confusing wording: our R&D team reads every suggestion. Nothing is offered in return, and we never publish reviews without permission.