How to Validate and Format Code Online
The Multi-File Code Validator lets you check syntax and auto-format code files in bulk. Upload an entire project folder or paste code directly, and the tool validates each file against its language's syntax rules, reports errors with line numbers, and produces clean formatted output for valid files.
Upload files by dragging them into the upload zone or clicking to browse. The tool auto-detects file types from extensions: .json files are validated with JSON.parse, .css files are checked for brace matching, .html files are parsed with DOMParser, .js files are checked with the Function constructor, and .csv files are validated for column consistency. You can also paste code directly and select the type manually.
The validation report shows each file's status at a glance: valid files display a green checkmark with file statistics (line count, character count), while invalid files display a red warning with specific error messages and line numbers. For valid files, an expandable section shows the formatted output with proper indentation.
This tool is particularly useful before deployments. A single malformed JSON config file can crash a Node.js server. An unclosed CSS brace can break an entire stylesheet. A JavaScript syntax error can prevent a module from loading. Validating all config files in a single operation catches these issues before they reach production.
Understanding Syntax Validation by Language
JSON Validation: The tool uses JSON.parse(), which is the definitive JSON validator. If JSON.parse throws an error, the JSON is invalid — period. The error message includes the character position, which the tool converts to a line number for easy debugging. Common JSON errors: trailing commas (not allowed in JSON), unquoted keys (JSON requires double quotes), single quotes (JSON requires double quotes), and missing brackets.
CSS Validation: The tool checks brace balance — counting opening and closing braces to detect mismatches. While not a full CSS linter, brace mismatch is the most common CSS error and the hardest to spot visually in large stylesheets. The tool also applies basic formatting: each rule gets its own line, properties are indented inside braces.
HTML Validation: The tool uses DOMParser to parse HTML and checks for parser errors. It also compares the count of opening and closing tags to detect unclosed elements. While not a full W3C validator, it catches the structural errors that cause rendering problems.
JavaScript Validation: The tool wraps your code in a new Function() constructor, which triggers JavaScript's parser without executing the code. This catches all syntax errors: missing semicolons, unmatched brackets, invalid operators, and reserved word misuse. Runtime errors (undefined variables, type mismatches) are not caught because they require code execution.
CSV Validation: The tool splits the input by newlines and compares each row's column count to the header row. If any row has a different number of columns, it is flagged as an error. This catches missing fields, extra commas, and unescaped delimiters that break CSV parsers.
Frequently Asked Questions
JSON (full parse validation and pretty-printing), CSS (brace matching and basic formatting), HTML (DOM parser validation and tag balance checking), JavaScript (syntax checking via Function constructor), and CSV (column count consistency). The tool auto-detects type from file extension.
The tool catches all parse-time syntax errors using the Function constructor. This includes missing brackets, invalid operators, malformed expressions, and reserved word misuse. It does not catch runtime errors because those require actual code execution. For comprehensive linting including style rules and best practices, use ESLint in your development environment.
Yes. Upload a folder and the tool processes all supported file types automatically. Unsupported file types (images, binaries, etc.) are silently skipped. The bulk report shows the validation status of every supported file in the folder, making it easy to scan for issues across an entire project.
No. All validation and formatting uses built-in browser APIs: JSON.parse for JSON, DOMParser for HTML, the Function constructor for JavaScript, and string splitting for CSV. No external services, APIs, or network requests are involved. Your code never leaves your browser.