Free Online JavaScript Checker
JavaScript Checker validates, lints, and analyzes your JavaScript code directly in your browser. We built six checking utilities into one workspace so you can catch syntax errors, detect code quality issues, measure complexity, locate console statements, review best practices, and find unused variables without installing any software or creating an account. Paste your JavaScript into the editor, upload a file from your computer, or drag and drop it onto the workspace. Your code stays on your device because all processing happens locally.
Syntax Checker
Parses JavaScript through the browser engine and reports syntax errors with line numbers and descriptive messages.
Code Linter
Scans for common quality issues including var usage, loose equality, eval calls, debugger statements, and empty catch blocks.
Code Statistics
Measures lines of code, function counts, variable declarations, cyclomatic complexity, and file size with visual dashboards.
Console Finder
Locates every console.log, console.warn, console.error, and other console method calls with line numbers.
Best Practices
Checks for template literal usage, parameter counts, timer string arguments, document.write, and nesting depth.
Unused Detector
Finds variables and functions that are declared but never referenced elsewhere in your code.
Online JavaScript Checker with Code Health Scoring
Writing JavaScript without a linter means bugs hide in plain sight until they crash production. Missing semicolons, loose equality comparisons, leftover debugger statements, and forgotten console.log calls are easy to overlook in code reviews. Our JavaScript Checker solves this by running your code through multiple checking passes and presenting results in a health score dashboard. You see a score from 0 to 100 that drops for every error, warning, and informational issue detected. Issues are grouped by severity with expandable sections so you can focus on the most critical problems first.
Upload JavaScript Files From Your Computer
You can load JavaScript into the workspace three ways: paste it into the code editor on the left, drag and drop a file anywhere onto the workspace, or click the Upload JS button. We accept .js, .mjs, .cjs, .jsx, .ts, and .tsx files. Because everything runs in your browser, there is no upload size limit and your files never leave your machine. This makes the checker safe for proprietary source code, API integrations with embedded credentials, and internal libraries you cannot share externally.
Real-time Checking with Visual Health Dashboard
As soon as you paste or upload code, the active checking tool processes it and displays results instantly. The right panel shows a circular health score that changes color from green through amber to red based on the number and severity of issues found. Below the score, issues are organized into collapsible groups for errors, warnings, and informational notices. Each issue displays the specific message, the line number where it occurs, and the rule name for easy reference.
Lint, Analyze, and Clean Up Your JavaScript Code
Beyond syntax checking, you often need deeper analysis of your JavaScript. The Code Linter combines syntax validation with quality checks for var declarations, loose equality, eval usage, and empty catch blocks. Code Statistics gives you a full metrics dashboard covering lines of code, function counts by type, variable declarations broken down by var, let, and const, and a visual cyclomatic complexity bar. The Console Finder locates every console method call so you can remove debugging output before deployment. The Best Practices checker flags string concatenation where template literals would be cleaner, functions with too many parameters, and deeply nested code blocks.
Find Unused Variables and Dead Code
The Unused Detector scans your code for variables and functions that are declared but never referenced elsewhere. It strips comments before counting references to avoid false negatives from commented-out usage. While it cannot catch every case of dead code, especially through dynamic property access, it catches the most common patterns and helps you keep your codebase clean and maintainable.
Your JavaScript Stays Private in Your Browser
We process everything locally using the browser JavaScript engine and pattern matching. No server requests, no cookies, no account required. You can safely check production source files, proprietary libraries, and internal code knowing that nothing is transmitted anywhere.
Related JavaScript and Developer Tools
If you work with JavaScript regularly, these free tools complement what you can do here:
- JavaScript Formatter -- format and beautify JavaScript with proper indentation
- JavaScript Minifier -- minify JavaScript to reduce file size
- JavaScript Obfuscator -- obfuscate JavaScript to protect source code
- JavaScript Optimizer -- optimize JavaScript for better performance
- JS Pretty Print -- pretty print JavaScript with syntax highlighting
- JS Formatter -- format JS code with configurable options
- JSON Formatter -- format, minify, validate, and transform JSON data