Free Online JavaScript Linter
JavaScript Linter scans your code for syntax errors, quality issues, and anti-patterns directly in your browser. We built six linting utilities into one workspace so you can validate syntax, catch common mistakes, measure complexity, locate leftover 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.
JavaScript Linter
Combines syntax validation with quality checks for var usage, loose equality, eval calls, debugger statements, and empty catch blocks. Shows fix suggestions.
Syntax Checker
Parses JavaScript through the browser engine and reports syntax errors with line numbers and descriptive messages.
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 for cleanup.
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 Linter with Severity Summary and Fix Suggestions
Writing JavaScript without a linter means bugs hide in plain sight until they crash in production. Loose equality comparisons, leftover debugger statements, forgotten console.log calls, and var declarations with unexpected hoisting behavior are easy to overlook in manual code reviews. Our JavaScript Linter solves this by running your code through multiple checking passes and presenting results in a summary dashboard. You see the total count of errors, warnings, and informational issues at a glance with a color-coded severity bar. Each issue includes the exact line number, the rule name, and a suggested fix so you know exactly what to change.
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 linter safe for proprietary source code, API integrations with embedded credentials, and internal libraries you cannot share externally.
Instant Results with Actionable Suggestions
As soon as you paste or upload code, the active tool processes it and displays results instantly. The right panel shows a summary strip with total issues broken down by severity. Below the strip, each issue is listed in a flat row format with a colored severity indicator on the left, the message in the center, and the line number, rule name, and fix suggestion on the bottom. This flat layout makes it easy to scan through many issues quickly without needing to expand or collapse groups.
Analyze, Clean Up, and Improve Your JavaScript Code
Beyond basic linting, you often need deeper analysis of your JavaScript. The Syntax Checker isolates pure syntax validation to quickly confirm whether code will parse without errors. 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 that shows whether your code is simple, moderate, or complex. The Console Finder locates every console method call so you can remove debugging output before deployment. The Best Practices tool flags string concatenation where template literals would be cleaner, functions with too many parameters, and deeply nested code blocks that hurt readability.
Detect 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, it catches the most common patterns and helps you keep your codebase lean and maintainable. You can copy the full lint report or download it as a text file for sharing with your team.
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 lint production source files, proprietary libraries, and internal code knowing that nothing is transmitted anywhere. The entire tool runs offline once the page has loaded.
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