Free Online JS Validator
JS Validator checks your JavaScript code for syntax errors, strict mode violations, and common code quality issues instantly in your browser. We built six validation utilities into one workspace so you can check syntax, validate strict mode compliance, lint for warnings, analyze code metrics, view highlighted source, and test module compatibility without installing software or creating an account. Paste your code 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.
JS Validator
Checks JavaScript syntax using the browser native parser and reports errors with line and column positions.
Strict Mode Check
Validates code against strict mode rules that catch undeclared variables, duplicate parameters, and restricted syntax.
Error Linter
Scans for syntax errors, strict mode violations, eval usage, console calls, debugger statements, and missing semicolons.
Code Stats
Counts total lines, code lines, blank lines, comments, functions, variables, nesting depth, and file size.
Syntax Highlight
Displays your code in a read-only viewer with full Dracula theme syntax highlighting for review and screenshots.
Module Check
Detects ES module syntax and lists all import and export statements found in your code.
Online JS Validator for Every Developer
Tracking down a syntax error in a large JavaScript file can waste hours when the error message from your build tool is vague or points to the wrong line. Our JS Validator runs your code through the browser native JavaScript parser and gives you the exact error message, line number, and column position in real time as you type or paste. The Health Bar at the top of the workspace shows a pulsing status dot, error and warning counts, a code health score from 0 to 100, and quick file statistics so you always know where your code stands.
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 File button in the toolbar. We accept .js, .jsx, .ts, .tsx, .mjs, and .cjs files. Because everything runs in your browser, there is no upload size limit imposed by a server and your files never leave your machine. This makes the validator safe for proprietary code, production bundles, and files containing sensitive configuration data.
Strict Mode and Error Linting
The Strict Mode Check prepends "use strict" to your code and validates it against the stricter parsing rules that modern JavaScript development relies on. The Error Linter goes further by scanning every line for common issues like eval() usage, console.log statements left in production code, debugger breakpoints, alert() calls, and possible missing semicolons. Each issue is listed with its severity, line number, and a clear description so you can fix problems systematically.
Analyze Code Metrics and Module Compatibility
Beyond checking for errors, understanding your code structure helps you write better JavaScript. The Code Stats tool counts total lines, executable code lines, blank lines, comment lines, character count, function declarations, variable declarations, maximum nesting depth, and file size. High nesting depth or low comment ratios can signal areas that need refactoring. The Module Check tool detects whether your code uses ES module syntax (import/export) and lists every module statement it finds, helping you understand compatibility between script and module contexts.
View Syntax-Highlighted Source Code
The Syntax Highlight tool renders your JavaScript in a read-only code viewer with full Dracula theme coloring. Keywords, strings, numbers, comments, operators, and function names are all color-coded for easy scanning. This is useful when you want to review code without accidentally editing it, take screenshots for documentation, or present code in meetings and tutorials.
How to Use JS Validator
Getting started takes seconds. Open the workspace and choose one of the six validation tools from the sidebar. Paste your JavaScript into the left editor panel, upload a file using the toolbar button, or drag and drop a file onto the workspace. Results appear instantly in the right panel. The Health Bar between the toolbar and editor panels shows your code status at a glance with a pulsing indicator, error and warning badges, and a health score gauge that updates in real time.
Step by Step
First, select the validation tool you need from the sidebar. Second, load your JavaScript by pasting, uploading, or dragging a file. Third, review the results in the right panel. For the Error Linter, click individual issues to see line numbers and descriptions. For Code Stats, review the metric cards to understand your code structure. Use the Copy or Download buttons in the toolbar to save results for your records.
Related JavaScript and Code Tools
If you work with JavaScript regularly, these tools complement what you can do here:
- JS Beautifier -- format, beautify, minify, and validate JavaScript code
- JavaScript Compress -- compress and strip JavaScript for production
- JavaScript Obfuscator -- protect JavaScript with obfuscation
- JavaScript Validator -- validate, format, minify, and analyze JavaScript
- JSON Formatter -- format, minify, validate, and transform JSON data
- HTML Formatter -- format and beautify HTML documents