JavaScript Code Checker: Free Online Security and Syntax Analysis

CodeSlick is a free JavaScript code checker that goes beyond syntax validation. While most JS validators catch missing semicolons or bracket mismatches, CodeSlick performs deep security analysis on your JavaScript code—identifying vulnerabilities that lead to data breaches, account takeovers, and supply chain attacks.

When you paste JavaScript code into CodeSlick's scanner, it runs 28 security checks covering:

• Injection vulnerabilities: SQL injection, NoSQL injection, command injection, and code injection via eval(), Function(), and template literal misuse.
• Cross-site scripting (XSS): DOM-based XSS through innerHTML, document.write(), and unsafe event handler assignments.
• Prototype pollution: Recursive merge functions and deep clone operations that allow attackers to modify Object.prototype.
• Hardcoded secrets: API keys, tokens, passwords, and connection strings embedded directly in source code.
• Insecure dependencies: Known malicious npm packages and vulnerable dependency patterns.
• Broken authentication: Weak JWT handling, missing token validation, and insecure session management.

Every finding includes a CWE identifier, CVSS severity score, and a direct mapping to the OWASP Top 10:2025 categories. CodeSlick also flags AI-generated code patterns, detecting hallucinations and insecure suggestions from tools like Copilot and ChatGPT.

CodeSlick's JavaScript analyzer combines static security analysis with AI code detection—a capability no other free JavaScript checker offers.

Security Analysis

The 28 checks are organized by attack class. Here is what CodeSlick catches that typical linters miss:

// CodeSlick flags: Command Injection (CWE-78, CVSS 9.8)
const exec = require('child_process');
exec.execSync('git log --author=' + req.query.user);

// CodeSlick flags: Prototype Pollution (CWE-1321, CVSS 7.5)
function merge(target, source) {
  for (let key in source) {
    target[key] = source[key]; // No __proto__ check
  }
}

// CodeSlick flags: DOM XSS (CWE-79, CVSS 6.1)
document.getElementById('output').innerHTML = userInput;

AI Code Detection

CodeSlick uses 150 signals to detect AI-generated JavaScript, including:

• Hallucination patterns: Calls to non-existent APIs, fabricated npm packages, and invented method names that LLMs commonly produce.
• LLM fingerprints: Repetitive comment styles, overly verbose variable names, and boilerplate structures characteristic of GPT-4, Claude, and Copilot output.
• Security anti-patterns: AI models frequently generate code with eval(), disabled CORS, and hardcoded credentials—CodeSlick catches all of these.

Both the security checks and AI detection run in a single scan, completing in under 3 seconds.

JavaScript vulnerabilities have caused some of the most widespread security incidents in recent years:

• event-stream (2018): A malicious npm package injected cryptocurrency-stealing code into a dependency used by millions. CodeSlick's malicious package detection covers 66 known malicious packages plus real-time OSV.dev lookups.
• Prototype pollution in Lodash: CVE-2019-10744 affected every application using _.defaultsDeep() with untrusted input, enabling denial of service and potential RCE. CodeSlick detects unsafe recursive merge patterns.
• ua-parser-js supply chain attack (2021): Three malicious versions of a package with 8 million weekly downloads installed cryptominers and password stealers. CodeSlick's dependency scanner flags known compromised packages.

These incidents share a pattern: they exploit JavaScript-specific behaviors (prototype chain, npm dependency resolution, dynamic typing) that generic code checkers do not understand. CodeSlick's checks are purpose-built for JavaScript's unique threat model.

CodeSlick's JavaScript code checker is completely free to use. No account required, no rate limits on the web scanner, and no data retention—your code is analyzed in memory and never stored.

What you get with every scan:

• 28 security checks with CWE, CVSS, and OWASP mapping for each finding
• AI code detection identifying hallucinations, LLM fingerprints, and insecure AI patterns
• Secrets detection across 38 patterns (AWS keys, GitHub tokens, Stripe keys, and more)
• AI-powered fix suggestions that generate secure replacement code for every vulnerability found

For teams, CodeSlick also offers a GitHub App that scans every pull request automatically and a CLI for pre-commit hooks in CI/CD pipelines. Start with the free web scanner at /analyze to see CodeSlick's JavaScript checks in action.