JavaScript Error Checker

About JavaScript Error Checker

The JavaScript Error Checker is a browser-based development tool for finding syntax problems and a small set of common source-code patterns. It is useful when you have pasted or edited JavaScript and want a quick first check before testing the code in a real application, or before you run it through a JavaScript obfuscator.

The checker uses the browser's JavaScript parser to validate whether the supplied source can be parsed as JavaScript. It also performs conservative text-based checks for patterns that developers commonly review before production deployment.

The tool does not attempt to replace a full development environment, linter, type checker, test suite, or browser debugger. Its purpose is to provide a quick and accessible first-pass analysis.

Why Check for Errors Before You Use a JavaScript Obfuscator

A JS obfuscator has to read and understand your code before it can transform it. That means it parses the source in much the same way a browser does. If the script contains a syntax error, such as a missing bracket or an unexpected token, the obfuscator can fail with an error message or produce output that does not run.

Running the checker first is a simple way to avoid that. A good workflow looks like this:

1. Check your script here and fix any syntax errors.
2. Clean up the warnings that matter for production, such as debugger statements, leftover console calls and TODO comments.
3. Obfuscate the fixed script with the ObfuscatorJS JavaScript obfuscator tool to obfuscate JavaScript code.
4. Check again by pasting the output back here to confirm it still parses.
5. Test the final script in your real application.

What This Tool Checks

• JavaScript syntax errors that prevent the source from being parsed.
• debugger statements.
• Common console method calls.
• eval() usage.
• document.write() usage.
• TODO and FIXME comments.
• Character and line statistics.

How To Use the JavaScript Error Checker

1. Paste the JavaScript source code into the input editor.
2. Click Check JavaScript.
3. Review the syntax result and any source-level warnings.
4. If an error location is available, inspect the indicated line and nearby code.
5. Correct the source and run the checker again.
6. When the script is clean, open a JS obfuscator online to protect it.

You can also press Ctrl + Enter on Windows or Linux, or Cmd + Enter on macOS, to run the check without clicking the button.

Understanding JavaScript Syntax Errors

A syntax error means the JavaScript parser cannot understand the structure of the supplied source. Common examples include a missing closing bracket, an unexpected token, an incomplete expression, an incorrectly formed string, or invalid syntax in a declaration.

For example, this code contains a missing closing parenthesis:

function greet(name) {
    console.log("Hello, " + name;
}

Correcting the expression makes the structure parseable:

function greet(name) {
    console.log("Hello, " + name);
}

The exact error message and reported location depend on the JavaScript parser implemented by the browser.

Syntax Errors vs. Runtime Errors

These two types of problems are different. A syntax error prevents JavaScript from being parsed correctly. A runtime error occurs after code is running and may depend on variables, browser APIs, network responses, user actions, timing, or other application state.

For example, this is syntactically valid:

const user = null;
console.log(user.name);

However, attempting to access name from null can produce a runtime error when the code executes. A syntax checker alone cannot determine every runtime problem.

Source-Level Warnings and Code Protection

In addition to syntax validation, this tool looks for a small number of source patterns that may deserve review. These are warnings or informational notices rather than proof that the code is wrong. Several of them are also relevant when you want to protect JavaScript source code.

• debugger: useful during development, but often removed from production code. Some obfuscators also offer separate debug-protection options.
• console: logging may be intentional, but it can reveal how your code works in browser developer tools. Many JavaScript code obfuscator tools can disable console output in the protected build.
• eval(): dynamic code evaluation can introduce security, debugging, and maintenance concerns, and it can also limit what an obfuscator is able to transform.
• document.write(): older code may use it, but modern applications often use DOM APIs instead.
• TODO / FIXME: these comments can indicate unfinished work, and comments in general may expose hints about your logic if they are left in a published file.

Should You Check Obfuscated JavaScript for Errors?

Yes. After you obfuscate JS, paste the output into this checker. If it still parses without a syntax error, the transformation produced valid JavaScript. If it does not, try different obfuscation options or check that the original script was error-free.

Keep in mind that a successful syntax check only proves that the code can be parsed. It does not prove that the obfuscated script behaves exactly like the original, so always run your tests on the final file.

Pros and Cons

What This Checker Does Not Detect

A successful syntax check does not mean that an application is bug-free. This tool does not fully analyze application logic, asynchronous behavior, network requests, DOM state, API responses, browser compatibility, performance, memory usage, or runtime exceptions.

For larger projects, use this checker as an initial validation step and then use appropriate development tools such as a linter, browser developer tools, automated tests, and project-specific validation.

Does the Checker Execute Your Code?

No. The supplied source is passed to the browser's JavaScript parser for syntax validation, but the resulting function is never invoked by this tool. Source-level checks are performed separately against the text of the supplied code.

Because this tool does not intentionally execute the supplied source, it should not be treated as a JavaScript runtime or application testing environment.

Code Privacy and Browser Processing

The checking operation is performed locally in the browser and does not require a server-side JavaScript checking API. This means the checker itself does not need to upload your source code to perform syntax validation.

As with any website, you should still avoid pasting passwords, API keys, private credentials, or other sensitive information into online tools. This is also why secrets should never be hidden in client-side code in the first place: obfuscation makes code harder to read, but it does not encrypt JavaScript, and a browser must still be able to run it.

Frequently Asked Questions