Javascript Validator

How to use it

The javascript validator a great tool to point out any errors or warnings you may have in your javascript code.

Add your codeCopy and paste your javascript code into the editor above, or use Open file to load a .js file. Example code fills the editor with a short script that has a few common problems.

Choose your optionsIf you want to customize your results, click on Options. You can say where the code will run (browser, NodeJS, jQuery), pick the JavaScript version, and choose which warnings to show.

ValidateThe code is checked as you type. You can also click Validate code, or press Ctrl+Enter, to check it straight away.

Read the resultsA syntax error is shown with its line, column and the lines around it. Warnings are listed under it. Click any of them to jump to that spot in your code.

Fix and check againGenerally warnings are not that critical, but any errors should be fixed to make your javascript code work correctly. The smallest error can stop a whole script from running on your website.

Good to know

Errors and warnings

An error means the code cannot run at all. A warning means it can run, but something looks risky or untidy, such as a missing semicolon or a variable that is never used.

"Not defined" warnings

If a variable comes from somewhere else, such as $ from jQuery or require from NodeJS, tick the matching option under Assume so it is not reported.

Your code is never run

The code is only read, never executed, so it is safe to check code you do not trust.

Frequently asked questions

What does this validator check?

First it checks that the syntax is valid, using the same rules a browser uses, and shows the exact line and column of the first mistake. If the syntax is valid it then looks for warnings, such as undefined or unused variables, missing semicolons and risky code, using JSHint.

Does it run my code?

No. The code is only read and analysed, never run.

Why do I get a warning for code that works?

Warnings are advice, not errors. For example, JavaScript adds missing semicolons for you, so the code works, but a missing semicolon can cause surprises in some cases. You can switch off warnings you do not want under Options.

It says the syntax is valid, but the lint step could not read all of it. What does that mean?

The syntax check understands the newest JavaScript, but JSHint, which finds the warnings, is older and does not know some newer features, such as class fields. The code is still valid. Warnings after the point where JSHint stopped are not shown.

What does the JavaScript version option do?

It sets which language features are allowed. If you pick ES5, newer features such as let, arrow functions and async are reported as errors. Leave it on Latest unless you need to support older browsers.

Is my code uploaded?

No. The checking is done in your browser and nothing is sent anywhere.

Related tools