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JavaScript validator

Paste JavaScript and see every mistake with its line and a plain fix.

PrivateNo uploadsRuns in your browser

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Checking…

Reading 17 lines

Try a real example
PrivateNo uploadsRuns in your browser

Problems

Show

Errors break the page. Warnings may misbehave. Tips make it better. Tap one to see it in the code.

Nothing to fix. Nice clean code.

How to check JavaScript for errors

  1. Paste or open your code

    Paste a script or a function, open a .js file, or try a sample. The check runs as you type.

  2. Read the list

    Each problem shows its line and column, what is wrong and how to fix it. Tap one to jump to it in the editor.

  3. Fix and save

    Press Fix what can be fixed safely, look over the changes, then apply them. Copy or download the clean code.

Good to know

  • A syntax error stops the whole script. Fix the first one first: the rest often go away with it.
  • Use === and !== to compare. == turns "1" into 1 first, which hides mistakes.
  • Declare every name with const or let. Without it, the name quietly becomes a global.
  • A missing semicolon is not an error. JavaScript adds them for you, and Tidy the layout adds them too.
  • Names one letter away from a real one, like docment, are flagged because they fail when the line runs.

Where people use it

  • Script will not run

    Find the syntax error and the line it is on.

  • Check a snippet before use

    Paste code from a lesson and check it before you add it to your page.

  • Learning to code

    Common mistakes come with a plain fix, so you learn as you go.

  • Then minify

    Make the clean script smaller for your site.

    HTML, CSS and JavaScript minifier

  • Try a pattern

    Test the regular expressions in your code.

    Regex tester online

Why this one

It points to the exact line and says what to change in simple words.

  • Line number and a plain fix for each problem.
  • Catches syntax errors and common mistakes.
  • Your code never leaves your device.

What you can change

  • HTML, CSS or JavaScript
  • Show errors, warnings or tips
  • Also tidy the layout when fixing

People find this page by searching

  • javascript validator
  • js error checker
  • check javascript syntax online
  • javascript lint online
  • js syntax checker
  • find javascript errors
Cite this tool
  • APA
    sdev.in. (n.d.). JavaScript validator [Web tool]. https://www.sdev.in/tools/javascript-validator
  • MLA
    "JavaScript validator." sdev.in, https://www.sdev.in/tools/javascript-validator.
  • Chicago
    sdev.in. "JavaScript validator." https://www.sdev.in/tools/javascript-validator.
  • BibTeX
    @misc{sdev-javascript-validator,
  title = {JavaScript validator},
  author = {{sdev.in}},
  howpublished = {\url{https://www.sdev.in/tools/javascript-validator}},
  note = {Web tool}
}

Questions people ask

What does a JavaScript validator check?

First it reads the code like a browser would and explains any syntax error in plain words, pointing at brackets that are not closed. Then it looks for mistakes that only show up when the code runs: names used without being declared, typos in names, changing a const, duplicate keys, code after return, = used in an if, and checks against NaN that are always false.

Does it run my code?

No. It only reads the code, so it is safe to check code that deletes files or sends messages. Nothing is run and nothing is sent anywhere.

What does Fix what can be fixed safely change?

Only lines that are already broken and have one clear repair: a misspelled name one letter away from a real one, a typeof check against a misspelled type, x === NaN turned into Number.isNaN(x), and leftover debugger lines. Tidy the layout also formats the code with even spacing and semicolons. You see every change first.

Can it check TypeScript or JSX?

JSX is understood. TypeScript type annotations are not plain JavaScript, so they show up as syntax errors here.

Is my code uploaded anywhere?

No. The checks run in a background worker inside your browser. Your code never leaves your device, so it is safe to check private work.

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