JavaScript Validator & JS Syntax Checker Online
Check JavaScript syntax with the Acorn parser. Paste code or open a file and see the first syntax error with its line and column. Runs in your browser.
Find the syntax error that stops your script
One missing bracket stops a whole script from running, and the browser console does not always point to the right place. This validator parses your code with Acorn and tells you whether it is valid JavaScript. If it is not, you get the first error Acorn hits, with its line and column.
Worked example: a missing parenthesis
function total(items) {
return items.reduce((sum, item) => sum + item.price, 0;
}Validating this reports an unexpected token on line 2. The parser reaches the semicolon while it is still inside the reduce( call, where only a comma or a closing parenthesis can come next. Add ) after the 0, validate again, and the result says the code parsed cleanly as an ES module.
The reported position is where the parser got stuck, which is often just after the real mistake. Look at the reported line and the line above it first.
Common errors and what they usually mean
| Message | Likely cause |
|---|---|
| Unexpected token | A missing ), ], } or comma, or an extra one, near the reported position. |
| Unterminated string constant | A quote opened with ' or " is not closed on the same line. |
| Unterminated template | A template literal is missing its closing backtick. |
| Unexpected keyword | A reserved word such as class or default is used as a name. |
| Identifier has already been declared | The same name is declared twice with let, const or class in one scope. |
| 'return' outside of function | A return at the top level of the file. |
Why it parses your code as a module
The validator reads code as an ES module, so import, export and top-level await are accepted. A leading #! line, as in Node scripts, is also accepted. Modules always use strict mode, which rejects some old script syntax: with statements, legacy octal numbers such as 010, and await used as a variable name. If an older script that runs in a plain <script> tag is rejected for one of these reasons, the error is real for module code but may not affect the old script.
What a clean result does and does not mean
- It means the code follows JavaScript grammar and can be parsed.
- It does not mean the code works. Undefined variables, misspelled properties, wrong argument types and logic errors are valid syntax.
- Only the first error is shown. Acorn stops at the first problem, so fix it and validate again.
- JSX and TypeScript fail. Tags such as
<App />and annotations such as: stringare not standard JavaScript.
A quick routine
- Paste the code or open a .js, .jsx or .mjs file.
- Click Validate JavaScript and go to the reported line. Columns count from 0, so add 1 to match most editors.
- Fix and repeat until the code parses cleanly, then run it or its tests to catch logic errors.
The check runs in your browser and the pasted code is not sent to ToolJiffy. To tidy the code afterwards, use the JavaScript formatter. Use the JavaScript minifier to shrink it for production, or the JSON formatter for JSON data.
Frequently Asked Questions
What does the JavaScript validator check?
It parses your code with Acorn, an MIT-licensed JavaScript parser used by tools such as webpack. If the code breaks a grammar rule, it shows Acorn's error message with the line and column. If parsing succeeds, it confirms the code is valid as an ES module using current ECMAScript syntax.
Does it fix my code?
No. It reports the first syntax error it finds and where it is. You make the fix, then validate again to find the next error, if any.
How do I read the position in the error?
An error such as "Unexpected token (2:56)" means line 2, column 56. Acorn counts columns from 0, so the character is the 57th on that line in most editors.
Why is code that runs in my browser rejected?
The validator parses code as an ES module, and modules always use strict mode. Older script-only syntax, such as with statements, legacy octal numbers like 010, a top-level return or using await as a variable name, is rejected in modules.
Does it find bugs that are not syntax errors?
No. Undefined variables, typos in property names, wrong types and logic mistakes are valid syntax, so they pass. A linter such as ESLint or running your tests catches those.
Can I validate JSX or TypeScript?
No. JSX tags and TypeScript type annotations are not standard JavaScript, so Acorn reports them as syntax errors. Check them with your project's own compiler.
Is my code uploaded?
No. Acorn runs as JavaScript in your browser, and the pasted code is not sent to ToolJiffy. The page itself still loads ordinary site resources, analytics and ads.