JavaScript Validator

These two failure modes need different tools and it is worth keeping them apart. A syntax error means the parser cannot read your file, so nothing in it runs — the failure is total, immediate, and usually easy to fix once you know where it is. A logic error means the code parses and runs perfectly while doing the wrong thing, which is far more expensive because it can reach production and behave incorrectly for a long time before anyone notices. Validation eliminates the first category quickly; tests, types, and code review are what address the second.

A validator is a diagnostic, and the better outcome is never needing it. An editor with JavaScript language support underlines an unbalanced bracket the moment you type it. A formatter that refuses to run is itself a signal that the file does not parse. ESLint on commit stops broken code entering the repository, and a build step fails loudly rather than deploying a file that will not load. Use this tool for code arriving from elsewhere — a CMS field, an email, a generated file — and let your editor handle the rest.

It catches structural syntax problems: brackets, braces, and parentheses that do not balance, and strings or template literals that are never closed. Those are the mistakes that stop a file parsing at all, and because a parse failure means nothing in the file runs, they present as a script that does absolutely nothing. What it cannot catch is anything about meaning — a typo in a property name, a variable used before it exists, an await that was forgotten, or a condition that is inverted are all perfectly valid syntax and will only show up at runtime or in a linter.

No, and they solve different problems. This checks whether the code parses. ESLint parses it too, then applies hundreds of rules about correctness and style — unused variables, unreachable code, missing await, unsafe comparisons, patterns your team has agreed to avoid. ESLint is what you want configured in your editor and running on commit for code you own. A standalone syntax check is for a snippet from somewhere else, where setting up a linter is disproportionate.

A syntax error is found before any of the file runs, so nothing in it executes — including whatever would have logged an error. In a browser the failure is reported in the console, but it is easy to miss if the console is closed or if the page appears to work because other scripts loaded fine. The symptom is a feature that simply does not respond, with no visible error, which is why checking that a file parses is a sensible first step when something has stopped working entirely.

It handles current JavaScript, including classes, modules, async and await, optional chaining, and template literals. TypeScript is a different language: type annotations, interfaces, and generics are not valid JavaScript, so a .ts file will produce errors that are not real problems. Use the TypeScript compiler for TypeScript. Similarly, JSX is not plain JavaScript and needs a JSX-aware parser.

No. Everything runs in your browser as JavaScript — your code is never uploaded, never logged, and never sent to a server. You can verify that by watching the Network tab in devtools while you work, or by disconnecting from the internet after the page loads and confirming the tools still function. Nothing you paste is executed either; it is only ever parsed as text.