JSON Validator
Free JSON Validator to check JSON syntax against RFC 8259 and pinpoint the exact line and reason for every error.
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The JSON Validator instantly checks whether your JSON is correctly formed and, if it isn't, points you to the exact line and reason — a trailing comma, a wrong quote, an unclosed bracket. Stop squinting at a wall of data hunting for the one broken character; paste, validate, and fix in seconds. Free, private, and processed entirely in your browser.
Find the Error, Not Just "Invalid"
A single misplaced character can make an entire JSON document unparseable, and unhelpful tools just tell you it's "invalid" without saying where. This validator does the opposite: it checks your JSON against the official standard (RFC 8259) and reports the specific line and position of every problem, so you go straight to the broken token instead of scanning hundreds of lines by hand.
How to Use It
- Paste your JSON into the input.
- Validate to check the syntax.
- Fix and recheck at the flagged line until it passes.
The Errors It Catches
| Mistake | Why it's invalid |
|---|---|
| Trailing comma | Not allowed after the last item — the most common error |
| Single quotes | JSON requires double quotes for keys and strings |
| Unquoted keys | Every property name must be in double quotes |
| Missing commas | Items must be separated by commas |
| Unmatched brackets | Every { and [ must be properly closed |
The Classic Trap: JSON Is Not a JavaScript Object
This catches developers constantly. JSON looks like a JavaScript object, but it's stricter. In JSON you must use double quotes for every key and string — single quotes fail. Trailing commas are forbidden. Comments aren't allowed at all. And undefined and NaN aren't valid values. So a snippet you copied straight out of your JavaScript code will often fail validation for reasons that feel surprising until you know the rules.
Know the Building Blocks
Valid JSON is made of just a few pieces: objects (key–value pairs in curly braces), arrays (ordered lists in square brackets), strings (always double-quoted), numbers, the booleans true and false, and null. That's the complete set — anything else isn't JSON. Once these are second nature, most errors become obvious at a glance.
Validate Before You Ship
Malformed JSON doesn't fail gracefully — one bad character makes the whole document unreadable to the parser, so an API rejects the entire payload, a build breaks, or an import silently fails. Validating before you send a request or deploy a config catches these problems while they're still easy to fix. It's especially worth doing with files like package.json and tsconfig.json, where a stray comma can derail your tooling.
Private and Instant
Validation happens in your browser, so your JSON — including API responses, tokens, and sensitive configuration — never leaves your device. Free, with no signup and no limits. Paste, validate, and get back to building.
JSON Validator FAQs
What does a JSON validator do?
It checks whether your JSON is syntactically correct according to the JSON standard (RFC 8259), and if not, tells you exactly what's wrong and where. Instead of a vague 'invalid' message, a good validator reports the specific line and position of each error — a missing comma, an unclosed bracket, a wrong quote — so you can fix it in seconds rather than hunting through the whole document.
Why is my JSON invalid when it looks like a JavaScript object?
Because JSON is stricter than JavaScript object syntax. In JSON, all keys and all strings must use double quotes (not single quotes), trailing commas after the last item are forbidden, comments aren't allowed, and values like undefined and NaN are invalid. Code that's perfectly fine as a JavaScript object literal often fails as JSON for exactly these reasons.
What are the most common JSON errors?
The big ones are: a trailing comma after the last element in an object or array (the single most frequent mistake), single quotes instead of double quotes, unquoted property keys, missing commas between items, and unmatched or missing brackets and braces. Unescaped newlines or tabs inside strings also break validation. A validator flags each with its exact location.
What are the building blocks of valid JSON?
Valid JSON consists of objects (key–value pairs in curly braces), arrays (ordered lists in square brackets), strings (always in double quotes), numbers, the booleans true and false, and null. Anything outside this set — comments, functions, dates as objects, undefined — is not valid JSON. Knowing these building blocks makes most errors obvious.
Does the validator change my data?
No. Validation only checks correctness; it doesn't alter your content. If you also choose to format the JSON, that adds indentation and line breaks for readability but leaves the actual keys, values, and structure exactly the same — only whitespace changes.
Why does valid JSON matter so much for APIs?
Because a single syntax error makes the entire document unparseable — the receiving system rejects all of it, not just the bad line. APIs, config files, and data imports fail completely on malformed JSON. Validating before you send or deploy catches these errors early, preventing broken requests, failed builds, and hard-to-trace bugs.
Can I validate configuration files like package.json?
Yes, and it's a smart habit. Files like package.json and tsconfig.json are JSON, and a stray comma or missing brace can break your build or tooling. Pasting them into a validator before committing catches syntax problems early, saving you from cryptic errors later in the pipeline.
Is it private and free?
Yes. Validation runs in your browser, so your JSON never leaves your device — safe for API responses, tokens, and sensitive configs. It's free with no signup and no limits.