JSON Validator – Validate JSON Online (Free Tool)
Free online JSON Validator tool to validate your JSON data and find errors quickly. Get detailed error messages with line numbers and position indicators to help fix invalid JSON syntax instantly. Checks for common JSON errors like missing brackets, trailing commas, unquoted keys, and malformed strings. All validation happens locally in your browser ensuring your data remains private and secure.
What is JSON Validator?
JSON Validator parses your input against the strict JSON grammar (RFC 8259) and tells you exactly where the first syntax error is. Use it to confirm an API response is shaped the way you expect before parsing, or to debug hand-written config files that the runtime rejects with a vague message. Validation happens in the browser, so private payloads stay on your machine.
Why use this tool?
- Works offline after the first page load
- Lightweight page weight that loads quickly even on slow connections
- Works on desktop, tablet, and mobile in any modern browser
- Works with very large inputs (multi-megabyte JSON, long regex patterns, big tables)
- No telemetry on the tokens, payloads, or code you paste in
How to use
- Paste your JSON data into the input field
- Click "Validate" to check if your JSON is valid
- View validation results and error details if any
Examples
Catch a missing comma
Input: {"a":1 "b":2} The validator points to "b" and reports an unexpected string token where a comma was expected.
Detect a trailing comma
Input: {"items":[1, 2, 3,]} The validator flags the trailing comma — valid in JavaScript object literals but illegal in JSON.
Verify before parsing
Paste an API response, validate, then copy with confidence into JSON.parse() in your code without runtime surprises.
Common use cases
- Debugging "Unexpected token" errors thrown by JSON.parse
- Validating config files (package.json, tsconfig.json) before commit
- Sanity-checking webhook payloads in production logs
- Pre-flight checking JSON fixtures used by automated tests
- Teaching JSON syntax — the error pointer makes the rule concrete
Troubleshooting
- Validator says "Unexpected token '" near a string.
- JSON requires double quotes around keys and string values. Replace single quotes with double quotes throughout the document.
- Says comments are not allowed.
- JSON does not support // or /* */ comments. Strip them or switch to JSON5/JSONC if your tooling supports it.
- Numbers like 1.0e500 marked invalid.
- JSON allows exponents but values must fit in IEEE 754 double precision (max ~1.8e308). Reduce magnitude or store as a string.
Frequently Asked Questions
JSON validation checks if your JSON data follows the correct syntax rules. It verifies proper bracket matching, correct use of quotes, valid data types, and proper structure.
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