JSON Validator

Validate JSON online and find errors fast

A JSON validator checks whether a block of text is well-formed JSON according to the official standard (RFC 8259). It is the fastest way to find out why an API request is being rejected, why a configuration file refuses to load, or why a data import is failing. Paste your JSON or upload a file and this validator instantly tells you whether it is valid — and if not, it pinpoints the exact line and column where the problem is, so you can fix it in seconds instead of scanning the whole document by hand.

Everything runs in your browser, so even confidential payloads — webhook bodies, access tokens, private configuration — are validated locally and never uploaded to a server.

What makes JSON valid?

Valid JSON follows a small, strict set of rules. The document must be a single value: usually an object wrapped in curly braces { } or an array wrapped in square brackets [ ]. Every object key must be a double-quoted string. Values may only be strings, numbers, booleans (true/false), null, objects or arrays. Items are separated by commas, and crucially there must be no trailing comma after the last element. Strings use double quotes — never single quotes — and special characters inside them must be escaped. Comments are not allowed.

How to read a JSON validation error

When JSON fails to parse, the parser stops at the first character it cannot make sense of and reports its position. This validator converts that position into a human-readable line and column. Remember that the reported spot is where parsing failed, which is frequently just after the real mistake — a missing comma on the previous line, for example, is detected at the start of the next token. Read the message and the surrounding lines together and the fix usually becomes obvious.

Common reasons JSON is invalid

• Trailing commas after the last array or object element.
• Single quotes instead of double quotes around keys or strings.
• Unquoted keys (valid in JavaScript, invalid in JSON).
• Missing commas, brackets or braces.
• Comments (// or /* */), which JSON does not support.
• Invalid values such as NaN, undefined or numbers with leading zeros.
• Unescaped control characters or a stray byte-order mark at the start of the file.

Why validate before you ship?

Invalid JSON fails loudly in some systems and silently in others. APIs return 400 errors, config loaders crash on boot, and data pipelines can drop malformed records without warning. Validating at the boundary of your system — as data comes in, and before it goes out — catches these problems early with a clear, actionable message. It is one of the cheapest habits you can adopt to make an application more robust, and this tool makes it effortless.