How to Check JSON Syntax Errors and Format Correctly

JSON looks simple until it breaks. A missing comma, an extra bracket, or a trailing comma after the last item — any of these will cause parsers to reject the entire document. If you're getting "unexpected token" errors or your API is returning 400 responses, the JSON is almost certainly malformed somewhere.

This guide covers how to identify exactly what's wrong, fix it, and format your JSON so it's both valid and readable.

Why JSON Is Strict About Syntax

Unlike HTML, which browsers try to render even with errors, JSON parsers have zero tolerance for malformed input. The spec (RFC 8259) is deliberately rigid. This means one typo in a 10,000-line file will cause the whole thing to fail silently or throw a cryptic error.

The strictness exists for a reason: JSON is designed for machine-to-machine communication, and ambiguity would create inconsistency across implementations. So when your JSON breaks, it breaks completely — and you need to find exactly where.

The Most Common JSON Syntax Errors

Before reaching for a validator, knowing what to look for speeds up debugging:

Trailing commas — JSON does not allow a comma after the last item in an object or array. JavaScript does, which trips up developers who write JSON by hand.