JSON Validator

Two kinds of valid

Syntactic validity means the document parses: braces balance, strings are double-quoted, no trailing commas. Semantic validity means the document has the shape your system expects: required fields present, types correct, values within permitted ranges. A payload can be perfectly parseable and still be wrong for your API. This validator handles both, syntax first and then optional JSON Schema validation.

Reading a syntax error

Parse errors are reported at the point the parser gave up, which is often after the real mistake. A missing comma between two objects is reported at the start of the second object. An unterminated string swallows everything until the next quote, so the error surfaces far below the line at fault. When the reported position looks innocent, check the line above it.

JSON Schema

JSON Schema describes the expected structure of a document: which properties are required, what types they take, patterns strings must match, numeric bounds, and how arrays and nested objects are constrained. It is the practical way to validate API request bodies and configuration files, and it is what OpenAPI uses to describe payloads.

The most useful keyword and the most commonly forgotten is additionalProperties: false. Without it a schema silently accepts unknown fields, so a typo in an optional property name passes validation and is then ignored at runtime. That failure mode is hard to debug precisely because nothing complains.

Duplicate keys

The specification says object keys should be unique but does not require parsers to reject duplicates. Most parsers, including the browser's, keep the last occurrence and discard earlier ones without warning. That hides real bugs when a document is assembled by concatenation or templating. This validator reports duplicate keys explicitly rather than silently collapsing them.

Everything stays local

Validation runs in your browser. Config files and API payloads routinely contain secrets, and sending them to a remote validator to check a missing brace is an unnecessary disclosure.