JSON Validator

A JSON validator checks whether a block of text is well-formed JSON and, when it is not, tells you exactly where the problem is. It parses the text against the JSON grammar defined in RFC 8259 and ECMA-404, and either confirms the document is valid or stops at the first character that breaks the rules, reporting the line, the column, and the character offset so you can jump straight to the mistake instead of reading the whole file by eye.

The reason a dedicated validator exists, rather than just calling the language built-in parser, is that the error a built-in parser gives you is often useless. JSON.parse in a browser reports "Unexpected token in JSON at position 148" with no line number, and the position it names is frequently a few characters past the real fault. This tool walks the text itself, keeps track of line and column as it goes, and phrases the error in terms of what it expected: a comma after a member, a closing brace, a double-quoted key. A trailing comma, a single-quoted string, an unescaped tab inside a string, a leading zero on a number, and two members with no comma between them are all called out by name.

Validity is only half of what you usually need to know, so a valid document also comes back with a report. The tool lists any duplicate keys, which the standard permits but which almost always mean a bug, because a normal parser keeps only the last value and drops the rest silently. It counts objects, arrays, keys, and each kind of scalar, measures the maximum nesting depth, and reports the size in both characters and UTF-8 bytes, so you can see the shape of an unfamiliar payload at a glance. When the JSON is valid it is also pretty-printed, giving you a clean, indented copy in the same step.

Everything runs in your browser. The text you paste is parsed in JavaScript on your own machine and is never uploaded, logged, or stored, which matters because the JSON people most need to validate is often an API response or a config file carrying tokens, keys, or personal data. Because there is no server round trip, the validator is instant and keeps working with the network disconnected once the page has loaded.