JSON Validator

A validator answers a narrow question: will a strict parser accept this text? The rules are short and set by RFC 8259. Strings and keys use double quotes. There are no comments. The last item in an object or array has no comma after it. Numbers have no leading zeros, no leading plus sign, and a digit on both sides of a decimal point. The only bare words are true, false and null, in lower case. This page checks exactly those rules, so if it says valid, any standard JSON parser will read the text.

Most invalid JSON fails for the same handful of reasons, and each has its own message here. A comma before } or ] is usually left behind after deleting the last item. A missing comma usually sits at the end of the line above the one named, so the message says so. Single quotes and bare keys come from JavaScript object literals. Curly quotes come from pasting through a chat app or a word processor. True and None come from Python's print. A leading zero, as in a PIN like 0412, is not a valid number, and the hint suggests putting it in quotes so the zero is not lost.

Brackets that do not match are the slowest mistake to find by eye in a long file. When a { or [ is never closed, the page points at the bracket that was opened, not at the end of the file. When the closing bracket is the wrong kind, as in a list closed with }, it says which kind was expected. When there is something after a complete value, such as a second object without a comma or a stray }, it says the JSON was already finished, which usually means two documents were pasted together.

Valid is not always the same as safe to use. The page also reports duplicate keys, which RFC 8259 discourages because programs disagree on which copy wins, and integers too large for JavaScript to hold exactly. A file with a byte order mark at the start, common when saved from Windows tools, is accepted and noted, since some servers reject it. None of these notes change your data; they tell you what another program might do with it.

Validation happens on this device. The text in the box and any file you open are read in the browser tab, and no part of them is sent to TaskNinja or a third party. There is nothing to log in to and nothing is saved when you close the tab.

If you only need to know whether the JSON is valid, you can close the page once you see the result. If you copy or download the formatted output, that copy lives on your device like any other file.