JSON CLEAN
A JSON formatter and validator for developers who paste API responses and need the formatted output to be the same data they put in.
Formatting looks harmless until it changes your data. If a formatter reads JSON with the browser's built-in parser and writes it back out, a very large ID can lose precision and the order of keys can shift. The built-in error messages also differ from browser to browser.
The decision was to keep the input exactly as written. JSON Clean reads the text with its own parser, keeps each number, string and literal in its original form, and keeps object keys in their original order. Formatting then only changes whitespace.
Because it tracks its own position while reading, an error points to the exact line and column, with a specific message: a trailing comma, a missing colon, an unterminated string, a bad escape sequence. You can format with two spaces, four spaces or tabs, or minify. It runs entirely in the browser.
The limit: duplicate keys pass through without a warning, very deeply nested input falls back to a generic error at the start of the text, and there are no tests for the parser yet. A custom parser is only as trustworthy as its tests.
Next I would run it against a public JSON test suite, check that formatting and then reading back always gives the original data, and add a warning for duplicate keys.