JSON Formatter and Validator pretty-prints JSON with a 2-space, 4-space, or tab indent, minifies it to one line, and can sort object keys recursively. Invalid JSON is reported with the parser's message and a best-effort line and column.
Everything runs in the page. Nothing you paste is uploaded.
No. Keys are sorted recursively inside objects; arrays keep their order.
Browsers word the parser error differently and do not all report a position, so the line and column are a best effort.
No. Parsing and formatting run in JavaScript in the open tab.
For valid JSON it summarises the structure: how many objects and arrays, and the deepest nesting level.