How does it decide between an object and an array?

When the next path segment is a number — either a delimiter-separated index like roles.0 or a bracket index like roles[0] — that level is built as an array. Otherwise it is built as an object, and numeric-keyed objects are normalised into real arrays.

Does it round-trip with the flattener?

Yes. Output from the JSON flattener feeds straight back into this tool. Match the delimiter and array style you used when flattening to reconstruct the original structure.

Is my JSON uploaded anywhere?

No. Unflattening runs entirely in your browser as JavaScript. Your JSON is never sent to a server.

JSON Unflattener

Rebuild nested JSON from flat dot-path keys — privately in your browser.

JSON unflattener

Paste a flat JSON object whose keys are dot paths (like user.name or roles[0]) and get back the original nested structure of objects and arrays. A path segment that is a number becomes an array index, so flat data round-trips cleanly with the JSON flattener.

Set the delimiter to match how the keys were built — a dot by default — and bracket indices are detected automatically. Everything runs in your browser, so your JSON never leaves your device.