What separator formats are supported?

Any single or multi-character separator is accepted. The default is a dot (.). Common alternatives are underscore (_), double underscore (__), or forward slash (/). Set it to match whatever was used when the JSON was originally flattened.

How are array keys handled?

Keys with bracket-index notation such as items[0].name and items[1].name are automatically converted into a proper JSON array: {"items":[{"name":...},{"name":...}]}. Sparse index gaps are filled with null.

What if two keys conflict — e.g. "a" and "a.b"?

If a key path tries to set a property on a value that is not an object (e.g. "a" is already a string and "a.b" tries to nest inside it), the deeper key wins and overwrites the earlier value. The tool shows a warning in that case.

Is my JSON data sent to a server?

No. All processing is entirely client-side in your browser. Your data never leaves your device.

Is there a size limit?

There is no enforced limit — everything runs locally. Very large files (several MB) may be slow depending on your hardware.

Can I unflatten an array at the root level?

Yes. If your flat object uses only numeric bracket keys at the root (e.g. [0].name, [1].name) the tool produces a root-level array instead of an object.

JSON Unflatten

Paste flat dot-notation JSON and instantly expand it back into a deeply nested object.

Flat JSON Input

Separator

Pretty-print output

Nested Output

Paste flat JSON and click Unflatten

Copied!

Summary

JSON Unflatten is the reverse of a JSON flattener: it takes a single-level object whose keys use dot notation (e.g. user.address.city) and reconstructs the original nested hierarchy. It also handles bracket-index array notation (e.g. items[0].name), automatically creating proper JavaScript arrays. All processing happens in your browser — your data never leaves your device.

How it works

Paste or type your flat, dot-notation JSON into the input panel.
Choose the separator used in the keys (default is a dot).
Click Unflatten to rebuild the nested object.
Review the expanded nested structure in the output panel.
Click Copy Output to copy the result to your clipboard.

Use cases

Reverse output from a JSON flattener back to its original form.
Reconstruct nested config objects from flat environment variable maps.
Convert spreadsheet column headers back to nested API request bodies.
Rebuild nested log payloads from flat search-index documents.
Debug dotted-path key mappings in template engines or ORMs.
Prepare flat database row data for nested JSON API responses.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu