Delimiter: Custom: Auto-detect arrays (numeric keys)

Features

🔀 Reverse Flattening

Converts dot-notation keys to nested objects
Restores original JSON hierarchy
Handles deeply nested structures
Preserves all value types

📋 Array Detection

Auto-detects numeric keys as array indices
Reconstructs ordered arrays correctly
Optional — can be disabled
Works with mixed object/array structures

⚙️ Flexible Delimiters

Dot (.) — most common default
Underscore (_) — common in env vars
Slash (/) — URL-path style keys
Custom delimiter for any format

JSON Unflatten Guide

How Unflattening Works

Flat JSON uses dot-separated keys to represent hierarchy. This tool splits each key on the delimiter and rebuilds the nested structure.

// Input (flat)
{
  "user.name": "Alice",
  "user.age": 30,
  "tags.0": "js",
  "tags.1": "go"
}

// Output (nested)
{
  "user": { "name": "Alice", "age": 30 },
  "tags": ["js", "go"]
}

Step-by-Step Tutorial

Paste your flat JSON into the input panel, or click Load Sample to try an example.
Choose your delimiter — the character that separates key segments (default is .).
Leave Auto-detect arrays checked if numeric segments like .0, .1 should become array indices.
Click Unflatten JSON to generate the nested output.
Copy or download the result using the buttons in the output panel.

Common Use Cases

Config files: Restore nested configuration objects from flat env-style keys.
Database round-trips: Reconstruct JSON documents that were flattened for storage in relational tables.
Form data: Convert flat HTML form field names (e.g. address.city) back into a structured payload.
API transformation: Reverse flatten operations applied by libraries like flat or lodash.set.
Logging pipelines: Restore original event objects from flattened log entries.

FAQ

Q: What happens if a key contains the delimiter character as part of the actual name?

A: The tool will split on every occurrence of the delimiter. If your keys contain literal dots (or whatever delimiter you chose), switch to a different delimiter or use a custom one that does not appear in your key names.

Q: Can I unflatten deeply nested structures?

A: Yes. There is no hard nesting limit. Keys like a.b.c.d.e will produce a five-level deep nested object.

Q: When should I disable array detection?

A: Disable it when your original data intentionally used numeric strings as object keys (e.g. {"scores.1": 100} should remain {"scores": {"1": 100}} rather than an array).

Where flat keys come from

Flat, dotted keys appear wherever a system can only store one level of key and value: environment variables, spreadsheets, form fields, analytics exports and key-value stores. Unflattening restores the nested structure your application expects.

Tips and common pitfalls

Round trip safely with our JSON Flatten tool, using the same delimiter in both directions.
If real key names contain dots, such as domain names, pick a different delimiter like __ or /.
Numeric segments such as items.0.name can become array indexes. Turn this off if those numbers are really IDs.
Check for conflicts: a and a.b cannot both hold values, since a cannot be a value and an object at once.

JSON Unflattener