JSON Unflattener

Rebuild nested JSON from a flat object whose keys are paths (user.name, tags[0]). The reverse of flattening — bracket indexes become arrays and dotted segments become nested objects.

What unflattening does

This is the inverse of flattening: it reads a single-level object whose keys are paths and rebuilds the original nesting. {"user.name":"Asha","tags[0]":"admin"} becomes an object containing a nested user object and a real tags array. Data that arrives flat — from spreadsheets, form submissions, environment-style configs or log exports — leaves as structured JSON your code can consume.

Worked example

Input:

{
  "user.name": "Asha",
  "user.address.city": "Pune",
  "tags[0]": "admin",
  "tags[1]": "editor",
  "active": true
}

Output:

{
  "user": { "name": "Asha", "address": { "city": "Pune" } },
  "tags": ["admin", "editor"],
  "active": true
}

When to use it

• Restoring structure after editing flattened data in a spreadsheet.
• Converting dotted-key exports from logging or analytics systems into real JSON.
• Building nested request payloads from flat form-style field names.
• Completing a flatten → edit → unflatten round trip with the JSON Flattener.

Limitations to know

• Keys must be consistent: if one key claims a is a value and another needs a.b to exist, the later key wins and a warning explains what was overwritten — the tool never fails silently.
• Whether tags.0 means an array index or an object key named “0” is your call via the numeric-segments option; data with year-keys like "2024" usually needs it off.
• Sparse indexes (only tags[3] present) produce null slots for the missing positions, because arrays have no gaps in JSON.

Common errors and fixes

• “The input must be a flat JSON object” — the tool received an array or a scalar; the input needs to be one object whose keys are the paths.
• Everything ended up under one giant key — the data uses a different delimiter (slashes or underscores); set the matching delimiter option.
• Numbers became array indexes unexpectedly — turn off “treat numeric segments as array indexes” to keep numeric object keys as keys.

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