JSON Unflatten: Dot Notation to Nested JSON

Rebuild nested data with JSON unflatten

JSON unflatten is the reverse of flattening. It takes an object whose keys are paths, such as "order.customer.name", splits each key on the separator and writes the value into a nested structure. It is the quickest way to turn a flat export, a translation file or a key-value dump back into readable JSON.

Example

Input:

{
  "order.id": 1001,
  "order.items.0.sku": "MUG-01",
  "order.items.1.sku": "CAP-02"
}

Output:

{
  "order": {
    "id": 1001,
    "items": [
      { "sku": "MUG-01" },
      { "sku": "CAP-02" }
    ]
  }
}

Bracket indexes work too: "user.tags[0]": "admin" and "user.tags[1]": "beta" produce "tags": ["admin", "beta"], so output from JSON flatten in either array style can be pasted straight back.

Options explained

• Key separator: the string that splits a key into segments. Set it to __ to expand {"db__host": "localhost", "db__port": 5432} into {"db": {"host": "localhost", "port": 5432}}.
• Numeric keys: Become arrays (default) turns segments like 0 and 1 into array positions.
• Numeric keys: Stay object keys keeps them as strings. Use this when numbers are identifiers, not positions.
• Indent: 2 spaces, 4 spaces or tabs.

Edge cases

Numbers that are really IDs

With the default setting, {"http.ports.80": "web", "http.ports.443": "tls"} creates an array with 444 slots, 442 of them null. Switch Numeric keys to Stay object keys and you get the intended {"http": {"ports": {"80": "web", "443": "tls"}}}. The same applies to HTTP status codes, years and database IDs.

Conflicting keys

If the input has both "a": 1 and "a.b": 2, the scalar is replaced by an object when the deeper key is processed, because a cannot be a number and an object at once. Keys are applied in the order they appear.

Input must be a flat object

Arrays and bare values are rejected with a clear message. Values themselves can be anything, including nested objects, which are copied as they are.

Unflatten vs. other tools

Unflatten works on JSON input. If your dotted keys live in a Spring application.properties file, use Properties to JSON, which parses the file format and nests keys in one step. For environment variables joined with __, the .env to JSON converter has a nesting option.

Where flat JSON comes from

• Translation files. Tools for i18n often store keys like "nav.home.title". Unflatten them when a library expects nested objects.
• Spreadsheet exports. Column headers such as address.city turn into flat keys after CSV to JSON; unflatten each record to get nested objects back.
• HTML forms. Fields named shipping.street or items[0].qty are serialized as flat pairs, and unflattening shows the intended structure.
• Logs and analytics. Many logging pipelines flatten events so each path becomes a searchable field.

Privacy

Everything runs locally in your browser; the JSON you paste is not uploaded.