---
title: "Flatten & Unflatten JSON (Dot Notation, CSV) | JSON Lark"
description: "Flatten nested JSON into dot or bracket path keys and unflatten it back, round-trip safe even for keys with dots. CSV output too. Runs in your browser."
url: https://jsonlark.me/json-flatten/
---

Sheet 08 Flatten

# Flatten and unflatten JSON

Turn nested JSON into one level of path keys such as `address.geo.lat`, as JSON or CSV, and turn path keys back into nested JSON.

## How to use it

Paste nested JSON to get a flat object whose keys are paths, or switch to **Unflatten** and paste path keys (one object, or an array of them) to rebuild the nesting. **Use output as input** flips the result back for a quick round trip.

- **Separator and array indexes**: Keys are joined with the separator you choose; array indexes are written `tags[0]`, or `tags.0` with the separator.
- **Keys that contain the separator**: A key that would be ambiguous is quoted as a JSON string in brackets: `settings["theme.dark"]`. The same goes for empty keys, keys with brackets or quotes, and digits-only keys when indexes use the separator. That is what makes flatten and unflatten exact inverses.
- **Empty objects and arrays**: Kept as values (`"notify": []`), so they survive the round trip.
- **CSV**: An array of objects becomes one row per item, with every path key that appears as a column; anything else becomes a single row. Cells are quoted per RFC 4180.

## Questions

**What if two keys collide when unflattening?**

You get an error naming the key, for example when a.b needs a to be an object but another key set a to a number. Missing array positions, such as [0] and [2] without [1], are filled with null and the status lists them.

**Can I unflatten a CSV file?**

Unflatten reads JSON, so convert the CSV to an array of objects first, for example with the [CSV to JSON converter on JSONMender](https://jsonmender.me/csv-to-json/), and paste the result: its path-key headers become nested objects again.

**Is my JSON uploaded?**

No. Flattening runs in this tab and nothing you paste leaves it. Page views are counted with Cloudflare Web Analytics, which sets no cookies and never sees your data.