What does flattening nested JSON mean?

Flattening walks every branch of a nested JSON object or array and collapses each leaf value into a single key that records its full path, joined by a delimiter. For example the nested value at order then customer then name becomes the flat key order.customer.name. The result is a one-level object of path/value pairs that is easy to scan, diff, search, or load into a spreadsheet.

How are arrays handled?

Array items are addressed by their numeric index. You choose the notation: dotted index produces items.0.sku and items.1.sku, while bracket notation produces items[0].sku. Both round-trip losslessly, because the unflatten step recognises a numeric token as an array index and rebuilds an array rather than an object.

Can I turn the flat keys back into nested JSON?

Yes. Switch the mode toggle to Unflatten, paste a flat object such as a.b.c equals 1, and the tool rebuilds the nested structure. Use the same delimiter and array notation you flattened with so the paths are parsed the same way. The Swap button moves the current output into the input and flips the mode in one click for quick round-trips.

How does the CSV export work?

Export flat CSV writes a two-column file — key and value — with one row per leaf. It uses the Papa Parse library entirely in your browser, quoting fields so commas, quotes, and newlines inside values stay intact. Nested objects or arrays that survive as leaf markers are written as their compact JSON text so the CSV is still valid.

What happens to empty objects and arrays?

By default the Keep empty option is on, so an empty object becomes a leaf marked {} and an empty array becomes []. This preserves the original shape when you unflatten. Turn the option off if you only care about real values and want empty containers dropped from the flat output.

Is my data sent anywhere?

No. All parsing, flattening, unflattening, and CSV generation happen locally in your browser using JavaScript. Nothing is uploaded to a server. Your most recent input and your delimiter, array-notation, and empty-handling settings are saved in your browser local storage so the tool restores them next time.

Nested JSON Flattener

The Nested JSON Flattener turns awkward, deeply nested JSON into a clean, one-level map of dot-notation keys and their values — and reverses the process just as easily. It is built for developers, data engineers, QA testers, and analysts who need to inspect a payload, diff two API responses, build a flat config, or move JSON into a spreadsheet without hand-editing brackets. Paste any object or array, and every leaf value gets a key that spells out its full path, like order.customer.name or order.items.0.sku. Switch to Unflatten and the same tool rebuilds the original nested structure from those flat keys, so the conversion is a true round-trip rather than a one-way dump.

How it works

The flattener walks the entire JSON tree recursively. Each time it reaches a primitive value — a string, number, boolean, or null — it records the joined path as a key. Objects contribute their property names; arrays contribute their numeric index. You control two things that matter for downstream tools: the delimiter that joins path segments (a dot by default, but you can use a slash, underscore, or anything short), and the array notation. Dotted indexes produce items.0.price; bracket notation produces items[0].price. The Keep empty option decides whether empty objects and arrays are preserved as explicit {} and [] leaf markers so the shape survives a round-trip.

Unflattening reverses all of this. It splits each key back into path segments, normalising bracket notation to indexes first, then walks down the structure creating objects or arrays as needed. A numeric token tells it to build an array; anything else builds an object. If every top-level key is numeric, the root itself collapses back into an array. Because the logic is symmetric, flattening then unflattening with the same settings returns exactly what you started with.

Example

Given an order object with a nested customer and a two-item array, flattening produces keys such as order.id, order.customer.name, order.customer.tags.0, order.items.0.sku, order.items.0.qty, and order.items.1.price. The summary line reports the number of leaf keys and the maximum nesting depth, and the scrollable table previews every pair.

Flat key	Value
`order.id`	`A-1007`
`order.paid`	`true`
`order.customer.name`	`Ada Lovelace`
`order.items.0.sku`	`BK-01`
`order.items.1.price`	`3.2`

Click Export flat CSV and those rows download as a quoted two-column spreadsheet ready for Excel, Google Sheets, or a database import — generated locally, with nothing ever leaving your browser.