Flatten & Unflatten
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Flatten deep nesting into a single level

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Common questions answered

What is JSON flattening?

JSON flattening transforms a nested object into a single-level object where every value is accessible through a dot-notation key path. For example, {"user": {"name": "Alice"}} becomes {"user.name": "Alice"}. Flattened JSON is useful for importing data into spreadsheets, flat-schema databases, or analytics tools that do not support nested structures.

This tool handles all JSON types during flattening: nested objects become dot-separated paths, arrays use bracket notation (items[0], items[1]), and primitive values remain as leaf values. You can also unflatten — converting dot-notation keys back into nested structures. This round-trip capability makes it useful for transforming data between systems with different requirements.

{
  "user": {
    "name": "Alice",
    "address": {
      "city": "Portland"
    }
  },
  "tags": ["a", "b"]
}

{
  "user.name": "Alice",
  "user.address.city": "Portland",
  "tags[0]": "a",
  "tags[1]": "b"
}

Get the most out of this tool

• Flatten JSON before importing into spreadsheets — each dot-notation key becomes a column header.
• Use bracket notation for arrays: "users[0].name" is more readable than "users.0.name" in flattened output.
• Flatten deeply nested API responses to quickly see all available fields without expanding the tree manually.

JSON flattening in depth

JSON flattening converts a nested JSON object into a single-level object where nested keys are represented as dot-separated paths. For example, {"address": {"city": "Portland"}} becomes {"address.city": "Portland"}. This transformation is useful when working with systems that do not understand nested JSON — many spreadsheet tools, SQL databases, and analytics platforms work only with flat key-value structures.

The flattening algorithm performs a depth-first traversal of the JSON tree. For each leaf value (a string, number, boolean, or null), it constructs the full path from root to leaf by joining all parent keys with a separator (usually a dot). For arrays, the index is included in the path: {"items": ["a", "b"]} becomes {"items.0": "a", "items.1": "b"}. This preserves all values in a lossless way — the original structure can be reconstructed from the flat form.

The separator character matters for usability. Dot notation (field.subfield) is the most common convention and is what JSONPath expressions use. Bracket notation (field[subfield]) mimics JavaScript property access. For CSV column headers, underscore separation (field_subfield) avoids the dot character that some tools treat specially. This tool uses dot notation by default but can be adjusted for compatibility with different target formats.

Elasticsearch, the popular search and analytics engine, uses dot notation extensively. When you index a document with nested fields in Elasticsearch, it internally flattens them. Defining explicit mapping for nested fields versus using Elasticsearch's flattened field type is a common architectural decision — using this tool to preview the flattened form helps you understand what Elasticsearch will store internally.

The reverse operation — unflattening — reconstructs the nested structure from the flat key-value form. This is useful when you have CSV or tabular data with dot-separated column headers that you want to convert back to nested JSON. The unflatten operation splits each key on the separator, creates the nested path of objects, and assigns the value at the leaf. This tool supports both flatten and unflatten operations.

Environment variables are inherently flat — environment variable names cannot contain dots or brackets in most shells. When application configuration is stored as nested JSON but needs to be exported as environment variables, flattening with underscore separation and uppercase conversion produces the standard format: {"db": {"host": "localhost"}} becomes DB_HOST=localhost. This pattern is used by many twelve-factor app configurations.

When developers use this tool

Additional frequently asked questions