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JSON to Flow

Generate Flow classes/structs automatically from a JSON object.

100% Client-Side Local Execution
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What is JSON to Flow?

This free online tool turns a sample JSON payload into a ready-to-use Flow type alias for Flow, so you can use this json to flow types generator tool to go straight from raw JSON to a typed type alias without writing the boilerplate yourself. It is built around Flow's actual idiom — a Flow type — rather than a generic one-size-fits-all class, and pairs naturally with Flow's static type checker for real-world (de)serialization. Use it for json to flowtype online or any time you need json to flow interface, whether you're scaffolding a new Flow project or mapping an existing API response.

Technical Execution

Paste or upload a sample JSON object or array and the tool walks its structure field by field, inferring a concrete Flow type for each value — mapping basic JSON types to `string`, `number`, and `boolean` as appropriate. For nested data, it emits a separate named Flow type for every nested JSON object, and types arrays as `Array<T>` of the matching element type. On the naming side, it preserves the original JSON key names exactly, matching Flow's structural typing approach to describing existing object shapes. For optional data, fields seen as `null`, or missing from some sample objects, are typed with a maybe type (`?string`) or made optional with a `?:` property modifier depending on whether the key itself is ever absent.

Zero Data Storage Guarantee

Everything runs client-side in your browser: the sample JSON you paste in — which may contain real field names, endpoint shapes, or structure from a private API or project — is parsed and converted entirely on your device and is never uploaded to any server. Nothing you enter is logged, stored, or transmitted anywhere, so you can safely paste a real payload from an internal or unreleased API to generate an accurate Flow type. Closing or refreshing the tab clears everything, since no copy of your data ever left your machine in the first place.

How to Use JSON to Flow (Step-by-Step Guide)

  1. Copy a representative sample of your JSON (the more fields and nested objects it includes, the more complete the generated type alias will be).
  2. Paste it into the input panel of the JSON to Flow tool, or click "Upload" to load it from a json file.
  3. Review the auto-detected root name, and rename it if you want the generated Flow type to use a specific class or type name instead of the default.
  4. Click "Generate" to produce the Flow type alias, which appears instantly in the output panel with syntax highlighting.
  5. Click "Copy" to copy the generated code to your clipboard, or "Download" to save it as a `.js.flow` file.
  6. Paste the result into your Flow project and adjust field names or types if your data has edge cases the sample didn't cover.

Frequently Asked Questions (FAQ)

Q: How does this tool handle nested objects and arrays when converting JSON to Flow?
A: The tool emits a separate named Flow type for every nested JSON object, and types arrays as `Array<T>` of the matching element type That keeps the generated code organized the way a Flow developer would structure it by hand, instead of flattening everything into one giant type alias.
Q: What happens to fields that are nullable or missing from some records?
A: For optional data, fields seen as `null`, or missing from some sample objects, are typed with a maybe type (`?string`) or made optional with a `?:` property modifier depending on whether the key itself is ever absent. If your sample only has one example object, fields that could be null in other records might not be detected as optional — paste a few varied samples (or an array of objects) for the most accurate result.
Q: Is this better than writing the type alias by hand?
A: For anything beyond a trivial, flat payload, yes — writing Flow types by hand for a nested JSON payload means manually tracking every maybe type and optional property, and that upkeep grows quickly as the API response changes. The generator produces the same result in seconds and gives you a correct starting point to refine, rather than a blank page.
Q: Is my JSON data safe to paste into this tool, especially if it's from a private API?
A: Yes. Everything runs client-side in your browser: the sample JSON you paste in — which may contain real field names, endpoint shapes, or structure from a private API or project — is parsed and converted entirely on your device and is never uploaded to any server. Nothing you enter is logged, stored, or transmitted anywhere, so you can safely paste a real payload from an internal or unreleased API to generate an accurate Flow type. Closing or refreshing the tab clears everything, since no copy of your data ever left your machine in the first place.
Q: Can I use this as a convert json to flow type?
A: Yes — the tool is commonly used as a convert json to flow type, and works the same way whether you're converting a one-off response for a prototype or generating model code for a production project.
Q: Does the tool rename fields to match Flow naming conventions?
A: Yes. It preserves the original JSON key names exactly, matching Flow's structural typing approach to describing existing object shapes, so the generated code reads naturally in Flow while still deserializing the original JSON correctly.
Q: What Flow library does the generated code work with?
A: The output is written to pair cleanly with Flow's static type checker, the most common choice for Flow developers handling JSON, so you can drop it into an existing project without changing your serialization setup.
Q: Does this tool work for deeply nested or large JSON samples?
A: Yes. The tool recurses through every level of nesting it finds and generates a complete set of linked type alias definitions, not just the top level — though for very large samples, trimming it down to one representative record per array is usually enough to get an accurate result.
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