JSON to Flow Type Generator
Generate Flow type definitions from any JSON structure
JSON Input
Flow Output
What Is JSON to Flow Type Generator?
Flow is a static type checker for JavaScript developed by Meta (Facebook). It uses type annotations — written in a syntax similar to TypeScript — to catch type errors at build time without changing your runtime code. The JSON to Flow Type Generator reads a JSON document and produces Flow type alias declarations that exactly match its shape.
Unlike TypeScript, Flow integrates directly into existing JavaScript files via a comment pragma (// @flow) and is stripped at build time by Babel. This makes it ideal for teams that want incremental static typing. This tool generates ready-to-use Flow type aliases — no external library needed.
How to Convert JSON to Flow Types
Follow these three steps to generate Flow type definitions from your JSON data.
Paste or Upload JSON
Paste your JSON into the left panel, click Upload to load a file, or click Sample to see an example. The tool will immediately generate Flow types in the right panel.
Review the Generated Flow Types
The right panel shows Flow type declarations. Nested objects become separate named types. Arrays are typed as Array<T>. Null values produce a union with null.
Copy or Download
Click Copy to send the types to your clipboard, or Download to save a .js.flow file. Paste the // @flow pragma at the top of your JS file and import the types.
Common Use Cases
React & React Native Apps
Flow is widely used in React and React Native projects. Generate prop types and state types directly from your API response JSON to add compile-time safety without migrating to TypeScript.
API Response Typing
When integrating a third-party REST API, paste a sample response and get Flow types instantly. This eliminates hand-authoring verbose type declarations and reduces human error.
Legacy JavaScript Migration
Incrementally add Flow types to a large legacy JavaScript codebase. Generate types from JSON config files or data fixtures to bootstrap type coverage file by file.
Configuration File Typing
Complex JSON configuration files — build configs, feature-flag configs, app settings — can be typed with Flow so that any misconfiguration is caught before reaching production.
Frequently Asked Questions
What is the difference between Flow and TypeScript?
Both are JavaScript static-type systems. Flow is a checker that annotates plain JS files via a pragma; TypeScript is a compiled superset of JS. Flow types are stripped by Babel at build time. TypeScript has broader ecosystem support, while Flow is still preferred by some Meta-stack teams.
Are exact object types generated?
By default the generator produces inexact object types (a Flow default). You can manually add {| ... |} syntax to make them exact, which prevents extra properties. See the Flow object type docs for details.
How are arrays handled?
Arrays are typed as Array<T> where T is inferred from the first element. Empty arrays produce Array<mixed> since the element type cannot be determined.
Does my JSON leave my browser?
No. All processing happens entirely in your browser. Your JSON data is never sent to any server.
Can I use the output directly in my project?
Yes. Copy the generated types into a .js.flow declaration file or paste them at the top of your JavaScript file after the // @flow pragma. You may need to adjust names to match your codebase conventions.
Related Tools
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