JSON to Flow Types Converter

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Bring Static Type Safety to Your Flow Projects with Automatic JSON Conversion

Flow, Facebook's static type checker for JavaScript, offers a powerful alternative to TypeScript for teams already invested in the Flow ecosystem. But when you're working with APIs that return JSON data, manually defining Flow type annotations for every response shape is monotonous and error-prone. The JSON to Flow Types Converter takes a sample JSON payload and generates accurate Flow type definitions automatically, saving you from the drudgery of hand-typing every property and nested object.

Whether you're integrating a third-party REST API, processing webhook payloads, or defining shapes for your own backend responses, this tool produces Flow types that match your data exactly. Strings become string, numbers become number, booleans become boolean, arrays get properly typed, and nested objects generate their own named type aliases. The result is clean, readable Flow code that you can paste directly into your project.

How the Conversion Process Works

Paste any valid JSON into the input area - a simple object, a deeply nested structure, or an array of records. The JSON to Flow Types Converter analyzes the structure recursively. For each object, it creates a Flow type alias with every property annotated. Nullable values are detected and marked with ? syntax. Arrays are typed based on their contents - if all elements share the same shape, you get a clean Array<ItemType> rather than a generic Array<mixed>.

The converter handles real-world JSON complexity gracefully. Mixed-type arrays, null values, empty arrays, and deeply nested hierarchies all produce sensible Flow annotations. You won't get perfect types for every edge case - no automated tool can infer runtime semantics from a single sample - but you'll get a solid foundation that's 90% done, leaving you to fine-tune only the ambiguous parts.

Why Flow Types Still Matter in a TypeScript-Dominated World

It's true that TypeScript has captured the majority of the market for JavaScript static typing. But Flow remains actively maintained and is deeply integrated into Meta's internal codebase, React Native's core, and several other major open-source projects. If your team uses Flow, switching to TypeScript just because more tooling exists for it isn't always practical or desirable.

The challenge is that most online type-generation tools target TypeScript exclusively. Finding a reliable JSON to Flow Types converter is surprisingly difficult. This tool fills that gap, giving Flow developers the same convenience that TypeScript developers have enjoyed for years.

Practical Use Cases for JSON-to-Flow Conversion

Consider a scenario where you're consuming a REST API that returns complex nested responses. Without types, Flow can't help you catch property access errors at development time. With the JSON to Flow Types Converter, you paste a sample API response, get the types, and immediately gain autocomplete and error checking for every field access in your codebase.

Another common use case is migrating untyped JavaScript to Flow. If your codebase has JSON fixtures, mock data files, or configuration objects without type annotations, running them through this converter gives you a head start on adding type coverage. Instead of spending hours defining types from scratch, you generate them from existing data and refine as needed.

Teams working with GraphQL and Flow also benefit. While GraphQL codegen tools primarily target TypeScript, you can take a sample query response and convert it to Flow types here, then integrate those types into your data layer manually.

Browser-Based, Private, and Instant

The JSON to Flow Types Converter runs completely in your browser. Your JSON data never touches a server, which is critical when working with sensitive API responses that contain user data or proprietary business information. The conversion happens instantly, regardless of how large your JSON payload is, because it's pure client-side computation.

No accounts, no rate limits, no installation. Paste your JSON, copy your Flow types, and get back to building your application with confidence that your types match your data.