JSON to Flow Converter - Generate Flow Types Online for Free

Online JSON to Flow Converter: Generate Static Types Instantly

Boost your JavaScript productivity with our JSON to Flow converter. While many projects have migrated to TypeScript, Flow remains a powerful static type checker for many large-scale JavaScript codebases, including those at Meta. This tool allows you to paste a sample JSON object or a JSON Schema and instantly generate clean, accurate Flow type definitions, ensuring your data structures are strictly typed and error-free.

Why Use Flow Types for Your JSON Data?

Flow helps you find errors in your JavaScript code before they run. Manually defining types for large JSON payloads is tedious; our tool automates this process.

Maintain Large JavaScript Codebases

For projects already utilizing Flow, keeping type definitions in sync with API responses is a challenge. By generating Flow types directly from JSON samples, you ensure that your components and utility functions handle data with 100% type certainty.

Catch Errors at Development Time

Flow’s static analysis can catch null pointer exceptions and property mismatches that standard JavaScript would only reveal at runtime. Converting your JSON to Flow types allows the type checker to protect your app's data flow from end to end.

Key Features of Our JSON to Flow Converter

Our tool is built to handle the specific syntax and requirements of the Flow type system.

1. Intelligent Type Mapping

Our converter automatically maps standard JSON data types to the appropriate Flow primitive types:

• string → string

• number → number

• boolean → boolean

• null → null

• array → Array<T>

2. Support for Maybe Types (Optionality)

In Flow, optional or nullable properties are handled using "Maybe" types (indicated by a leading ?). Our tool analyzes your JSON Schema or sample data to automatically apply the ? prefix to properties that are not required, matching Flow’s strict null-checking logic.

3. Nested Objects and Exact Object Types

The converter handles deeply nested JSON structures by creating separate type aliases. You can also choose to generate Exact Object Types (using {| |} syntax) to prevent extra properties from being added, ensuring even stricter type validation.

How to Convert JSON to Flow Types

1. Input Your JSON: Paste your raw JSON data or JSON Schema into the input editor.

2. Configure Options: (Optional) Set your base type name (e.g., UserType) and choose between regular or exact object types.

3. Generate Flow Code: The tool will instantly process the structure and display the Flow definitions.

4. Copy and Paste: Click the "Copy" button to add the types to your .js or .flow files.

Technical Insights: JSON to Flow Mapping

Handling Arrays and Collections

Our tool identifies the contents of arrays. If an array contains a single type, it generates Array<string>; if it contains mixed types, it creates a union type like Array<string | number> to maintain type integrity.

Type Aliasing vs. Inline Types

To keep your code readable, our converter prefers Type Aliases. Instead of deeply nesting types within one large block, it breaks down complex objects into smaller, reusable type definitions.

Frequently Asked Questions (FAQ)

Is this tool compatible with the latest version of Flow?

Yes! We use the modern Flow syntax, ensuring compatibility with current versions of the Flow type checker and Babel presets.

Does this tool support JSON Schema?

Absolutely. You can paste a standard JSON Schema (Draft 4, 7, etc.), and the tool will map the constraints and requirements to Flow types.

Is my data secure?

Yes. Your data never leaves your browser. All conversion and type-checking logic is performed locally via JavaScript, ensuring your proprietary data structures remain private.