Convert JSON to TypeScript Types
Paste a JSON example and generate a TypeScript Root type from its nested structure. Use it to bootstrap API response types, fixtures, SDK prototypes, and frontend models without uploading your data.
🔒 Your data stays in your browser. FixTools does not upload this JSON for processing.
How to use JSON to TypeScript
- Step 1
Paste a representative JSON object or array into the editor. Use a sample that includes the fields and nested structures you expect to handle.
- Step 2
Click Generate TypeScript. FixTools infers primitive, object, array, and null types from the supplied values.
- Step 3
Copy the generated Root type into your project, then refine optional properties, unions, enums, nullable fields, and reusable interfaces based on your real API contract.
JSON to TypeScript example
A nested API sample can be converted into a strongly typed starting point for frontend or SDK work.
Example input
{"id":42,"name":"Ada","active":true,"roles":["admin"]}Example output
export type Root = {
"id": number;
"name": string;
"active": boolean;
"roles": string[];
};What this tool does
The generator inspects the runtime shape of your sample JSON and maps strings, numbers, booleans, nulls, arrays, and nested objects into a TypeScript type expression. It is designed as a fast starting point rather than a complete schema-inference engine.
Common use cases
- • Bootstrap TypeScript types from a REST or GraphQL JSON response
- • Create typed fixtures while prototyping a frontend
- • Convert sample webhook payloads into development types
- • Draft SDK response models before formal schema generation
- • Explore unfamiliar API data structures
How it works
The tool recursively maps JavaScript value types to TypeScript. Objects become inline object types, arrays infer their element type from the first sample item, and null is preserved as the null type. The output uses quoted property names so keys that are not valid TypeScript identifiers remain representable.
Limitations and edge cases
Sample-based inference cannot know what values are possible but absent from the sample. A field missing from one response may actually be optional; an array may contain multiple shapes; a value that is currently null may normally be a string; and a string may represent a finite enum. Production types should be checked against API documentation, JSON Schema, OpenAPI, or multiple representative samples.
Related concepts
Type inference from samples
A sample shows observed values, not the complete contract. Inference is strongest when several representative payloads are available.
TypeScript type vs. JSON Schema
TypeScript describes types for the compiler. JSON Schema is a runtime-oriented vocabulary for validating JSON data and expressing constraints.
JSON to TypeScript FAQ
Can I generate a TypeScript interface from JSON?
This tool generates a TypeScript Root type from sample JSON. You can convert or refactor the inline object types into named interfaces if that better matches your project style.
How are arrays converted to TypeScript?
The current generator infers the array element type from the first item in the sample. Mixed-shape arrays may require manual union types.
Are optional properties detected automatically?
No. A single JSON sample cannot prove that a property is optional. Mark fields optional only when your API contract or multiple samples support that conclusion.
What happens to null values?
A sample value of null is inferred as the null type. In production, the real type may be a union such as string | null, which should be added manually if appropriate.