JSON to TypeScript Interface Generator
Generate TypeScript interface declarations from JSON. Infers correct TypeScript types for strings, numbers, booleans, arrays, nested objects, and null — producing ready-to-use type definitions for your API responses and data models.
Infers TypeScript types from JSON value types (string, number, boolean, null, array, object).
Generates nested interface declarations for deeply nested JSON objects.
Handles arrays by inferring the element type from the first array item.
Produces clean, PascalCase interface names.
Output is ready to paste into TypeScript source files or declaration files.
How to Use
Paste a JSON object into the input field
The generator infers types from the values and outputs TypeScript interfaces
Copy the interface declarations into your .ts or .d.ts file
Adjust optional fields (add ?) where keys may be absent in real data
API Integration
- Type API response payloads
- Generate interfaces from Swagger/OpenAPI examples
- Type-safe data fetching with fetch or axios
Frontend Development
- Type Redux store slices from API data
- Generate form data interfaces
- Type React component props from JSON configs
TypeScript Ecosystem
- React
- Next.js
- Angular
- Vue 3
- Node.js
- Deno
Mark fields that are sometimes missing in real API responses as optional with ? — the generator assumes all fields are required.
If a JSON array is empty, the element type cannot be inferred — replace unknown[] with the correct type after generating.
For union types (a field that can be string | number), add the union manually after generating the base interface.
Review generated interfaces against full API documentation and add optional markers (?) for fields that may be absent.
Replace null with the actual union type (string | null, number | null) once you know which fields are nullable.
For empty arrays in the JSON sample, replace the generated unknown[] with the correct element type immediately.
Frequently Asked Questions
Find answers to common questions about our tools and services.
Understanding JSON to TypeScript Interface
TypeScript has become the dominant language for large-scale JavaScript development, and its central value proposition is compile-time type safety. When consuming REST APIs, processing JSON configuration files, or handling data from external sources, TypeScript developers need accurate type definitions that describe the shape of the incoming data. Without proper types, API response objects are typed as any, which defeats the purpose of TypeScript entirely — IDE autocompletion fails, refactors become unsafe, and type errors that would be caught at compile time instead surface as runtime exceptions. Generating TypeScript interfaces from real JSON examples is the fastest way to bootstrap accurate type definitions when a formal OpenAPI or JSON Schema specification is unavailable.
The type inference algorithm for JSON-to-TypeScript conversion maps each JSON value type to its TypeScript equivalent through a set of direct correspondences. JSON strings become TypeScript string. JSON numbers (both integers and floats, since JSON does not distinguish them) become TypeScript number. JSON booleans become TypeScript boolean. JSON null becomes TypeScript null. JSON objects become TypeScript interface declarations with each key as a required property typed by its value's inferred type. JSON arrays become TypeScript array types (ElementType[]) where the element type is inferred from the first array element. These mappings handle the vast majority of real-world API data structures without ambiguity.
Nested JSON objects are handled by generating separate named interfaces for each distinct object shape, which is both cleaner and more reusable than inline anonymous types. A JSON response with a top-level user object that contains an address object with a city field produces three interfaces: Address (containing city: string), User (containing address: Address), and Root (containing user: User). This hierarchical decomposition matches the style recommended in TypeScript documentation and produces interfaces that can be individually imported, extended, or reused across the codebase. Interface names are derived from the JSON key names in PascalCase convention.
A practical limitation of JSON-sample-based type generation is that the generator can only infer what is present in the provided example. If a field is sometimes null and sometimes a string in real API responses, but the provided example only shows the string case, the generated type will be string rather than string | null. If some fields are optional (sometimes present, sometimes absent), the generator will mark them as required since they appear in the sample. Developers should treat the generated interfaces as a starting point and refine them based on the full API contract documentation, adding union types, optional markers (?), and generics as needed to accurately represent the complete data model.