JSON to TypeScript Interface Generator

Reverse-engineering a TypeScript type from a JSON example is one of the most common one-off tasks for a frontend or full-stack engineer. The API doesn't have an OpenAPI spec, the team's docs are out of date, but you have a sample response — the fastest path to compiler-checked code is generating a starter interface from that sample, then refining.

What automatic generation gets right

• Object shapes — every key becomes a property.
• Primitive types (string, number, boolean, null).
• Arrays of homogeneous primitives.
• Nested objects (recursively).
• Optional vs required (best-effort, based on which keys appear in all samples).

What it can't infer (and what you must add)

• Union types. A field that's sometimes string, sometimes null, sometimes missing — generators guess string | null | undefined but can't know if other variants exist.
• String literal unions. status: "active" in your sample looks like string, but the API might emit "active" | "pending" | "cancelled". You have to know the domain.
• Enums. Generators don't promote literal unions to enums — you decide.
• Nullable arrays. Empty arrays [] get any[] or never[] depending on the tool. Always specify the element type manually.
• Date strings. "2026-05-03T12:00:00Z" looks like string. If you parse it to a Date later, type the field as Date after parsing.
• Numeric IDs that should be branded. A type alias like type UserId = number & { __brand: 'UserId' } prevents passing a UserId where a PostId is expected.

Workflow recommendations

1. Capture multiple sample responses — one with every optional field populated, one minimal.
2. Run the generator on each. Diff the results.
3. Manually merge the differences into a final interface, marking optional fields with ?.
4. Tighten string fields to literal unions where you know the domain.
5. Run your own code against the type and let the compiler surface mismatches.
6. If the API is documented in OpenAPI/JSON Schema, prefer generating from that — it's authoritative.

When to use Zod or io-ts instead

For external APIs you don't control, runtime validation matters as much as compile-time types. Schema libraries like Zod, io-ts, and ArkType produce both at once: you write a schema, you get a TypeScript type AND a runtime validator. The validator catches API drift you wouldn't notice at compile time.

This generator is a starting point — not the finish line. It runs entirely in your browser, so confidential payloads stay local.

Frequently Asked Questions