jsontype

JSON → TypeScript, instantly. Paste JSON on the left and TypeScript interfaces appear on the right as you type. Copy them, or Save & share for a /s/:id link that reopens the exact JSON and types.

One tool does all of it. Bun runs the TypeScript server, serves it with Bun.serve(), bundles the frontend with Bun.build, tests it with bun test, and stores shares in bun:sqlite. There are no runtime dependencies.

bun install
bun run dev        # build + serve on :3000, rebuilds and restarts on change
bun run test       # inference, parse-error and share round-trip tests
bun run typecheck

How it works

File	What it does
src/infer.ts	Hand-written inference. It runs in the browser for the live preview and on the server when a share is saved.
src/jsonError.ts	Finds where a parse failed (line, column, reason). Browsers word JSON.parse errors differently, and Safari gives no position.
src/app.ts	Routes: /, /s/:id, POST /api/share, GET /api/share/:id.
src/db.ts	bun:sqlite with no ORM: one table and two prepared statements.
build.ts	Bundles src/client/* and inlines it into one HTML page (≈9 KB gzipped) that the server imports.
server.ts	Bun.serve() entry point.

Inference rules:

• Nested objects become named interfaces.
• null next to a value gives T | null.
• A key missing from some items in an array of objects becomes optional (key?).
• [] becomes unknown[].
• Mixed primitive arrays become a union: (string | number)[].
• Identical shapes share one interface.
• Objects used as lookup tables become Record<string, T>. For example, npm's packages map in a 6,800-line package-lock.json becomes about 50 lines of types. An object counts as a lookup table when all of its values share one type and its keys look like data (paths, ids, dates or headers), vary from sample to sample, or map to many values of identical shape.

Share ids are a hash of the content, so the same JSON always gets the same link. A /s/:id page embeds the saved JSON and types in the HTML, so it renders on first paint with no extra fetch.