JSON to JavaScript (JSDoc)
Paste a JSON payload and get JSDoc typedef blocks describing its shape. JavaScript has no type declarations of its own, so a typedef is what editors read for autocompletion and what tsc checkJs validates against — and because it is only a comment, it costs nothing at runtime. Fields missing from some records in an array are marked optional with JSDoc bracket syntax, and nested objects each get their own typedef.
How it works
Paste a representative JSON sample and the JavaScript declarations appear on the right. The types are inferred from what is actually in the sample, so the more complete your example, the better the result — one array element with an extra field is enough to have that field marked optional rather than missed.
JavaScript has no type declarations, so the useful output is a JSDoc typedef block: editors read it for autocompletion, and running tsc with checkJs will type-check against it. It costs nothing at runtime because it is only a comment.
An optional property is marked by bracketing its name — @property {string} [note] — which is JSDoc’s own syntax and not the same as making the type nullable.
Generation runs entirely in your browser — nothing you paste is uploaded.
Frequently asked questions
Why does a JSON to JavaScript tool produce comments?
JavaScript has no syntax for declaring the shape of an object, so the only way to describe it is a JSDoc comment. Editors read those for autocompletion and go-to-definition, and the TypeScript compiler will check against them when run with checkJs — all without adding a single byte to what ships.
How are optional fields marked in JSDoc?
JSDoc marks an optional property by bracketing its name, as in @property {string} [note] — which is different from making the type nullable, and is what the generator emits for any field missing from some records in the sample.
Can I use these typedefs with TypeScript?
Yes — running tsc with the checkJs option makes it type-check plain JavaScript files against their JSDoc, which is a common way to add safety to an existing codebase without converting it to TypeScript.
What happens to nested objects?
Every nested object gets its own typedef named after the key that held it, and the parent references it by name — so an array of objects contributes one typedef for its items rather than one per element.
Is my JSON uploaded anywhere?
Nothing is uploaded — the sample is parsed and the typedefs are generated entirely in your browser, which matters when the payload you are pasting is real data.
Related tools
JSON to TypeScript Converter
This JSON to TypeScript converter turns a sample payload into exported interfaces.
JSON to Python Dataclass Converter
This JSON to Python converter generates typed dataclasses with snake_case field names.
JSON Formatter & Beautifier
This JSON formatter beautifies minified or messy JSON online with clean indentation, line numbers, and code folding.