Generate TypeScript interfaces from a JSON sample. Nothing is uploaded.
The JSON → TypeScript generator infers interfaces from a pasted sample and merges array element shapes into one type — inference runs entirely in the browser.
This generator turns a JSON sample — an API response, a config file, a fixture — into TypeScript interfaces you can paste straight into your code. Nested objects become their own named interfaces, and arrays of objects are merged into a single element type.
The JSON is parsed with your browser's JSON.parse, then the json-to-ts library walks the value and emits one interface per distinct object shape, starting from the root name you choose. Every step runs locally in this tab; the sample never leaves your device.
No. This page never sends your input to a server — formatting, decoding and hashing all run in this tab, on this device.How that works.
All elements of an array are merged into one interface. A property present in every element keeps its type, a property missing from some elements becomes optional (marked with ?), and the same property with different types across elements becomes a union like number | string. That is why a sessions array whose items differ produces an interface with optional members.
The generator only sees the sample you paste. If a property is absent from one array element in the sample, it is typed as optional. Feed it a sample where every element carries the property, or remove the ? by hand when you know the API guarantees it.
Interfaces describe object shapes. The underlying library accepts a JSON object or an array of objects; a bare string, number, boolean or mixed array has no members to describe, so the tool shows an error instead. For those, a plain type alias like type Root = string is all you need.
A property that is null in the sample is typed as null, because null is all the sample proves. If the field is really a nullable string, widen it to string | null yourself.
No. Parsing and interface generation run entirely in this tab — LocalFirstTools has no server that receives your sample, which matters when it comes from a production API response.
Browse alljson toolsor thefull tool directory.