JSON to TypeScript Interface Generator

About this tool

Paste a sample JSON object or array and get matching TypeScript interface declarations (or type aliases), including nested objects and arrays. This saves the tedium of hand-writing types for an API response or config file you already have real data for.

Types are inferred from a single sample — if a field is sometimes a different type or missing in other real responses, double-check the generated type against your actual data. The result is syntax-highlighted (toggle "Raw" for a plain copyable textarea); everything runs locally in your browser.

Frequently asked questions

How are arrays typed?

An array of objects is merged into one nested interface using the union of keys seen across its elements — keys not present in every element are marked optional (?). An array of primitives (numbers, strings) becomes type[], or a union like (string | number)[] if the values are mixed.

What happens with null values?

A field that is null in your sample is typed as null — you'll likely want to widen it manually to something like string | null once you know the field's real type from other samples.

Why does a property name have quotes around it in the output?

JSON keys that aren't valid TypeScript identifiers (containing spaces, starting with a digit, etc.) are automatically wrapped in quotes, since TypeScript requires that for such property names.

Is my JSON uploaded anywhere?

No — parsing and type generation both happen entirely in your browser.