How does the tool decide a field is optional?

When the JSON root is an array of objects, the tool compares the keys present across every item. Any key that does not appear in all items is marked optional with a `?` in the generated interface.

What TypeScript type does it use for numbers and strings?

Plain JSON types map directly: strings become `string`, numbers become `number`, booleans become `boolean`, and `null` becomes `null` (combined into a union if a field can also hold another type).

How are nested objects handled?

Each nested object gets its own named interface, generated from the property path (for example `address` becomes an `Address` interface), and the parent field references it by name.

What happens with an empty array or an array of mixed types?

An empty array is typed as `any[]` since there is no data to infer from. An array containing different primitive types produces a union type like `(string | number)[]`.

Is my JSON sent to a server?

No. Parsing and type generation run entirely in your browser with plain JavaScript. Your data never leaves your device.

JSON to TypeScript

Paste JSON and instantly get matching TypeScript interface definitions with nested types and optional fields detected.

Input JSON

Root interface name

Use interface (uncheck for type)

TypeScript Output

Paste JSON and click Generate

Copied!

Summary

JSON to TypeScript reads a JSON value and generates the matching TypeScript interface definitions, including nested object interfaces, array element types, and union types for arrays with mixed shapes. When an array of objects is scanned, fields that are missing from some entries are marked optional so the generated types stay accurate instead of forcing every field to be required. The result is ready to paste straight into a `.ts` file.

How it works

Paste or type JSON into the input panel, or load a sample.
Set a root interface name (defaults to RootObject).
Click Generate to parse the JSON and build TypeScript interfaces.
Nested objects become their own named interfaces; arrays of objects are scanned across all items to detect optional fields.
Review the generated interfaces in the output panel.
Click Copy to copy the TypeScript code to your clipboard.

Use cases

Generate types for a REST API response before writing a fetch client.
Scaffold interfaces for a new TypeScript project from sample data.
Check whether a JSON payload has fields that are sometimes missing.
Document a third-party API response shape as TypeScript types.
Speed up adding type safety to an existing JavaScript codebase.
Turn a config file into a typed settings interface.

Frequently Asked Questions

Last updated: 2026-07-13 · Reviewed by Nham Vu