JSON to TypeScript Interface Generator — Free Online | TimeToProgram — Free

How to use the JSON to TypeScript Interface

Generate TypeScript interfaces in one step:

Paste your JSON

Paste a JSON object (or array of objects) into the input. The generator uses the structure and value types to infer TypeScript types.

Set the root interface name

Choose the name for the root interface (default: 'RootObject'). Nested objects get their own named interfaces automatically.

Click Generate

The TypeScript output appears with properly typed interfaces. Arrays become typed arrays (string[], number[], NestedType[]).

Copy into your project

Copy the generated interfaces directly into your .ts or .d.ts file. Review and refine — for example, marking optional fields with ?.

Refine the output

Types are inferred from sample values. Fields that could be null in real responses should be changed to string | null or T | undefined after generation.

When to use this tool

Stop writing TypeScript interfaces by hand. Use this tool when:

→You receive a new API response and need to type it for your TypeScript project immediately
→You're adding TypeScript to an existing JavaScript project with existing JSON data structures
→You want to generate a base interface from a JSON payload and refine it with more specific types
→You're documenting an API and need TypeScript type definitions for the docs or SDK
→You're working with large JSON responses that have dozens of nested fields

Frequently asked questions

Q:How does type inference work?

The generator checks the JavaScript type of each value: strings become string, numbers become number, booleans become boolean, null becomes null, arrays become typed arrays, and nested objects become new named interfaces. Arrays of objects generate a named interface for the element type.

Q:What if my JSON has nullable fields?

If a field's value is null in the sample JSON, it's typed as null. In real APIs many fields can be either a value or null — after generating, manually update these to string | null, number | null, etc. as appropriate for your use case.

Q:Does it handle arrays of objects?

Yes. An array of objects generates a separate interface for the element type and types the field as InterfaceName[]. For example, {"users": [{"id": 1, "name": "Alice"}]} generates a User interface and types the field as users: User[].

Q:Can I use the output with Zod or other schema libraries?

The output is TypeScript interface syntax. For Zod schemas, use the generated interfaces as a reference to write z.object() definitions — or use our dedicated JSON to Zod Schema generator tool.