How are nested objects and arrays of objects typed?

Each nested object gets its own named interface derived from the key, and the parent references it. An array of objects is typed from its first element only, with the key name singularised, so tags yields a Tag interface.

What type is produced for null values and empty arrays?

A key whose sample value is null is typed literally as null, not as a union with another type. An empty array becomes any[], since no element exists to infer from.

What does the readonly modifier checkbox do?

When enabled, every generated property is prefixed with readonly. You can also set the root interface name, which defaults to Root and seeds the names given to nested types.

JSON to TypeScript – Free Online Type Generator

interface Root { readonly id: number; readonly name: string; readonly isActive: boolean; readonly address: Address; readonly tags: string[]; readonly metadata: Metadata; } interface Address { readonly street: string; readonly city: string; readonly zip: string; } interface Metadata { readonly created: string; readonly score: number; }

How to Use JSON to TypeScript

Follow this simple step-by-step guide to get the most out of our free online JSON to TypeScript:

1
Paste a JSON object or array into the JSON Input box on the left side.
2
Set the name of the top-level type in the Root interface name field below the boxes.
3
Tick or untick Use readonly modifier to control whether generated properties are readonly.
4
Copy the generated interfaces from the TypeScript Output panel with its copy control.

Key Features & Capabilities

Zero Uploads

Execution occurs 100% locally in your browser memory for maximum privacy.

Instant Latency

Zero network lag or server queues — get results instantaneously as you type.

Quick Export

One-click copy to clipboard or instant file download options built right in.

Technical Specifications & Privacy Guarantee

At utilitiestools.net, privacy and data security are core design principles. When using JSON to TypeScript, your inputs are parsed and processed locally via client-side JavaScript execution engine. No plain text, uploaded media, or API tokens are transmitted over the internet or logged into remote server infrastructure.

Common Practical Use Cases

API Client Work: turn a captured endpoint response into interfaces to type a fetch wrapper.
Migration to TypeScript: bootstrap types for a legacy JavaScript module from the fixtures it already ships.
SDK Development: draft interface stubs from a vendor's documented sample payload before the real types exist.
Data Pipelines: derive a row shape from one sample record to type a transform step.

Frequently Asked Questions

No. Every key present in the sample becomes a required property. Provide a sample containing all fields, then widen the properties that may be absent by hand afterwards.

JSON to TypeScript

How to Use JSON to TypeScript

Key Features & Capabilities

Zero Uploads

Instant Latency

Quick Export

Technical Specifications & Privacy Guarantee

Common Practical Use Cases

Frequently Asked Questions

Q: Are any properties marked optional with a question mark?

Q: How are nested objects and arrays of objects typed?

Q: What type is produced for null values and empty arrays?

Q: What does the readonly modifier checkbox do?

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