JSON to TypeScript Interface Converter - Free Online Tool
Paste any JSON and instantly get clean TypeScript interfaces - nested objects, arrays, and union types all handled automatically. Ideal for typing REST API responses, GraphQL payloads, and any external JSON data. The output uses interface syntax by default; rename to type if your project style guide prefers type aliases. No sign-up, no server, 100% private.
// TypeScript interfaces will appear here
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Last updated: May 25 2026
Reviewed by the QuickTooly Team
JSON to TypeScript Guide
Why Convert JSON to TypeScript Interfaces?
- Instant live preview: Interfaces appear as you type - no button press needed.
- Deep nesting support: Nested objects become their own named interfaces automatically.
- Smart array handling: Arrays of objects are merged into a single interface; missing keys become optional.
- Optional & readonly toggles: Make every field optional or readonly with a single click.
- 100% private: Your JSON never leaves your browser - no server, no logging.
- Custom interface name: Set any root interface name instead of the default
Root. - Copy with one click: Grab the generated TypeScript and paste it straight into your IDE.
What Is a TypeScript Interface?
A TypeScript interface is a compile-time contract that describes the shape of an object. Unlike runtime classes, interfaces are erased during transpilation and add zero overhead to your JavaScript bundle. They power IDE autocompletion, catch type mismatches early, and serve as living documentation for your data structures.
When working with REST APIs, GraphQL responses, or any external JSON data, converting the payload to a TypeScript interface means TypeScript can validate every property access, surface typos at compile time, and give your editor full autocomplete - instead of treating everything as any.
How This Converter Works
The converter walks the JSON tree recursively and maps each JSON type to its TypeScript equivalent:
| JSON Type | TypeScript Type |
|---|---|
| String | string |
| Number | number |
| Boolean | boolean |
| Null | null |
| Object | Named interface block |
| Array of objects | Merged interface + [] |
| Empty array | unknown[] |
How to Use This Tool
- Paste your JSON into the left panel - the converter runs instantly
- Set the interface name to match your domain model (e.g.
User,ApiResponse) - Toggle Optional to make every field
?:- useful when the API may omit fields - Toggle Readonly to add the
readonlymodifier - ideal for immutable data transfer objects - Click Copy and paste the interfaces into your TypeScript project
Tips for Better TypeScript Interfaces
Generated interfaces are a starting point - review them before committing. Consider narrowing wide types like string to literal unions (e.g. 'active' | 'inactive'), replacing number dates with Date or ISO string types, and splitting large interfaces into smaller composable ones. Also check that null fields align with your actual API contract.
Frequently Asked Questions
Does this tool send my JSON to a server?
No. All conversion happens locally in your browser using plain JavaScript. Your JSON data never leaves your device and is never stored or logged.
What happens with arrays of objects?
All objects in the array are merged into a single interface. Keys that appear in some objects but not others are automatically marked optional with ?:.
How are nested objects named?
Nested interfaces inherit a name from their parent interface plus the field key in PascalCase. For example, a user field inside Root produces an interface named RootUser, avoiding any naming conflicts.
Can I convert JSON arrays at the root level?
Yes. If the root value is an array of objects, the converter merges all items into a single root interface and exports a type Root = RootItem[] alias so you have both the array type and the item interface available.
What does the Optional fields toggle do?
It appends ? to every field name in every generated interface. This is useful when your API may omit fields in some responses and you prefer a permissive starting type that you can tighten later.
What does the Readonly fields toggle do?
It prepends the readonly modifier to every property, preventing accidental mutation. This is a common pattern for data transfer objects (DTOs) and API response types.
Should I use interface or type in TypeScript?
Both work for describing object shapes. interface supports declaration merging - useful when extending third-party library types - while type aliases are more flexible for unions, intersections, and mapped types. For plain API response objects either works. This converter outputs interface by convention; rename the keyword to type if your project style guide prefers type aliases.