JSON to TypeScript Interface Generator — Free Online
Generate TypeScript interface definitions from any JSON object using AI. Creates typed interfaces with optional fields, union types, and nested types. Free.
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JSON to TypeScript Interface Generator — Free Online
About JSON to TypeScript Interface Generator — Free Online
This tool generates TypeScript interface definitions from any JSON object using AI. It infers types from values, marks nullable fields as optional, uses union types for mixed arrays, and creates nested interfaces for nested objects.
JSON to TypeScript Type Mapping
| JSON value | TypeScript type | Example |
|---|---|---|
| string | string | "Alice" → string |
| integer / float | number | 42 → number |
| boolean | boolean | true → boolean |
| null | null or T | null | null field → optional |
| array of strings | string[] | ["a","b"] → string[] |
| array of objects | InterfaceName[] | nested array type |
| object | interface Nested { ... } | generates sub-interface |
| mixed array | string | number[] | union type for mixed values |
Interface vs Type Alias
| Feature | interface | type |
|---|---|---|
| Object shape | ✓ Preferred | ✓ Works |
| Extensible with extends | ✓ Yes | ✗ No (use intersection) |
| Declaration merging | ✓ Yes | ✗ No |
| Union / intersection | ✗ No | ✓ Yes |
| Best for JSON models | ✓ Recommended | Use for unions/primitives |
Frequently Asked Questions
How do I use a TypeScript interface to parse JSON?
TypeScript interfaces are compile-time only — they are erased at runtime. Use JSON.parse(text) as MyInterface to assert the type, or use a runtime validator like Zod (z.object(...).parse()) to validate the shape at runtime.
What is the difference between a TypeScript interface and a type for JSON?
For modeling JSON objects, both work. interface is preferred because it is extensible via extends and supports declaration merging. Use type when you need union types or the shape is not a plain object.
How do I handle optional fields in TypeScript from JSON?
Mark fields that may be missing or null as optional with the ? modifier: field?: string. This is equivalent to field: string | undefined. This tool marks fields with null values in the sample JSON as optional.
How do I generate a TypeScript interface from a JSON Schema?
Use json-schema-to-typescript (npm package) for JSON Schema → TypeScript. This tool works the other direction: from a live JSON object (not a schema), which is useful when you have an API response but no schema documentation.
Can TypeScript automatically infer JSON types?
TypeScript can infer types from JSON imports (with resolveJsonModule: true in tsconfig.json) but only for static files. For runtime API responses, you need to cast with as or use a runtime validator — TypeScript cannot verify shapes at runtime.
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