JSON to TypeScript

Example

Input:

{ "id": 1, "name": "Alice", "age": null, "tags": ["admin", "user"], "address": { "city": "Madrid" } }

Output:

interface User {
  id: number;
  name: string;
  age: number | null;
  tags: string[];
  address: Address;
}

interface Address {
  city: string;
}

Type mapping

Optional vs nullable

• foo?: string, the field may be absent from the object. undefined-checking applies.
• foo: string | null, the field is always present but may explicitly be null.
• foo?: string | null, could be absent OR null.

JSON itself does not have undefined, but APIs vary on how they signal absence. Match your API semantics:

• REST APIs typically omit missing fields -> ?:.
• GraphQL always returns every requested field -> | null.
• Some SDKs use both in different contexts.

Union vs literal types

If the tool sees the same string field with a small set of values across samples ("status": "pending", "active", "archived"), it can emit a string literal union:

status: "pending" | "active" | "archived";

Toggle “infer string literal unions” if you want this.

Common mistakes

• Inferring from one sample. Every field becomes required; nullability cannot be observed. Pass 5-10 varied samples for better types.
• Empty arrays. "tags": [] gives no type info, the generator emits unknown[]. Provide a sample with at least one element.
• Mixed-type arrays. [1, "two", true] produces (number | string | boolean)[]. Usually this means the JSON should be redesigned rather than typed.
• Numeric string keys. JSON {"1": "a", "2": "b"} is still an object in TypeScript (Record<string, string>), not an array. The generator handles this correctly.