Use interface (vs type) Export declarations Readonly properties

Features

🔷 TypeScript Types

Interface and type alias support
Nested object interfaces
Array type inference
Null-aware optional fields

⚙️ Configurable Output

Toggle interface vs type
Export declarations
Readonly modifiers
Nested type generation

🎯 Type Accuracy

string, number, boolean inference
unknown | null for null values
Type[] for arrays
unknown[] for empty arrays

TypeScript Guide

Interface vs Type

TypeScript interfaces and type aliases are both used to describe object shapes. Interfaces are extensible with declaration merging; type aliases support unions and intersections.

// Interface (extensible)
export interface User {
  id: number;
  name: string;
  active: boolean;
}

// Type alias (flexible)
export type User = {
  id: number;
  name: string;
  active: boolean;
};

Using Generated Types

import type { Root } from './types';

async function fetchUser(): Promise<Root> {
  const res = await fetch('/api/user');
  return res.json() as Root;
}

const user: Root = await fetchUser();
console.log(user.name); // type-safe access

Common Use Cases

API Responses: Type-safe REST API response handling
Config Files: Strongly-typed configuration objects
React Props: Component prop type definitions
Zustand/Redux: Typed state management stores
Form Data: Validated form submission payloads

FAQ

Q: When should I use interface vs type?

A: Use interface when you need declaration merging or plan to extend the type. Use type when you need union types, mapped types, or more complex compositions.

Q: Why are null fields typed as unknown | null?

A: When a JSON value is null, the actual non-null type cannot be inferred. unknown | null (simplified as the optional field pattern) signals that the field exists but the concrete type requires manual annotation.

Getting the most out of generated TypeScript types

Generated types are a starting point, not the final word. A single sample only shows the shape of one response, so treat the output as a draft that you tighten once you know which fields are optional, which can be null and which strings are really enums.

Tips and common pitfalls

Paste the largest, most complete response you have. Fields that are missing from the sample cannot appear in the generated types.
Replace broad string fields with union types where the API documents a fixed set of values, for example "draft" | "published".
Types disappear at runtime. If the data comes from outside your code, validate it as well, for example with a Zod schema generated by our JSON to Zod tool.
Keep generated types in their own file so you can regenerate them without losing hand-written code.

JSON to TypeScript Interface Generator