JSON から TypeScript へ：実例付き完全ガイド

Converting JSON to TypeScript interfaces is one of the most common tasks in modern web development. Whether you're consuming REST APIs, working with configuration files, or defining data models, having proper TypeScript types ensures type safety and better developer experience.

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Why Convert JSON to TypeScript?

TypeScript's type system catches errors at compile time that would otherwise surface at runtime. When you receive JSON from an API, TypeScript has no idea what shape that data takes — unless you define interfaces. Without types, you lose autocompletion, refactoring support, and compile-time error checking.

Basic Conversion Rules

The fundamental mappings from JSON types to TypeScript types are straightforward:

// JSON
{
  "name": "Alice",
  "age": 30,
  "active": true
}

// TypeScript
interface User {
  name: string;
  age: number;
  active: boolean;
}

Handling Nested Objects

Real-world JSON is rarely flat. When objects contain other objects, you should create separate interfaces for each level:

// JSON
{
  "id": 1,
  "name": "Alice",
  "address": {
    "street": "123 Main St",
    "city": "Springfield",
    "coordinates": {
      "lat": 39.7817,
      "lng": -89.6501
    }
  }
}

// TypeScript
interface Coordinates {
  lat: number;
  lng: number;
}

interface Address {
  street: string;
  city: string;
  coordinates: Coordinates;
}

interface User {
  id: number;
  name: string;
  address: Address;
}

Tip: Use separate interfaces rather than deeply nested inline types. It makes your code more readable and reusable.

Array Types

Arrays in JSON can contain uniform or mixed types. TypeScript handles both cases:

// Uniform array
{ "tags": ["typescript", "react", "next"] }
// -> tags: string[]

// Array of objects
{
  "users": [
    { "id": 1, "name": "Alice" },
    { "id": 2, "name": "Bob" }
  ]
}
// -> users: User[]

// Mixed array (rare but possible)
{ "data": [1, "two", true] }
// -> data: (number | string | boolean)[]

When an array contains objects of the same shape, define a dedicated interface. For heterogeneous arrays, use union types.

Null and Optional Fields

JSON APIs often return null for missing values, or omit fields entirely. TypeScript distinguishes between these two cases:

// JSON with null values
{
  "id": 1,
  "name": "Alice",
  "avatar": null,
  "bio": null
}

// TypeScript — nullable vs optional
interface User {
  id: number;
  name: string;
  avatar: string | null;    // field exists, value can be null
  bio?: string;              // field might not exist at all
  nickname?: string | null;  // might not exist OR might be null
}

Key difference: null means the field exists but has no value. undefined (optional with ?) means the field might not be present at all. Use | null for nullable fields and ? for optional fields.

Best Practices

// Good: runtime validation with Zod
import { z } from 'zod';

const UserSchema = z.object({
  id: z.number(),
  name: z.string(),
  email: z.string().email(),
  avatar: z.string().nullable(),
});

type User = z.infer<typeof UserSchema>;

// Validate API response
const user = UserSchema.parse(await res.json());

Automate the Conversion

Manually writing interfaces for large JSON payloads is tedious and error-prone. Use our JSON to TypeScript converter to instantly generate accurate interfaces from any JSON input. Paste your JSON, get TypeScript — it's that simple.

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