JSON to TypeScript Converter Online — Generate Typed Interfaces

Transform raw JSON payloads into strongly typed, modular TypeScript interfaces and type definitions.

Source JSON100% Client-Side Local

typescript OutputGenerated AST

export interface RootObject {
  status: string;
  statusCode: number;
  message: string;
  timestamp: string;
  data: Data;
}

export interface Data {
  id: string;
  username: string;
  role: string;
  active: boolean;
  metrics: Metrics;
}

export interface Metrics {
  logins: number;
  lastActiveHours: number;
  storageUsedMb: number;
}

Type Inference from JSON to TypeScript Interfaces & Types

Modern web applications rely heavily on TypeScript for compile-time verification, auto-completion, and refactoring confidence. However, bridging untyped REST endpoints, GraphQL responses, webhook events, and third-party JSON payloads into structured TypeScript contracts is often a manual, error-prone task.

FormJson's TypeScript engine automates this pipeline by analyzing runtime JSON shapes, resolving ambiguous primitive types, detecting optional properties across array samples, and extracting clean, decoupled interfaces adhering to strict compiler guidelines (strict: true, noImplicitAny: true).

JSON Runtime Values to TypeScript Static Types Mapping

JSON Data Value	Inferred TypeScript Type	Compiler Strategy & Nullability	Generated Code Sample
"Jane Doe"	string	Standard UTF-16 string primitive	name: string;
42 / 3.1415	number	Unified IEEE 754 double precision float	amount: number;
true / false	boolean	Standard boolean flag	isActive: boolean;
null	null \| any	Nullable indicator; merged with sibling types	avatarUrl: string \| null;
["alpha", "beta"]	string[]	Homogeneous array typed as element array	tags: string[];
[10, "text", true]	(number \| string \| boolean)[]	Heterogeneous array converted to union elements	values: (string \| number)[];
{"street": "1st Ave"}	Address (Interface)	Extracted as standalone named interface	address: Address;

Advanced Inference Strategies & Interface Extraction

01. Polymorphic Array & Union Inference

Real-world collections often contain objects with evolving schemas. If index 0 has {"id": 1, "tier": "gold"} and index 1 has {"id": 2, "credits": 500}, the inference engine unifies the signatures into an aggregated interface with optional fields:

export interface UserAccount {
  id: number;
  tier?: string;
  credits?: number;
}

02. Recursive Child Decoupling

Instead of producing fragile, unreadable inline anonymous types (e.g., data: { nested: { sub: boolean } }), the converter performs depth-first traversal and defines individual PascalCase interfaces for each level of nesting:

export interface SubRecord {
  sub: boolean;
}

export interface RootData {
  nested: SubRecord;
}

Integrating Generated Interfaces in React & Node.js Projects

Once your interfaces are generated, paste them into your codebase to establish end-to-end type safety across API boundaries:

A. Typing TanStack Query & React Component Props

Assign the root interface directly as the generic type parameter for useQuery or fetch, guaranteeing full IntelliSense inside JSX templates:

import type { UserProfileResponse } from './types/api';

export function UserCard({ userId }: { userId: string }) {
  const { data, isLoading } = useQuery<UserProfileResponse>({
    queryKey: ['user', userId],
    queryFn: () => fetch('/api/user/' + userId).then((r) => r.json())
  });

  if (isLoading || !data) return <div>Loading...</div>;
  return <h1>{data.profile.displayName}</h1>;
}

B. Express / Fastify Request Body Validation

Pair TypeScript interfaces with runtime validation libraries like Zod or TypeBox to guarantee that external client payloads match your expected schema before processing business logic.

Common Pitfalls & TypeScript Best Practices

! Null vs Undefined in API Payloads

In JSON, absent keys serialize to undefined in JavaScript, while explicit null fields serialize to null. Under TypeScript's strictNullChecks, string | null is not assignable to string | undefined. Ensure your interface captures explicit nullability where required.

! 64-Bit Integers & Large Numbers

JavaScript numbers are 64-bit floating point values (safe up to 2^53 - 1 or 9,007,199,254,740,991). If your JSON receives Twitter IDs, Snowflake identifiers, or blockchain uint256 integers, configure your backend to serialize them as strings or handle them via TypeScript bigint.

FAQ

How does the converter infer TypeScript types from raw JSON data?

The converter parses the JSON payload into an Abstract Syntax Tree (AST) and recursively maps JavaScript runtime primitives (string, number, boolean, null) to their corresponding static TypeScript types. It inspects object structures to generate named child interfaces and analyzes array elements to identify uniform types or heterogeneous union types.

How are nested objects and nested arrays structured in the output?

Deeply nested JSON objects are automatically decoupled and extracted into clean, top-level PascalCase interfaces (such as 'UserAddress' or 'OrderItem'). This eliminates deeply nested inline type definitions, maximizes interface reusability, and matches idiomatic TypeScript architecture.

How does the generator handle optional fields and polymorphic arrays?

When converting an array of objects where certain keys appear in some items but not others, the generator unifies the schema and marks missing attributes with the optional modifier '?' (e.g., 'middleName?: string'). For mixed arrays containing varied scalar types, it generates concise union types (e.g., '(string | number)[]').

Should I use 'interface' or 'type' aliases in my TypeScript project?

In modern TypeScript, both are highly capable. Interfaces are generally preferred for public API models and object contracts because they support declaration merging and provide cleaner compiler error messages. Type aliases are indispensable for unions, primitives, tuples, and intersection types.

Can I use the generated types directly in React, Next.js, and Node.js backends?

Yes. The generated interfaces are 100% standard TypeScript with zero external dependencies. You can paste them directly into your frontend React component props, TanStack Query fetch handlers, Next.js Server Actions, or Node.js/Express API route controllers.

Is my JSON payload transmitted to external servers during conversion?

No. All parsing, schema analysis, and TypeScript interface generation execute entirely inside your browser via local client-side JavaScript. No data is sent over the network, ensuring complete privacy for sensitive API payloads and internal models.