JavaScript to TypeScript Converter Online - Free JS to TS Tool

Last updated March 22, 2026

Features of Our JS to TS Converter

Type Inference

Automatically infers types from JavaScript code and adds appropriate TypeScript annotations.

Migration Tips

• Start with strict: false in tsconfig.json and enable strict mode gradually
• Use any as a temporary placeholder for complex types — replace them incrementally
• Install @types/ packages for third-party libraries: npm install @types/node @types/react
• The converter handles straightforward cases — complex dynamic patterns may need manual annotation
• Run tsc --noEmit after conversion to see remaining type errors
• Migrate one file at a time — TypeScript and JavaScript can coexist during migration

Paste your JavaScript code to instantly get TypeScript with type annotations. All conversion happens in your browser.

Interface Generation

Creates TypeScript interfaces for objects and complex data structures.

Instant Conversion

Convert JavaScript to TypeScript in real-time without server uploads.

100% Private

All processing happens in your browser. Your code never leaves your device.

Why Convert JavaScript to TypeScript?

• Type Safety: Catch errors at compile-time instead of runtime
• Better IDE Support: Enhanced autocomplete and IntelliSense
• Improved Maintainability: Self-documenting code with type annotations
• Refactoring Confidence: Safely refactor with type checking
• Team Collaboration: Clear contracts between code modules
• Modern Features: Access latest JavaScript features with backward compatibility

Conversion Examples

Example 1: Function with Parameters

JavaScript:

function add(a, b) {
  return a + b;
}

TypeScript:

function add(a: number, b: number): number {
  return a + b;
}

Common Conversion Patterns

Here are common JavaScript patterns and their TypeScript equivalents:

Variables & Constants

JS: const name = 'John';

TS: const name: string = 'John';

Arrays

JS: const numbers = [1, 2, 3];

TS: const numbers: number[] = [1, 2, 3];

Objects

JS: const user = { name: 'John', age: 30 };

TS: const user: { name: string; age: number } = { name: 'John', age: 30 };

Classes

JS: class Person { constructor(name) { this.name = name; } }

TS: class Person { name: string; constructor(name: string) { this.name = name; } }

Best Practices for Migration

1. Start with tsconfig.json: Configure TypeScript compiler options
2. Rename files gradually: Change .js to .ts one file at a time
3. Use 'any' initially: Add proper types incrementally
4. Enable strict mode: Gradually enable stricter type checking
5. Add type definitions: Install @types packages for libraries
6. Test thoroughly: Ensure functionality remains unchanged

Frequently Asked Questions

Q: Is the conversion 100% accurate?

A: Basic conversions are accurate, but complex code may need manual type adjustments. Always review and test converted code.

Q: Can I convert entire projects?

A: This tool is best for individual files. For full projects, use TypeScript's migration tools or convert files incrementally.

Q: Will my JavaScript code still work?

A: Yes, TypeScript is a superset of JavaScript. Valid JavaScript is valid TypeScript (though it may need type annotations).

Q: Do I need to install anything?

A: No, this online converter works entirely in your browser. For development, install TypeScript via npm.

JavaScript input:
const name = "Alice";
const age = 30;
const isActive = true;

function greet(user) {
  return `Hello, ${user.name}! You are ${user.age} years old.`;
}

TypeScript output:
const name: string = "Alice";
const age: number = 30;
const isActive: boolean = true;

interface User {
  name: string;
  age: number;
}

function greet(user: User): string {
  return `Hello, ${user.name}! You are ${user.age} years old.`;
}

JavaScript input:
const users = [
  { id: 1, name: "Alice", email: "alice@example.com" },
  { id: 2, name: "Bob", email: "bob@example.com" }
];

function getUserById(users, id) {
  return users.find(user => user.id === id);
}

const config = {
  apiUrl: "https://api.example.com",
  timeout: 5000,
  retries: 3
};

TypeScript output:
interface User {
  id: number;
  name: string;
  email: string;
}

const users: User[] = [
  { id: 1, name: "Alice", email: "alice@example.com" },
  { id: 2, name: "Bob", email: "bob@example.com" }
];

function getUserById(users: User[], id: number): User | undefined {
  return users.find(user => user.id === id);
}

interface Config {
  apiUrl: string;
  timeout: number;
  retries: number;
}

const config: Config = {
  apiUrl: "https://api.example.com",
  timeout: 5000,
  retries: 3
};

JavaScript input:
async function fetchUser(id) {
  const response = await fetch(`/api/users/${id}`);
  if (!response.ok) {
    throw new Error(`HTTP error: ${response.status}`);
  }
  const data = await response.json();
  return data;
}

async function createUser(userData) {
  const response = await fetch('/api/users', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify(userData)
  });
  return response.json();
}

TypeScript output:
interface UserData {
  name: string;
  email: string;
  [key: string]: unknown;
}

interface ApiUser {
  id: number;
  name: string;
  email: string;
  createdAt: string;
}

async function fetchUser(id: number): Promise<ApiUser> {
  const response = await fetch(`/api/users/${id}`);
  if (!response.ok) {
    throw new Error(`HTTP error: ${response.status}`);
  }
  const data: ApiUser = await response.json();
  return data;
}

async function createUser(userData: UserData): Promise<ApiUser> {
  const response = await fetch('/api/users', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify(userData)
  });
  return response.json() as Promise<ApiUser>;
}