JavaScript to TypeScript Converter - Convert JS to TS Online Free

Last updated March 22, 2026

JavaScript to TypeScript Converter

Convert JavaScript code to TypeScript automatically. This tool adds type annotations, converts var to let/const, adds interfaces for objects, and applies TypeScript best practices to your JavaScript code.

Conversion Features

• Type Inference: Automatically infers types from values
• Interface Generation: Creates interfaces for object parameters
• Modern Syntax: Converts var to let/const
• Function Types: Adds parameter and return types
• Array Types: Converts to typed arrays
• Null Safety: Adds optional chaining where appropriate

Migration Tips

1. Start with .ts Extension

Rename .js files to .ts and fix errors gradually

2. Enable Strict Mode

Use "strict": true in tsconfig.json for better type safety

3. Add Types Incrementally

Start with any types, then refine to specific types

4. Use Type Definitions

Install @types packages for third-party libraries

Migration Strategy

• Rename .js files to .ts one at a time — start with utility functions and types
• Use any as a temporary escape hatch — replace with proper types incrementally
• Install type definitions: npm install -D @types/node @types/express @types/react
• Run npx tsc --noEmit to check for type errors without compiling
• Enable strict: true in tsconfig.json once most files are converted
• Use // @ts-ignore sparingly for complex cases you'll fix later

Paste your JavaScript to get TypeScript with type annotations. All conversion happens in your browser — your code is never sent to any server.

Common Conversions

Variables

// JavaScript
var name = "John";

// TypeScript
const name: string = "John";

Functions

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

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

Objects

// JavaScript
const user = {
  name: "John",
  age: 30
};

// TypeScript
interface User {
  name: string;
  age: number;
}
const user: User = {
  name: "John",
  age: 30
};

Frequently Asked Questions

Q: Will my JavaScript code break in TypeScript?

A: No, all valid JavaScript is valid TypeScript. You can gradually add types without breaking existing code.

Q: Do I need to convert everything at once?

A: No, you can migrate file by file. TypeScript supports mixed JS/TS projects.

Q: Is this tool free?

A: Yes, completely free with no signup required. All processing happens in your browser.

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

function formatCurrency(amount, currency) {
  return `${currency}${amount.toFixed(2)}`;
}

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

function formatCurrency(amount: number, currency: string): string {
  return `${currency}${amount.toFixed(2)}`;
}

JavaScript:
function processOrder(order) {
  const total = order.items.reduce((sum, item) => {
    return sum + item.price * item.quantity;
  }, 0);

  return {
    orderId: order.id,
    total,
    status: 'processed',
    processedAt: new Date().toISOString()
  };
}

TypeScript:
interface OrderItem {
  price: number;
  quantity: number;
}

interface Order {
  id: string;
  items: OrderItem[];
}

interface ProcessedOrder {
  orderId: string;
  total: number;
  status: string;
  processedAt: string;
}

function processOrder(order: Order): ProcessedOrder {
  const total = order.items.reduce((sum: number, item: OrderItem) => {
    return sum + item.price * item.quantity;
  }, 0);

  return {
    orderId: order.id,
    total,
    status: 'processed',
    processedAt: new Date().toISOString()
  };
}

JavaScript:
function formatValue(value) {
  if (typeof value === 'string') {
    return value.toUpperCase();
  }
  if (typeof value === 'number') {
    return value.toFixed(2);
  }
  return String(value);
}

function createUser(name, email, role) {
  return {
    name,
    email,
    role: role || 'user',
    createdAt: Date.now()
  };
}

TypeScript:
function formatValue(value: string | number | boolean): string {
  if (typeof value === 'string') {
    return value.toUpperCase();
  }
  if (typeof value === 'number') {
    return value.toFixed(2);
  }
  return String(value);
}

type UserRole = 'admin' | 'user' | 'moderator';

interface NewUser {
  name: string;
  email: string;
  role: UserRole;
  createdAt: number;
}

function createUser(name: string, email: string, role?: UserRole): NewUser {
  return {
    name,
    email,
    role: role || 'user',
    createdAt: Date.now()
  };
}