JavaScript to TypeScript Converter Free Online Tool 2026

Last updated March 22, 2026

Example: Object and Interface Generation

// JavaScript:
function createUser(name, email, age) {
  return {
    id: Math.random().toString(36),
    name,
    email,
    age,
    createdAt: new Date()
  };
}

function getUserDisplayName(user) {
  return `${user.name} <${user.email}>`;
}

// TypeScript (converted):
interface User {
  id: string;
  name: string;
  email: string;
  age: number;
  createdAt: Date;
}

function createUser(name: string, email: string, age: number): User {
  return {
    id: Math.random().toString(36),
    name,
    email,
    age,
    createdAt: new Date()
  };
}

function getUserDisplayName(user: User): string {
  return `${user.name} <${user.email}>`;
}

Example: Class Conversion

// JavaScript:
class EventEmitter {
  constructor() {
    this.events = {};
  }

  on(event, listener) {
    if (!this.events[event]) this.events[event] = [];
    this.events[event].push(listener);
    return this;
  }

  emit(event, ...args) {
    if (this.events[event]) {
      this.events[event].forEach(l => l(...args));
    }
  }

  off(event, listener) {
    if (this.events[event]) {
      this.events[event] = this.events[event].filter(l => l !== listener);
    }
  }
}

// TypeScript (converted):
type Listener = (...args: unknown[]) => void;

class EventEmitter {
  private events: Record<string, Listener[]>;

  constructor() {
    this.events = {};
  }

  on(event: string, listener: Listener): this {
    if (!this.events[event]) this.events[event] = [];
    this.events[event].push(listener);
    return this;
  }

  emit(event: string, ...args: unknown[]): void {
    if (this.events[event]) {
      this.events[event].forEach(l => l(...args));
    }
  }

  off(event: string, listener: Listener): void {
    if (this.events[event]) {
      this.events[event] = this.events[event].filter(l => l !== listener);
    }
  }
}

Example: Async Function Conversion

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

async function fetchUserPosts(userId) {
  const [user, posts] = await Promise.all([
    fetchUser(userId),
    fetch(`/api/users/${userId}/posts`).then(r => r.json())
  ]);
  return { user, posts };
}

// TypeScript (converted):
interface User {
  id: number;
  name: string;
  email: string;
}

interface Post {
  id: number;
  title: string;
  body: string;
  userId: number;
}

interface UserWithPosts {
  user: User;
  posts: Post[];
}

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

async function fetchUserPosts(userId: number): Promise<UserWithPosts> {
  const [user, posts] = await Promise.all([
    fetchUser(userId),
    fetch(`/api/users/${userId}/posts`).then(r => r.json() as Promise<Post[]>)
  ]);
  return { user, posts };
}

Example: Generic Function Conversion

// JavaScript:
function first(arr) {
  return arr[0];
}

function last(arr) {
  return arr[arr.length - 1];
}

function findById(items, id) {
  return items.find(item => item.id === id);
}

// TypeScript (converted with generics):
function first<T>(arr: T[]): T | undefined {
  return arr[0];
}

function last<T>(arr: T[]): T | undefined {
  return arr[arr.length - 1];
}

function findById<T extends { id: unknown }>(items: T[], id: T['id']): T | undefined {
  return items.find(item => item.id === id);
}

Example: React Component Conversion

// JavaScript:
function UserCard({ user, onEdit, onDelete, showActions = true }) {
  return (
    <div className="user-card">
      <h3>{user.name}</h3>
      <p>{user.email}</p>
      {showActions && (
        <div>
          <button onClick={() => onEdit(user.id)}>Edit</button>
          <button onClick={() => onDelete(user.id)}>Delete</button>

)}
    </div>
  );
}

// TypeScript (converted):
interface User {
  id: number;
  name: string;
  email: string;
}

interface UserCardProps {
  user: User;
  onEdit: (id: number) => void;
  onDelete: (id: number) => void;
  showActions?: boolean;
}

const UserCard: React.FC<UserCardProps> = ({
  user,
  onEdit,
  onDelete,
  showActions = true
}) => {
  return (
    <div className="user-card">
      <h3>{user.name}</h3>
      <p>{user.email}</p>
      {showActions && (
        <div>
          <button onClick={() => onEdit(user.id)}>Edit</button>
          <button onClick={() => onDelete(user.id)}>Delete</button>
        </div>
      )}
    </div>
  );
};

Example: Type Errors Surfaced During Conversion

// JavaScript (no errors visible):
function processItems(items) {
  return items.map(item => item.value.toUpperCase());
}

processItems([{ value: 'hello' }, { value: null }]);
// Runtime error: Cannot read property 'toUpperCase' of null

// TypeScript (error caught at compile time):
interface Item {
  value: string | null;
}

function processItems(items: Item[]): string[] {
  return items.map(item => item.value.toUpperCase());
  // TS Error: Object is possibly 'null'. (line 2)
}

// Fixed TypeScript:
function processItems(items: Item[]): string[] {
  return items
    .filter((item): item is { value: string } => item.value !== null)
    .map(item => item.value.toUpperCase());
}

Migration Strategy

• Step 1: Rename .js files to .ts (or .jsx to .tsx for React)
• Step 2: Add "strict": false to tsconfig.json initially
• Step 3: Use the converter to add basic type annotations
• Step 4: Fix any type errors the compiler reports
• Step 5: Gradually enable stricter TypeScript options
• Step 6: Replace any types with specific types over time

Paste your JavaScript code to get a TypeScript version with inferred types, generated interfaces, and access modifier suggestions. The output is a starting point — review and refine the types for your specific use case.

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

function greet(name) {
  return `Hello, ${name}!`;
}

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

function greet(name: string): string {
  return `Hello, ${name}!`;
}