JavaScript to TypeScript Converter
Add enterprise-grade type safety to JavaScript codebases with AI-powered conversion. Automatically infers types from usage patterns, extracts reusable interfaces from object shapes, and generates TypeScript declarations. Transforms untyped function parameters to typed signatures, adds return type annotations, and creates union types for conditional branches—catching bugs before runtime.
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Issue Description
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How It Works
- Step 1: Paste your JavaScript source code including ES6+ features, class definitions, function declarations, and object literals that need type safety.
- Step 2: The AI analyzes variable assignments, function parameter usage, return statements, and object property access patterns to infer appropriate types.
- Step 3: Advanced type inference generates TypeScript interfaces from repeated object shapes, adds type annotations to functions, and applies union types for conditional logic.
- Step 4: Receive fully-typed TypeScript code with interfaces, type aliases, generics where applicable, and strict null checks ready for gradual refinement.
JavaScript vs TypeScript: Feature Comparison
| Feature | JavaScript | TypeScript |
|---|---|---|
| Type Safety | No compile-time checks | Full static type checking |
| IDE Support | Basic autocomplete | Rich IntelliSense, refactoring |
| Error Detection | Runtime errors only | Catch errors at compile-time |
| Documentation | JSDoc comments | Built-in type annotations |
| Tooling | Limited static analysis | Advanced type inference, analysis |
Code Conversion Examples
Example 1: Function Parameters and Return Types
function calculateTotal(items, taxRate) {
const subtotal = items.reduce((sum, item) => {
return sum + (item.price * item.quantity);
}, 0);
const tax = subtotal * taxRate;
return {
subtotal: subtotal,
tax: tax,
total: subtotal + tax
};
}
const cart = [
{ name: 'Widget', price: 10.99, quantity: 2 },
{ name: 'Gadget', price: 25.50, quantity: 1 }
];
const result = calculateTotal(cart, 0.08);
console.log('Total: $' + result.total.toFixed(2));interface CartItem {
name: string;
price: number;
quantity: number;
}
interface TotalResult {
subtotal: number;
tax: number;
total: number;
}
function calculateTotal(items: CartItem[], taxRate: number): TotalResult {
const subtotal = items.reduce((sum: number, item: CartItem) => {
return sum + (item.price * item.quantity);
}, 0);
const tax = subtotal * taxRate;
return {
subtotal: subtotal,
tax: tax,
total: subtotal + tax
};
}
const cart: CartItem[] = [
{ name: 'Widget', price: 10.99, quantity: 2 },
{ name: 'Gadget', price: 25.50, quantity: 1 }
];
const result: TotalResult = calculateTotal(cart, 0.08);
console.log('Total: $' + result.total.toFixed(2));Key Changes:
The converter extracts the CartItem interface from the array structure and TotalResult from the return object shape. Function parameters gain explicit types (CartItem[], number), and the return type is annotated as TotalResult. The reduce callback parameters get typed for better IDE support. Array variables receive explicit type declarations for compile-time safety.
Example 2: Class Methods and Generics
class DataStore {
constructor() {
this.items = [];
}
add(item) {
this.items.push(item);
}
find(predicate) {
return this.items.find(predicate);
}
filter(predicate) {
return this.items.filter(predicate);
}
getAll() {
return [...this.items];
}
}
const userStore = new DataStore();
userStore.add({ id: 1, name: 'Alice', active: true });
userStore.add({ id: 2, name: 'Bob', active: false });
const activeUsers = userStore.filter(user => user.active);
console.log(activeUsers);class DataStore<T> {
private items: T[];
constructor() {
this.items = [];
}
add(item: T): void {
this.items.push(item);
}
find(predicate: (item: T) => boolean): T | undefined {
return this.items.find(predicate);
}
filter(predicate: (item: T) => boolean): T[] {
return this.items.filter(predicate);
}
getAll(): T[] {
return [...this.items];
}
}
interface User {
id: number;
name: string;
active: boolean;
}
const userStore = new DataStore<User>();
userStore.add({ id: 1, name: 'Alice', active: true });
userStore.add({ id: 2, name: 'Bob', active: false });
const activeUsers: User[] = userStore.filter(user => user.active);
console.log(activeUsers);Key Changes:
The generic class is untyped in JavaScript but becomes DataStore<T> in TypeScript, allowing type-safe collections. Method parameters receive function type annotations ((item: T) => boolean), and return types are explicit (T | undefined, T[]). The User interface is extracted from usage patterns. The private items field gains an access modifier and type annotation for encapsulation and safety.
Frequently Asked Questions
How are JavaScript types inferred?
The converter analyzes usage patterns to infer types. Variable assignments determine basic types, function parameters infer from usage, return types come from return statements, and complex types use union types or any where ambiguous.
What happens to untyped code?
Ambiguous code gets the 'any' type initially for safety, allowing gradual typing. You can then refine these to specific types. The converter prioritizes compilation success, letting you strengthen types incrementally.
Can it create interfaces?
Yes! The converter extracts interfaces from object literals and function parameters. Repeated object shapes become reusable interfaces, improving type reusability and code documentation with proper TypeScript patterns.