Ask Claude to Add TypeScript Types to Your Untyped JavaScript Code

Migrating from JavaScript to TypeScript doesn't have to be a big-bang rewrite. Claude reads your JS files and adds types based on how the code is actually used.

"Add TypeScript types to src/services/userService.js — rename it to .ts and type everything"

Claude reads the function signatures, return values, object shapes, and how other files call it, then adds accurate types:

// Before: untyped JavaScript
function getUser(id) {
  return db.query('SELECT * FROM users WHERE id = ?', [id]);
}

// After: Claude adds types from actual usage
interface User {
  id: number;
  name: string;
  email: string;
  created_at: Date;
}

async function getUser(id: number): Promise<User | null> {
  return db.query<User>('SELECT * FROM users WHERE id = ?', [id]);
}

You can migrate incrementally:

# Start with the most important files
"Type the files in src/services/ — these are the core business logic"

# Type one module at a time
"Add types to the authentication module, including the middleware and helpers"

# Type API boundaries first
"Add TypeScript interfaces for all API request and response shapes"

# Generate types from API responses
"Read the API routes and generate TypeScript types for every endpoint's input and output"

Claude handles the nuances that make manual typing tedious:

• Union types — detects when a value can be string | null or User | undefined
• Generics — adds proper generic parameters to utility functions and collections
• External libraries — uses @types/* packages and suggests installing missing ones
• Gradual migration — uses // @ts-ignore or any sparingly where strict typing would require larger refactors
• Discriminated unions — creates proper tagged union types for state machines and action objects

For existing TypeScript projects with loose types, Claude tightens them:

"Find all uses of 'any' in the codebase and replace them with proper types"

TypeScript migration is a typing problem — and Claude is very good at typing.

via Claude Code