🛠️ Migrating from JavaScript to TypeScript: Strategies and Gotchas

If you're thinking about migrating your JavaScript codebase to TypeScript, you're not alone — and you're definitely on the right track. TypeScript offers type safety, better tooling, and a more confident developer experience. But a migration isn't just a simple "rename-all-the-files" process. It's a journey — and like any journey, it’s smoother with a map. 🗺️

In this post, I'll walk you through a practical migration strategy and highlight some common gotchas to avoid.

✅ Why Migrate to TypeScript?

🔍 Catch errors before they happen (at compile time)

✨ Enjoy better IntelliSense and code navigation

🛡️ Refactor with confidence

📚 Your types are your documentation

Let’s make your codebase future-proof.

🔄 Migration Strategies

1. Start with the TypeScript Compiler

Install TypeScript and initialize the project:

npm install --save-dev typescript
npx tsc --init

This creates a tsconfig.json file — the heart of your TS setup.

2. Enable JavaScript Compatibility

You can type-check your existing .js files without converting them yet:

// tsconfig.json
{
  "compilerOptions": {
    "allowJs": true,
    "checkJs": true,
    "outDir": "dist"
  },
  "include": ["src"]
}

3. Rename Files Gradually

Start renaming .js → .ts (or .jsx → .tsx) one file at a time.

Start with:

• Utility functions
• Isolated modules
• Non-UI logic

Avoid renaming everything at once — it’s easier to debug issues incrementally.

4. Add Type Annotations Where Needed

Start small. Focus on:

• Function arguments and return types
• Object shapes using interfaces or type aliases
• Constants and enums

type User = { name: string };

function greet(user: User): string {
  return `Hello, ${user.name}`;
}

5. Leverage JSDoc in .js Files

Want TS support without converting a file yet? Add JSDoc:

/**
 * @param {string} name
 * @returns {string}
 */
function greet(name) {
  return `Hello, ${name}`;
}

This is a great way to introduce typing without renaming.

6. Use any and @ts-ignore Sparingly

Yes, they can unblock you. But track them down and clean them up later.

// @ts-ignore
someLegacyCode();

7. Tighten the Compiler Gradually

Once you're confident, turn on stricter settings in tsconfig.json:

"strict": true,
"noImplicitAny": true,
"strictNullChecks": true

Enable them one at a time to avoid overwhelming yourself.

⚠️ Gotchas to Watch For

🧩 Dynamic Code Is Hard to Type
Things like eval, Object.assign, or deeply dynamic object keys are hard to infer. Consider refactoring those.

📦 Missing Types for Libraries
Use DefinitelyTyped when needed:

npm install --save-dev @types/lodash

If no types exist, you can write your own *.d.ts file.

🔀 Implicit any Types
You’ll hit this a lot if noImplicitAny is enabled:

function log(message) {
  console.log(message);
}
//       ~~~~~~~~ Error: Implicit any

⚛️ JSX Requires .tsx
Using React? Any file that includes JSX must use the .tsx extension.

Also, don’t forget to type your props:

type ButtonProps = {
  label: string;
};

function Button({ label }: ButtonProps) {
  return <button>{label}</button>;
}

🧪 Update Your Build + Test Tools
Make sure:

• Webpack or Vite is configured to handle .ts files
• Babel has @babel/preset-typescript
• Jest uses ts-jest or babel-jest

🧭 Your Migration Roadmap

Phase	What to Do
🧪 Enable Type Checking	Use allowJs + checkJs
🔀 Rename Incrementally	Convert .js → .ts in small batches
✍️ Add Types Slowly	Start with functions, constants, object shapes
⚙️ Configure Tools	Update build/test tooling
🧼 Clean Up	Remove any, @ts-ignore, etc.

✨ Final Thoughts

Migrating to TypeScript doesn’t have to be overwhelming. It’s not an all-or-nothing decision. Use a gradual, iterative approach — and before you know it, you'll have a fully typed, safer, and more enjoyable codebase.

Your future self (and your team) will thank you. 🙌

Thanks for reading! Have you tried migrating a JS project to TypeScript? Share your story or tips in the comments below.