🛠️ 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

✨ 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.