How do you gradually add TypeScript to an existing JavaScript project?

Migrating to TypeScript doesn't have to be all-or-nothing. A gradual approach minimizes risk and lets your team learn as you go.

Start with setup: add TypeScript and create tsconfig.json with loose settings. Set allowJs: true so JavaScript files work alongside TypeScript. Set strict: false initially—you'll tighten this later.

Rename files incrementally. Start with utility files and leaf components that have few dependencies. Rename .js to .ts (or .tsx for React). TypeScript will infer what it can and use 'any' for the rest.

Add types to new code from the start. When you write new files or significantly modify existing ones, use proper types. This stops the untyped codebase from growing while you migrate.

Create type definitions for common patterns. If your app has User, Product, or Order objects everywhere, define those types in a shared file. Import them as you migrate each file.

Tighten strictness gradually. Enable strict null checks first (catches the most bugs), then other strict options. Each time, fix the new errors before enabling more. Tools like ts-migrate can help automate some fixes.

Don't aim for perfect types immediately. Use 'any' as an escape hatch, then create cleanup tasks to improve typing later. Progress over perfection keeps momentum going.