Migrating a Legacy JavaScript Codebase to TypeScript - JSJ 680

In this episode, Dan and I (Steve) dove deep into what turned out to be a surprisingly complex, yet incredibly insightful topic: gradually migrating a massive legacy JavaScript project over to TypeScript. We're talking about nearly 1,000 JS files, 70,000+ lines of code, and years of developer history—all transitioning carefully to a typed, modern future.

Dan walked us through how he started by setting up the project for success before converting even one file—getting CI/CD ready, setting up tsconfig.json, sorting out test dependencies, dealing with mock leaks, and even grappling with quirks between VS Code and WebStorm debugging.

We talked tools (like TS-ESLint, concurrently, and ts-node), why strict typing actually uncovered real bugs (and made the code better!), and why it’s crucial not to touch any.js files until your TypeScript setup is rock solid.

Key Takeaways:

• Gradual migration is 100% possible—and often better—than ripping the bandaid off.
• TypeScript can and will catch bugs hiding in your JavaScript. Be prepared!
• Use VS Code extensions or TS-Node to support your devs’ tooling preferences.
• Don't underestimate the setup phase—it’s the foundation of long-term success.
• Start small: Dan's team converted just one file at first to test the whole pipeline.

If you’re sitting on a legacy JS project and dreaming of TypeScript, this episode is your blueprint—and your warning sign.

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