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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