Migrating a Legacy JavaScript Codebase to TypeScript - JSJ 680

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.

Become a supporter of this podcast: https://www.spreaker.com/podcast/javascript-jabber--6102064/support.

Episoder(737)

025 JSJ Require.js with James Burke

025 JSJ Require.js with James Burke

The panelists talk to James Burke about Require.js.Special Guest: James Burke . Support this podcast at — https://redcircle.com/javascript-jabber/donationsPrivacy & Opt-Out: https://redcircle.com/pr...

21 Aug 201235min

024 JSJ Strata.js with Michael Jackson

024 JSJ Strata.js with Michael Jackson

Support this podcast at — https://redcircle.com/javascript-jabber/donationsPrivacy & Opt-Out: https://redcircle.com/privacyBecome a supporter of this podcast: https://www.spreaker.com/podcast/javascri...

14 Aug 201243min

023 JSJ Phantom.js with Ariya Hidayat

023 JSJ Phantom.js with Ariya Hidayat

The panelists talk to Ariya Hidayat about Phantom.js.Special Guest: Ariya Hidayat. Support this podcast at — https://redcircle.com/javascript-jabber/donationsPrivacy & Opt-Out: https://redcircle.com...

7 Aug 201240min

022 JSJ Node.js on Azure with Glenn Block

022 JSJ Node.js on Azure with Glenn Block

The panelists talk to Glenn Block about Azure.Special Guest: Glenn Block. Support this podcast at — https://redcircle.com/javascript-jabber/donationsPrivacy & Opt-Out: https://redcircle.com/privacyB...

1 Aug 201256min

021 JSJ Weapons of Choice

021 JSJ Weapons of Choice

The panelists discuss their weapons of choice. Support this podcast at — https://redcircle.com/javascript-jabber/donationsPrivacy & Opt-Out: https://redcircle.com/privacyBecome a supporter of this p...

30 Jul 20121h 11min

020 JSJ Cloud9

020 JSJ Cloud9

The panelists talk about Cloud9. Support this podcast at — https://redcircle.com/javascript-jabber/donationsPrivacy & Opt-Out: https://redcircle.com/privacyBecome a supporter of this podcast: https:...

13 Jul 201255min

019 JSJ Browserify with James Halliday

019 JSJ Browserify with James Halliday

The panelists talk Browserify with James Halliday.Special Guest: James Halliday. Support this podcast at — https://redcircle.com/javascript-jabber/donationsPrivacy & Opt-Out: https://redcircle.com/p...

2 Jul 201240min

018 JSJ Agile Development

018 JSJ Agile Development

The panelists discuss Agile development. Support this podcast at — https://redcircle.com/javascript-jabber/donationsPrivacy & Opt-Out: https://redcircle.com/privacyBecome a supporter of this podcast...

23 Jun 201258min

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