Migrating a Legacy JavaScript Codebase to TypeScript - JSJ 680
JavaScript Jabber5 Juni

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.

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021 JSJ Weapons of Choice

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020 JSJ Cloud9

020 JSJ Cloud9

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

2 Juli 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: https://www.spreaker.com/podcast/javascript-jabber--6102064/support.

23 Juni 201258min

017 JSJ CoffeeScript with Jeremy Ashkenas

017 JSJ CoffeeScript with Jeremy Ashkenas

The panelists talk to Jeremy Ashkenas about CoffeeScript.Special Guest: Jeremy Ashkenas . 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/javascript-jabber--6102064/support.

20 Juni 201218min

016 JSJ SQL and NoSQL

016 JSJ SQL and NoSQL

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015 JSJ Open Discussion

015 JSJ Open Discussion

The panelists have an open discussion. 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/javascript-jabber--6102064/support.

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014 JSJ SVG and Data Visualization with Chris Bannon

014 JSJ SVG and Data Visualization with Chris Bannon

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