Routing at Scale with TanStack Router's Nicolas Beaussart | React Router Migration, Monorepos, PayFit

What do you do when React Router v5 is blocking your React 18 upgrade, your frontend spans 25 repos, and you have 5 different micro frontend strategies running in the same app?

In this episode of Señors at Scale, Dan Neciu sits down with Nicolas Beaussart-Hatchuel, Staff Engineer at PayFit and maintainer of TanStack Router. Nicolas shares the full story of migrating a 1.5 million-line codebase from React Router v5 to TanStack Router using the strangler pattern, without big-bang migrations and without stopping 18 engineers from shipping 15-20 PRs a day.

We also dive into the origin story of TanStack Router, why PayFit killed micro frontends entirely and moved back to a single monorepo, how building the whole app at once saved 25MB of JavaScript, and why his MCP experiments performed worse than simply letting AI agents read the code. Plus: what it really takes to go from senior to staff engineer.

Key Topics:

- How Nicolas got into coding and his first iframe-based micro frontend migration

- The origin story of TanStack Router and URL-as-state

- Migrating 1.5M lines from React Router v5 to TanStack Router

- The strangler pattern applied to frontend migrations

- Faking React Router providers to sync two routers on one URL

- Consolidating 25 repos into one monorepo

- Secret dependencies, Yarn v1 pain, and standardizing on Vite

- Why dropping per-library builds saved 25MB of JavaScript

- TypeScript Go in editors and its RAM cost

- MCP servers vs agents reading code directly

- Internal DevRel: winning engineers over with social proof

- Going from senior to staff: system design and finding problems worth solving

- What's next: TanStack Start v1 and parallel routes

Connect with Nicolas:

💼 LinkedIn: https://www.linkedin.com/in/beaussan/

🐦 X: https://x.com/beaussan

🐙 GitHub: https://github.com/beaussan

🌐 Website: https://beaussan.io

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💼 Dan's LinkedIn: https://www.linkedin.com/in/neciudan

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ADDITIONAL RESOURCES:

- TanStack Router: https://tanstack.com/router

- Engineering Management for the Rest of Us by Sarah Drasner

- Scaling Fast by Swizec Teller

- Swizec's newsletter: https://swizec.com

#TanStackRouter #React #Monorepo #MicroFrontends #FrontendArchitecture #TypeScript #StaffEngineer #SenorsAtScale

💬 Have you ever had to migrate a router in a live codebase? What pattern did you use? Drop it in the comments.