Senior React Developer

Senior React Developer - real-time, data intensive UI Engineering | Prague | hybrid | Senior IC We're hiring for a critical, high-profile project - a full replatforming of a large-scale, real-time data intensive application serving thousands of external clients worldwide. This is a genuinely complex, external-client-facing system - not an internal tool, not a CRUD app. The engineering problem The application processes and renders continuously updating, high-frequency data in real time, connected to numerous downstream data sources - and users are acting on that data live, as it changes. That's the actual engineering challenge here: building a UI that stays fast, correct, and stable under constant real-time data flow, at scale, for thousands of concurrent external users. You'll be building new features, resolving real issues surfacing from an active client migration, and shaping how this high-performance interface evolves. Team ~80 people across the globe on the project overall. UI engineering team specifically: 20 people globally, 5 in Prague. This is a senior individual contributor role - we need a battle-scarred UI Engineering veteran who's designed and built complex, large scale and most importantly data intensive, ideally real-time data, UIs. High potential juniors are unfortunately not a goto fit for this particular role. The one thing that matters most Genuine, hands-on experience building real-time or high-throughput, data-intensive UIs - interfaces where users act on continuously updating data. Standard business applications, however well built, aren't a match here - this is the single biggest filter for this role. What you'll need Deep React + TypeScript: custom hooks, advanced component patterns, performance optimisation, error boundaries, context architectureReal-time data experience: WebSockets, and UIs that stay performant under continuous high-frequency updatesTanStack React Query and Apollo Client (GraphQL) for server stateGenuine ownership of testing: Vitest, React Testing Library, Playwright — including test architecture, fixtures, and API mocking. 100% unit test coverage expected, owned by developers, not handed offReal architectural ownership — you've designed complex UIs from the ground up, have opinions on architecture and design, and can proactively guide technical direction. This is not a "pick up a ticket" role Interview process Live coding and a practical assignment, completed without AI coding tools - come ready to demonstrate your own hands-on skill directly. Is this you? If you've built real-time or heavily data-intensive frontends, genuinely own architecture and testing as part of your craft, and want deep technical ownership rather than a path into management - this is worth a conversation. If your background is mainly standard business applications or task-based delivery, this specific role won't be the right fit - but reach out anyway, we may have something better suited.