This role is best suited for a Senior Frontend Engineer with 5+ years of experience building and scaling production React applications, particularly in SaaS, fintech, or developer-focused products. The candidate should have deep expertise in React, frontend architecture, design systems, performance optimization, monitoring/observability, and cross-functional collaboration, with bonus experience in GraphQL, open-source projects, or data-intensive applications. They should be highly autonomous, comfortable with ambiguity, and capable of owning the frontend platform end-to-end while driving reliability, consistency, and long-term maintainability.

1. How would you design and scale a frontend application used by enterprise SaaS customers?
   I would focus on maintainable architecture, reusable components, strong state management patterns, observability, performance optimization, and a design system that scales across teams and features.

2. What are the key principles of building and maintaining a production design system?
   A design system should provide reusable, accessible, well-documented components with clear standards. It should balance consistency, flexibility, and ease of adoption across the organization.

3. How do you approach frontend observability and monitoring?
   I use error tracking, performance monitoring, logging, user session analysis, and custom metrics to quickly identify, diagnose, and resolve issues before they significantly impact users.

4. What techniques do you use to optimize React application performance?
   I use memoization, code splitting, lazy loading, efficient state management, virtualization for large datasets, and profiling tools to identify rendering bottlenecks.

5. How would you architect a frontend application that consumes GraphQL APIs?
   I would use a client such as Apollo, normalize and cache data efficiently, define clear query boundaries, handle loading/error states consistently, and minimize unnecessary network requests.

6. How do you handle ambiguity when product requirements are not fully defined?
   I clarify objectives, identify assumptions, propose solutions, create prototypes when necessary, and maintain close communication with stakeholders while continuing to make progress.

7. What challenges are unique to billing or fintech frontend applications?
   Challenges include handling complex financial data, ensuring accuracy, managing permissions and security, maintaining auditability, and presenting complicated workflows in a user-friendly way.

8. How do you ensure frontend reliability at scale?
   I combine automated testing, monitoring, strong TypeScript usage, code reviews, safe deployment practices, and continuous performance measurement to maintain reliability.

9. What qualities distinguish a senior frontend engineer from a mid-level engineer?
   Senior engineers take ownership beyond implementation by driving architecture, improving engineering standards, mentoring others, making tradeoff decisions, and proactively solving broader product and technical challenges.

10. How would you contribute to an open-source developer-focused product like Lago?
    I would focus on building maintainable systems, improving developer experience, writing clear documentation, engaging with community feedback, and ensuring the product remains scalable and reliable for a growing user base.

Lago is hiring a Senior Frontend Engineer to own and scale the frontend of one of the fastest-growing open-source billing platforms, trusted by companies like PayPal, Synthesia, and Mistral AI. In this fully remote Europe-based role, you'll work primarily with React and GraphQL to build high-performance product experiences, evolve the company’s design system, and improve frontend reliability, monitoring, and observability. You'll collaborate closely with product and backend teams while taking full ownership of major initiatives in a fast-moving startup environment that values autonomy, accountability, and technical excellence. With a salary of €60K–€90K, stock options, strong backing from investors including Y Combinator, and the opportunity to shape a product used by leading SaaS companies worldwide, this is an exceptional role for frontend engineers passionate about developer tools, open source, and fintech infrastructure.