Senior Frontend Engineer

1. How would you structure a large React 18 application with complex fintech business logic?

I would organize the application by business domains, separating UI, state management, API services, validation, and shared infrastructure. Clear boundaries, typed interfaces, reusable hooks, and well-defined data-flow patterns help keep the codebase maintainable as it grows.

2. What are the main benefits of using Redux Toolkit, and how would you use it in a production application?

Redux Toolkit reduces boilerplate through tools such as createSlice, createAsyncThunk, and built-in Immer support. I would use it for predictable global state, normalized entities, async workflows, and selectors that prevent unnecessary component re-renders.

3. How do you improve the performance of a React application?

I first measure performance using browser profiling and React DevTools, then optimize based on evidence. Common techniques include memoization, code splitting, lazy loading, virtualization, stable component props, efficient selectors, and minimizing unnecessary network requests and renders.

4. How would you design a type-safe API integration in TypeScript?

I would define explicit request and response types, centralize the API client, and handle loading, error, and success states consistently. Ideally, types would be generated or validated from the API contract, with runtime validation for untrusted external data.

5. What testing strategy would you apply to a React and TypeScript application?

I would use unit tests for pure functions and business rules, component tests for user-visible behavior, and integration tests for workflows involving state and APIs. Tests should focus on contracts and behavior rather than implementation details, with critical payment flows receiving especially strong coverage.

6. How do you ensure responsive and cross-browser-compatible interfaces?

I use semantic HTML, responsive CSS techniques, Material UI breakpoints, and progressive enhancement. I also test critical flows across supported browsers and viewport sizes, checking for differences in layout, form controls, JavaScript APIs, and accessibility behavior.

7. Which security risks should a frontend engineer consider in a fintech application?

Important areas include XSS prevention, CSRF protection where applicable, secure token handling, input validation, dependency security, clickjacking protection, and avoiding sensitive data in logs or local storage. The frontend should also rely on server-side authorization rather than treating client-side checks as security boundaries.

8. How would you handle complex asynchronous payment workflows in the frontend?

I would model the workflow as explicit states such as idle, submitting, pending, completed, failed, and retryable, rather than relying on scattered boolean flags. Idempotent requests, clear error mapping, polling or event updates where necessary, and protection against duplicate submissions are essential.

9. What factors do you consider when reviewing frontend architecture and code in a pull request?

I check correctness, readability, separation of concerns, type safety, performance, test coverage, accessibility, security, and consistency with established patterns. I also consider whether the solution scales to future requirements and whether the code is easy for other engineers to understand and maintain.

10. How do you estimate frontend tasks and handle uncertainty during Agile planning?

I break the work into technical and user-facing steps, identify dependencies and risks, and compare the scope with similar completed tasks. When uncertainty is high, I propose a short investigation or spike, document assumptions, and communicate estimates as ranges rather than false precision.