Jonas Keller

Refactored React/TypeScript dashboard components (code-splitting, memoization, list virtualization), reducing bundle size by 34% and improving Time to Interactive from 3.2s to 1.1s for 60K+ DAU. Architected Node.js/Express microservices with multi-tier Redis caching, reducing API latency by 68% (410ms to 130ms) at 1.5M+ daily requests. Redesigns in MongoDB schemas and aggregation pipelines improved indexing and reduced DB CPU usage by 40%, speeding report queries by 3.5x. Built authentication with rotating refresh tokens, RBAC, and pre-signed URL uploads to offload work to an upload server. Increased Jest/RTL test coverage from 55% to 87% and aligned CI/CD with containerized services using Docker for parity.

Built multi-step user onboarding and workflow builder in React.js/TypeScript, managing complex local/global state with Redux Toolkit and improving customer trial conversion by 22%. Developed 15+ modular REST endpoints with Node.js/Express, adding structured input validation (Zod) and error-handling middleware to prevent malformed client requests. Integrated third-party payment and notification services using Stripe webhooks for automated subscription billing and SendGrid for transactional email triggers. Designed relation-like Mongoose data models with optimized population paths and virtual fields, resolving N+1 query bottlenecks and reducing profile page load time by 28%. Authored end-to-end and unit test suites using Jest/ Supertest/ React Testing Library and participated in PR reviews and sprint planning within a 6-person Agile team.

Converted Figma wireframes into responsive React components using modern CSS/Tailwind, ensuring pixel-perfect layouts and consistent rendering across mobile, tablet, and desktop browsers. Connected frontend forms to internal Express.js REST APIs using Axios, adding client-side input validation and custom loading/error states to improve user feedback. Resolved 50+ backlog UI and functional bugs reported in Jira using Chrome DevTools and Git for debugging, submitting clean pull requests for senior review. Assisted senior developers with backend maintenance, writing basic Mongoose queries for user data filtering and maintaining organized Postman collections for API documentation.