Sergiy L. — Senior Full-Stack Engineer

Work Experience

Senior Full-Stack Engineer, Fintech platform

Duration: June 2022 - Present

Summary: Development of a fintech platform including a manager portal, multi-step payment forms, payment gateway integrations, and a merchant CRM system. The platform supports high load transaction dashboards and integrates multiple payment providers to facilitate seamless financial operations.

Responsibilities:

• Architected and developed the manager portal using React and Zustand for state management with role-based access control.
• Built multi-step payment forms supporting multiple currencies and provider APIs with React Hook Form and Zod validation.
• Designed and implemented payment gateway integration layer handling OAuth flows, webhooks, and transaction synchronization using Node.js and Express.
• Developed merchant CRM with Next.js (App Router), server-side rendering, and live transaction feed via Socket.io.
• Created NestJS microservices with Prisma/PostgreSQL and MongoDB for audit logs.
• Implemented server state management with TanStack Query for caching and background refetching.
• Optimized API latency by 45% through Redis caching, query indexing, and cursor-based pagination.
• Styled UI consistently with Tailwind CSS and shadcn/ui component library.
• Achieved over 80% test coverage on payment modules using Jest, React Testing Library, and Cypress.
• Containerized services with Docker and deployed to AWS with S3 for document storage.

Technologies: React, Next.js (App Router), TypeScript, Zustand, TanStack Query, TanStack Table, React Hook Form, Zod, shadcn/ui, Tailwind CSS, Node.js, Express, NestJS, Prisma, PostgreSQL, MongoDB, Redis, Socket.io, Docker, AWS (S3), Jest, React Testing Library, Cypress.

Full-Stack Engineer, ElastX (Payment service provider)

Duration: July 2019 - June 2022

Summary: Development of an internal dashboard and CRM system for a payment service provider to manage merchants and analyze transaction data. The project included real-time transaction monitoring, analytics charts, and a React Native companion app for merchant notifications.

Responsibilities:

• Built internal dashboard in React with custom hooks for data fetching and complex grids with sorting, filtering, and CSV export.
• Created a shared component library using Material-UI and styled-components for dashboard and CRM modules.
• Structured Redux store with feature-based slicing and Redux Thunk for scalable state management.
• Developed payment analytics charts with Recharts showing transaction volumes, merchant KPIs, and revenue breakdowns.
• Shipped React Native companion app for merchant notifications and quick transaction approvals.
• Implemented REST APIs in Node.js/Express with Sequelize ORM for MySQL supporting complex search and aggregations.
• Added real-time updates via React Query polling with WebSocket fallback for live transaction monitoring.
• Maintained Vue.js 2 admin panel with MobX for system configuration and cross-service data synchronization.
• Optimized initial load time by 55% using React.lazy code splitting and route-based chunking.
• Set up JWT authentication with Axios interceptors for token refresh and role-based access control.

Technologies: React, Redux, Redux Thunk, TypeScript, React Query, React Native, Next.js, MobX, Vue.js 2, Node.js, Express, Sequelize, MySQL, JWT, Axios, Recharts, Material-UI, styled-components, Jest.