Serhii is available for hire
Hire SerhiiSerhii Onenko
Verified Expert in Engineering
Full-stack Web Developer
Puerto de la Cruz, Spain
Toptal member since May 22, 2019
Bio
Sergiy is an adept full-stack developer with experience at renowned companies like Cityblock and NewMotion (Shell Recharge). Honing his expertise at Europe's premier electric car charge network, he has successfully developed apps from inception, enhanced mature projects, and contributed to several startups, three of which have flourished. Sergiy excels in aligning systems to Google's high standards.
Portfolio
Messageworks
React, Next.js, PostgreSQL, Clerk, Sentry, Notion, Amazon Web Services (AWS)...
Cityblock
TypeScript, React, React Apollo, Apollo Server, Cypress, Scrum, Mentorship...
Toptal Projects
Chrome, React, Sketch, Figma, PostgreSQL, Node.js, Express.js, Ionic, Angular...
Experience
- Web Development - 12 years
- Data Visualization - 8 years
- TypeScript - 5 years
- React - 3 years
- Chrome Extensions - 2 years
- Solidity - 1 year
- Claude Code - 1 year
Preferred Environment
Git, Scrum, React, Next.js, PostgreSQL, TypeScript, Claude Code, Codex, Cursor AI, OpenRouter
The most amazing...
...contribution I've had was with Cityblock, a Google subsidiary's healthcare app. Enhanced Europe's top EV charging network at NewMotion (Shell).
Work Experience
CTO | Software Architect
2025 - 2026
Messageworks
- Re-architected and rebuilt a fragile founder-built MVP into a maintainable production SaaS, gradually replacing the original codebase while keeping the product operational.
- Architected a TypeScript monorepo with Next.js, React, tRPC, PostgreSQL, and Drizzle, optimizing a small team for end-to-end type safety and fast product iteration.
- Designed multi-stage LLM pipelines with versioned prompts, retries, caching, telemetry, and parallel execution, including a seven-run synthetic-audience evaluation workflow.
- Implemented defense-in-depth multi-tenant security with Clerk, CASL, service-level ownership checks, and PostgreSQL RLS across account and organization data.
- Provisioned dev, staging, and production AWS environments with Terraform, ECS Fargate, RDS, ALB, ECR, SSM, CloudWatch alarms, and OIDC-based CI/CD.
- Built an isolated Playwright E2E system with per-spec database clones, six permission personas, and 227 declared browser tests covering core product and authorization flows.
- Created AI-assisted engineering workflows with reusable coding-agent commands, specialized review/security agents, PRD-driven implementation, and automated E2E repair loops.
- Enabled a non-technical founder to implement product changes with coding agents while I retained hands-on ownership of architecture, difficult cases, reviews, infrastructure, and production reliability.
Technologies: React, Next.js, PostgreSQL, Clerk, Sentry, Notion, Amazon Web Services (AWS), Terraform, Architecture, Software Architecture
Senior Engineer
2022 - 2023
Cityblock
- Contributed to success by suggesting improvements and actively driving innovation within the team. My suggestions for architectural enhancements were implemented, allowing other developers to build upon the foundation I created.
- Took on the responsibility of mentoring and guiding fellow developers, assisting them in improving their skills and achieving their full potential. By fostering a collaborative environment, I promoted knowledge-sharing and teamwork.
- Improved the Scrum process and implemented strategies that enhanced efficiency and effectiveness. The team achieved streamlined workflows and improved project delivery timelines through my insights.
- Recognized the importance of thorough testing. I optimized test speed, resulting in faster iterations and increased overall development productivity.
- Developed new features, resolved bugs, and enhanced GraphQL solutions. Collaborated with designers and leaders to seamlessly integrate design requirements, offering continuous, valuable input.
Technologies: TypeScript, React, React Apollo, Apollo Server, Cypress, Scrum, Mentorship, Architecture, GraphQL, REST, Healthcare IT, Healthcare
Freelance Developer
2019 - 2022
Toptal Projects
- Created the skeleton for a complex TypeScript/React Chrome extension and built a great development team that took the extension to the next level, porting it to mobile platforms.
- Created the functional design for a cross-platform application, "Startup in Stealth Mode."
- Configured an AWS deployment process, Amazon EC2, Bash Script, PostgreSQL, and SSL.
- Collected requirements for every next feature set and made demos and standups.
- Implemented the application, including authentication (Google and email), authorization, file upload, messaging, and user management.
Technologies: Chrome, React, Sketch, Figma, PostgreSQL, Node.js, Express.js, Ionic, Angular, Web Development, REST APIs, JavaScript, SQL, Bootstrap, TypeScript, Git, Protractor, Karma, Sass, RxJS, Chrome Extensions, Scrum, iOS, Docker, MongoDB, Bash, Amazon Web Services (AWS), Xcode, CSS3, HTML5, Data Visualization, Maps, JSON Web Tokens (JWT), Integration Testing, Testing, Unit Testing, Webpack, Functional Programming, Ionic 4, OOP Designs, WebSockets, Coaching, Leadership, Safari Extensions, Kanban, WatchOS, Architecture, Domain-driven Design (DDD), OS X, Visual Studio Code (VS Code), ECMAScript (ES6), Bcrypt, Flutter, React Native, CSS, GraphQL, Python
Technical Lead
2017 - 2019
NewFirePartners
- Designed and developed the micro frontend architecture for the existing Angular 5 app.
- Created a better development process and introduced new build commands that allow switching between development environments without needing to dig tons of documentation.
- Improved all team (30+ developers) development speed by reducing application build time in development mode five times.
- Created and contributed to the core library used across all pluggable applications.
- Shared knowledge with the team and distributed teams.
Technologies: Node.js, Webpack, Angular, AngularJS, TypeScript, ECMAScript (ES6), JavaScript, Web Development, REST APIs, Bootstrap, Git, Protractor, Karma, Sass, Chart.js, RxJS, Scrum, Meteor, Bash, Jira, Integration Testing, Testing, Unit Testing, Dependency Injection, E2E Testing, Functional Programming, Google Chart API, OOP Designs, WebSockets, Coaching, Leadership, Safari Extensions, Architecture, Domain-driven Design (DDD), OS X, IntelliJ IDEA, Visual Studio Code (VS Code), CSS
Co-founder and Architect
2017 - 2018
Pubium
- Designed and developed an application from scratch, including delegation to other developers. Fully controlled step-by-step implementation, introducing the Scrum process.
- Created the back end for the management portal, including the roles system.
- Designed the restaurant portal's front-end development for other developers, allowing them to register restaurants and manage tables, days, hours, and reservations.
- Designed and implemented POC for the RESTfull back-end app.
- Created the notifications application for push notifications across all existing infrastructure.
- Set up all development and production environment,s including Docker and Swarm.
- Integrated the application with socials and mail services.
- Coached and translated the most important knowledge to the team.
Technologies: Docker, React, Silhouette, Slick, PostgreSQL, MongoDB, Node.js, Play Framework, Scala, Web Development, REST APIs, SQL, Bootstrap, TypeScript, Algorithms, Git, Express.js, Google Maps, Sass, Scrum, Arduino, Raspberry Pi, Realm, Bash, C++, CSS3, HTML5, Akka Actors, Dependency Injection, Akka Streams, Webpack, Functional Programming, OOP Designs, WebSockets, Coaching, Leadership, SBT, Kanban, Docker Swarm, WatchOS, Architecture, Domain-driven Design (DDD), OS X, IntelliJ IDEA, ECMAScript (ES6), CSS
Senior Web Developer
2015 - 2016
IDMC
- Architected and developed the front end for web-based healthcare applications from scratch.
- Designed and developed the REST layer for the application.
- Added a complex system of roles, permissions, and restrictions for users.
- Developed Elasticsearch-based aggregations and translated them to visualizations and tables.
- Created a complex filter system using tables and charts.
- Contributed to Kibana, where we needed additional features.
- Implemented a system that alerts if microorganisms evolve resistance to certain antibiotics.
Technologies: React, AmCharts, AngularJS, ScalaTest, Elasticsearch, PostgreSQL, Akka HTTP, Scala, Web Development, REST APIs, JavaScript, Bootstrap, Algorithms, Git, Google Maps, Sass, Chart.js, Scrum, Bash, Jira, CSS, CSS3, Akka, Data Visualization, Maps, Akka Actors, Akka Persistence, Dependency Injection, Akka Streams, Leaflet, Functional Programming, Apache Camel, Google Chart API, OOP Designs, WebSockets, Leadership, HL7, SBT, Kibana, Kanban, Gulp, Bower, HL7 FHIR Standard, Architecture, Domain-driven Design (DDD), OS X, IntelliJ IDEA, Healthcare IT
Software Developer
2012 - 2015
Shellcharge
- Designed and developed a RESTful application to enable partner integrations with our services.
- Contributed to the main web application, Account.shellrecharge.com. It allows users to manage their charge points, shows available charge points on the map, manages subscription plans, etc.
- Contributed to the central application responsible for managing the behavior of charging points.
- Computed payments for charge point usage, thus contributing to the application.
Technologies: MySQL, Lift, Kendo UI, HAL, Akka, PostgreSQL, Slick, spray, CSS, AngularJS, Scala, Web Development, REST APIs, JavaScript, SQL, Bootstrap, jQuery, Algorithms, Git, Google Maps, Less, Scrum, Bash, Linux, Jira, Data Visualization, Maps, Akka Actors, Akka Persistence, Integration Testing, Testing, Unit Testing, Dependency Injection, E2E Testing, Leaflet, Functional Programming, OOP Designs, Regression Testing, SBT, Kanban, RabbitMQ, Architecture, Domain-driven Design (DDD), OS X, IntelliJ IDEA
Lead JavaScript Developer
2011 - 2012
Samsung R&D Ukraine
- Designed and crafted a mobile UI library using JavaScript, HTML5, and CSS3 for an e-book RIA framework, which was integrated with C#.
- Acted as the lead developer, chief architect, and team lead.
- Enhanced proprietary libraries for the Tizen operating system using C++.
- Enhanced the Kies Air mobile app by adding compatibility for additional screen resolutions, devices, and features like photo previews.
Technologies: NetBeans, Subversion (SVN), HTML5, CSS3, PHP, JavaScript, Web Development, REST APIs, jQuery, Git, C++, C, Linux, Data Visualization, Dependency Injection, OOP Designs
Lead Full-stack Developer
2011 - 2011
OS-Direct
- Designed a web app for team management with features like photo uploads, data entry, chart visualization, and PDF reports. Incorporated a robust role system for varied users.
- Developed a web-based application for live streaming conferences featuring a pre-moderated chat and a sophisticated role management system.
- Built a data visualization application with comprehensive filtering and grouping capabilities for in-depth analysis.
- Developed landing pages for esteemed, major clients.
Technologies: CSS, MySQL, PHP, MODx, Ext JS, Web Development, REST APIs, JavaScript, SQL, jQuery, Data Visualization, Dependency Injection, OOP Designs
Web Developer
2010 - 2011
Lumostestprep
- Designed a web solution for educators to create courses and share articles.
- Integrated video uploading and course content editing features.
- Linked the platform with a student portal for course access and test-taking.
Technologies: CSS, MySQL, PHP, JavaScript, Ext JS, Web Development, SQL, jQuery, Data Visualization, Dependency Injection
Experience
NewMotion
NewMotion or Shell Recharge Solutions. Initially, I was a front-end developer; during the project, I became a full-stack developer for the most prominent app in the company. In the end, I was focused on the back end exclusively, and I was in charge of the newly created microservice using Scala and Akka.
Senior Engineer
https://www.cityblock.com/As a senior developer at Cityblock, I drove team innovation, proposed architectural improvements, and mentored peers. I refined our Scrum process for better efficiency, optimized testing speed, and worked on new features and bug fixes. Collaborating with designers and leadership, I ensured design integration and played a pivotal role in achieving project milestones.
Certifications
DECEMBER 2018 - PRESENT
Hacking with WatchOS 4
Udemy
NOVEMBER 2018 - PRESENT
Complete Sketch 5 Course
Udemy
NOVEMBER 2018 - PRESENT
The Complete iOS Bootcamp Development
Udemy
JANUARY 2014 - PRESENT
Principles of Reactive Programming
Coursera
JUNE 2013 - PRESENT
Functional Programming Principles in Scala
Coursera
Skills
Libraries/APIs
REST APIs, jQuery, React, Node.js, Chart.js, Google Chart API, Akka Streams, Leaflet, Google Maps, RxJS, Babylon.js, Slick, spray
Tools
Git, ScalaTest, Karma, Webpack, YouTrack, Knex.js, Claude Code, Jira, IntelliJ IDEA, Xcode, Subversion (SVN), Figma, Sketch, Gulp, RabbitMQ, Bower, Canvas 2D, SBT, Kibana, Docker Swarm, Notion, React Apollo, Codex, Sentry, Terraform
Languages
CSS3, HTML5, JavaScript, TypeScript, Scala, SQL, CSS, Sass, Less, ECMAScript (ES6), Bash, C++, C, PHP, GraphQL, Python, Solidity, Swift
Frameworks
Angular, JSON Web Tokens (JWT), Bootstrap, Express.js, Ionic, Play 2, AngularJS, Protractor, Apache Camel, Ionic 4, Next.js, Ext JS, Play Framework, Akka, Kendo UI, Lift, Chrome, Flutter, React Native, Cypress
Platforms
Godot, Visual Studio Code (VS Code), OS X, Linux, NetBeans, MODx, Amazon Web Services (AWS), Arduino, Raspberry Pi, WatchOS, iOS, Docker, Meteor, DigitalOcean, Clerk
Storage
PostgreSQL, Elasticsearch, MySQL, MongoDB, Realm
Paradigms
Dependency Injection, Testing, Unit Testing, E2E Testing, Functional Programming, Scrum, Kanban, HL7 FHIR Standard, REST
Industry Expertise
Healthcare
Other
Web Development, AmCharts, Data Visualization, Maps, Chrome Extensions, Front-end, Algorithms, Akka Actors, Akka Persistence, Integration Testing, Safari Extensions, Architecture, Non-fungible Tokens (NFT), ClickUp, Silhouette, HAL, Akka HTTP, Regression Testing, Machine Learning, Coaching, Leadership, OOP Designs, HL7, WebSockets, Domain-driven Design (DDD), Crypto, Bcrypt, Apollo Server, Mentorship, Full-stack, Cursor AI, OpenRouter, Healthcare IT, Software Architecture
Collaboration That Works
How to Work with Toptal
Toptal matches you directly with global industry experts from our network in hours—not weeks or months.
1
Share your needs
Discuss your requirements and refine your scope in a call with a Toptal domain expert.
2
Choose your talent
Get a short list of expertly matched talent within 24 hours to review, interview, and choose from.
3
Start your risk-free talent trial
Work with your chosen talent on a trial basis for up to two weeks. Pay only if you decide to hire them.
Top talent is in high demand.
Start hiring