Denys Halkin

Senior Front-end Engineer | React, Angular, React Native, Expo, Capacitor | Architecture, delivery, CI/CD

LinkedIn: linkedin.com/in/denis-galkin-b3a234b6

Location: Poland | Open to remote B2B opportunities worldwide and remote employment (UoP) in Poland

Professional Summary

Senior Front-end Engineer with 10+ years of experience building production web and cross-platform mobile applications. Deep frontend experience across React and Angular, from AngularJS 1.5 through Angular 15, complemented by strong mobile delivery with React Native, Expo/EAS, Capacitor, and Apache Cordova. Focused on frontend architecture, cross-platform feature ownership, API contracts, release engineering, observability, and long-term support of products used by large audiences. Experienced in using AI-assisted development workflows to improve planning, implementation, debugging, and delivery quality.

Career Highlights

Delivered products serving 15M+ monthly active users, 40,000+ B2B clients, and mobile apps with more than 1M installs.
Owned complex features from early design drafts and system design through API contracts, implementation, rollout planning, release automation, monitoring, and production support.
Designed cross-platform web/mobile solutions including React Native WebView bridges, phased feature-flag rollouts, API migrations, compliance flows, and reusable upload/preview systems.
Built and evolved production Angular platforms from AngularJS 1.5 through Angular 15, including Nx monorepos, booking widgets, shared libraries, and hybrid mobile applications.
Combined hands-on engineering, architecture planning, mentoring, AI-assisted delivery workflows, and cross-team coordination across product and consulting environments.

Skills

Frontend: React, AngularJS 1.5, Angular 2-15, Next.js, Vue.js, TypeScript, JavaScript, HTML, CSS, Sass, Stylus, MUI, Storybook
Mobile: React Native, Expo/EAS, Capacitor, Apache Cordova, Ionic, PWA, WebView bridges, deep links, push notifications
Architecture & Delivery: system design, feature planning, API contracts, API migrations, release management, mentoring, cross-team coordination, CI/CD ownership
Integrations: REST APIs, WebSocket, SSE, Firebase Functions, feature flags, mapping APIs, localization, attribution and advertising SDKs, financial and identity-service integrations
State, Forms & Quality: React Query, Redux, RxJS, NgRx, Zod, ESLint, Jest, Jasmine, Puppeteer, Sentry, WCAG
Release, Tooling & AI: GitHub Actions, GitLab CI, fastlane, GitHub Environments, conventional commits, semantic versioning, Claude, Codex, local LLM experiments, Git, Node.js, Docker, NGINX, webpack, Gulp, Jenkins, Xcode

Work Experience

Selected part-time and consulting roles are marked where applicable.

10.2025 - Present: Algoteryx - Senior Front-end & Mobile Developer

Owned delivery across React web, React Native/Expo mobile, partner-facing workflows, and back-office systems, taking features from early product and design drafts to task breakdown, API contracts, implementation, rollout, and production support.
Designed and shipped a cross-platform registration funnel using an embedded WebView bridge between React Native and the web application, including postMessage protocol design, deep-link and universal-link callbacks, auth-token handoff, and platform-specific permission workarounds.
Planned and coordinated phased, risk-tagged feature-flag rollouts across web and mobile, combining WorkOS-managed flags with local feature toggles for controlled internal testing and safer production releases.
Built reusable React and React Native upload and preview systems with Zod validation, FormData handling, replace/remove/retry flows, camera capture, two-sided document validation, and previews for images, videos, text files, and PDF documents.
Migrated legacy API clients and endpoints on web and mobile to a new unified API layer, refactored shared request handling, and introduced Sentry-based API error monitoring through an interceptor.
Extended mobile observability with Sentry ANR and memory diagnostics, production sourcemap handling, and noise filtering for cleaner error tracking.
Implemented Firebase Functions proxying and server-side Meta Conversions API event logging to improve advertising event quality and attribution reliability.
Delivered finance and compliance-heavy product flows including invoice management, payout-type changes with OTP verification, regional payout rules, Polish bank-account validation, GDPR consent review, and Cloudflare Turnstile bot protection.
Designed and implemented release engineering automation for a React/TypeScript application: conventional commits, semantic versioning, changelog generation, tag-based staging builds, manual production deploys by release tag, GitHub Releases, merge queue protection, and CI validation for commit and PR governance.
Mentored frontend teammates, introduced best practices, improved code quality standards, and helped stabilize delivery as the frontend team structure changed.
Experimented with local LLMs and AI-agent workflows, documenting project-specific gotchas in agent-facing engineering guides to make AI-assisted development more reliable.
Core stack: React, React Native, Expo/EAS, TypeScript, MUI, Zod, React Query, GitHub Actions, Firebase Functions, Sentry, WorkOS, Cloudflare Turnstile.

05.2022 - 10.2025: KOBOGO.NG (Part-time) - Senior Front-end Developer

Led the development and successful launch of a mobile application built with React + Capacitor, reaching 130k active users and over 1M installs.
Took end-to-end ownership of the project across React, Capacitor, and Next.js applications: new features, legacy support, refactoring, release monitoring, and production maintenance.
Automated the deployment process through GitHub Actions CI/CD and fastlane, reducing release time and improving delivery stability.
Replaced the legacy React Native application by rewriting the product on Capacitor, enabling faster delivery of new features.
Implemented mobile push notifications with Firebase Cloud Messaging (FCM), supporting communication with more than 100k users.
Developed custom Capacitor plugins using vendor-specific SDKs.
Configured attribution and advertising SDK integrations, including Kochava, Adjust, AppsFlyer, and others.
Fully migrated the project from TSLint to ESLint with flat config.
Developed and maintained a live-update workflow with GitHub Actions, including real-time rollouts, rollback support, and Sentry-based monitoring.
Collaborated on integrations with financial APIs such as Mono and SmileID to support verification and loan-processing flows.
Core stack: React, Capacitor, Next.js, GitHub Actions CI/CD.

05.2022 - 10.2025: Altegio (Part-time) - Front-end Consultant

Joined after YCLIENTS in a part-time consulting role to evolve the front-end platform for international clients while working in parallel with KOBOGO.NG.
Localized booking widgets and web interfaces, adapting copy, date and number formatting, multi-currency price presentation, and timezone handling for each target market.
Adjusted map, address, and geolocation features to support Apple Maps, Google Maps, and other region-specific providers together with geo-permission flows.
Improved mapping cost efficiency through migrations to Apple Maps and OSM Leaflet, and explored self-hosted map tile deployment.
Created and adapted customer-facing email templates and communication flows, including SparkPost-based template work for international rollouts.
Partnered with product, legal, and support teams to tailor features for regional regulations, payment integrations, and customer expectations across multiple countries.
Maintained and evolved Angular applications in an Nx monorepo, including shared frontend solutions used across web and mobile surfaces for international rollouts.
Core stack: Angular, Capacitor, Nx monorepo, proprietary UI Kit.

01.2017 - 04.2022: YCLIENTS LLC - Front-end Developer

Developed and maintained key features for over 40,000 B2B clients and 15,000,000 MAU, including brands like Chop-Chop (android), GC Barbershop (ios, android), Lumberjack (ios, android), Kleopátra Szépségszalonok (ios, android) and many others.
Served as a key contributor with deep product knowledge, mentoring new developers and helping establish best practices across the team.
Led the hybrid mobile applications direction: automated app publishing, configured fastlane, worked with Apple and Google ecosystems, and wrote internal documentation.
Delivered the internal builder used to create or update about 1,000 iOS and Android applications.
Mentored junior and middle developers, improving code quality and team efficiency.
Developed and supported an Angular online booking widget for multiple platforms, including this live booking example. Introduced new features, wrote tests, coordinated REST API data, and grew with the platform from AngularJS 1.5 through Angular 15.
Set up GitLab CI and fastlane pipelines for automated mobile app deployment across testing and production stages.
Developed custom Apache Cordova and Capacitor plugins using vendor-specific SDKs.
Built and maintained internal npm packages, shared libraries, UI libraries, and the API layer, using Storybook to publish reusable components.
Delivered multiple push-notification features for iOS and Android, including a notification center, silent notifications, emergency flows, and APNs-to-Firebase migration.
Improved product quality and platform coverage through localization, timezone-sensitive features, WCAG-aware color handling, and Sentry release visibility.
Ensured GDPR compliance across product areas while supporting ongoing platform evolution experiments such as NativeScript and React Native.
Core stack: AngularJS/Angular (1.5 → 15), Apache Cordova/Capacitor, GitLab CI/CD, Storybook, custom npm libraries/UI kits.

09.2016 - 01.2017: Freelance - Manager/Programmer

Created and managed a small team for website development and support, handled project acquisition, and contributed hands-on implementation for clients from platforms like FL.ru and Upwork.

08.2015 - 09.2016: MY-apps Company - Technical Support

Supported clients via email and Skype, created and configured iOS apps, prepared bug reports, and collaborated closely with the development team.

05.2014 - 08.2015: Independent Academic Project Work

Delivered programming and mathematics projects independently, strengthening problem-solving, technical writing, and practical implementation skills at the beginning of my career.

Education

Kharkiv National University of Radio-electronics

Specialization: Information Security Technologies

Period: 2014 - 2018

Languages

English: B1+ (actively improving toward B2)

Ukrainian: Native

Russian: Native

Interests

Technology, automation, practical AI adoption in engineering workflows, finance, travel, cars