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Simon Flack
Verified Expert in Engineering
React Developer
Jelenia Góra, Poland
Toptal member since May 29, 2020
Bio
Simon is a senior full-stack product engineer with experience across web, mobile, and back-end systems. He thrives in founder-led and small-team environments where judgment, ownership, and pragmatic delivery matter. His recent work includes marketplace stabilization, KPI/finance analytics, healthcare mobile apps, native integrations, CI/CD, testing, and AI/LLM prototyping with React, Next.js, Node.js, Python/Django, PostgreSQL, Supabase, and Stripe.
Portfolio
CampGenie
TypeScript, React, Next.js, Supabase, PostgreSQL, Stripe, Playwright, PostHog...
Iconico Travel Inc
Next.js, React, JavaScript, Azure, Node.js, Docker, Claude, Claude Code...
Arena Labs, Inc
HTML, JavaScript, CSS, HTML5, React, Python, Docker Compose, Tailwind CSS...
Experience
- React - 11 years
- JavaScript - 10 years
- Node.js - 7 years
- TypeScript - 6 years
- Full-stack - 5 years
- Mobile App Development - 4 years
- Playwright - 4 years
- Capacitor - 3 years
Work Experience
Senior Product Engineer
2026 - 2026
CampGenie
- Stabilized critical family and camp flows across onboarding, search, booking, Stripe payments, RLS security, analytics, monitoring, and admin operations.
- Introduced Playwright E2E coverage, local Supabase workflows, RLS hardening, health checks, and analytics instrumentation to improve release confidence under a tight seasonal launch window.
- Built operational tooling for a non-technical founder, including admin list and detail pages, booking and transaction drilling, direct program editing, Slack ops alerts, and centralized transactional emails.
- Extended the product model to support direct and referral partnerships, custom-date mini camps, holiday-shortened schedules, per-session capacity enforcement, and richer family search across camp, program, and location data.
Technologies: TypeScript, React, Next.js, Supabase, PostgreSQL, Stripe, Playwright, PostHog, Email Automation, GitHub Actions, E2E Testing, Sentry, Stripe API, Claude Code, AI-generated Code, Code Review, Code Auditing, Auth, Model Context Protocol (MCP), Analytics, Dashboards, System Architecture, Workflow Automation, AI Tools, Data Modeling, Database Structure, Scalability, Scalable Architecture, CRM, Agentic Coding, Role-based Access Control (RBAC), Software Engineering, Technical Architecture, Multi-tenant Architecture, Server-side Rendering (SSR), Monorepos, NPM, OAuth 2, OAuth, Startups, Fintech, Agile Software Development, Leadership, Education Technology (Edtech), Front-end Architecture, Software Design, Web Applications
JavaScript Developer
2025 - 2026
Iconico Travel Inc
- Set up CI/CD and platform foundations for a Next.js/Azure system, then shipped operational fixes across booking creation, partner selection, email/send status, reminders, contact history, and price/cost workflows.
- Architected and shipped an end-to-end KPI analytics platform from scratch.
- Built declarative KPI definitions, auto-materialization, KPI admin tooling, chart/tooltip improvements, Excel exports, snapshot backfills, and data-refresh/reconciliation fixes.
- Optimized KPI snapshot generation with bulk-write batching and lightweight field-projection loaders, reducing database round-trips by up to 50x.
- Designed and built finance workflows for client payments, multi-currency support, commissions, invoice/proforma handling, supplier-payment tracking, and payment-time reconciliation.
Technologies: Next.js, React, JavaScript, Azure, Node.js, Docker, Claude, Claude Code, OpenCode, Sentry, AI-generated Code, Code Review, Code Auditing, Auth, Model Context Protocol (MCP), Analytics, Dashboards, System Architecture, Workflow Automation, AI Tools, Data Modeling, Database Structure, Scalability, Scalable Architecture, CRM, Agentic Coding, Role-based Access Control (RBAC), Software Engineering, Technical Architecture, Server-side Rendering (SSR), NPM, Fintech, Agile Software Development, Front-end Architecture, Software Design, Web Applications
Senior Engineer
2021 - 2024
Arena Labs, Inc
- Delivered a cross-platform healthcare app with Next.js and Capacitor, shipping to web, iOS, and Android stores, used daily by clinicians.
- Led front-end efforts while actively shaping Python/Django back-end logic and overall system architecture.
- Designed and implemented a cross-platform Bluetooth plugin (Swift and Java) for biometric data capture, featuring an async API, robust error handling, and queuing—replacing unstable contractor code and enabling reliable device integration.
- Mentored developers across the stack via code reviews, diagrams, and asynchronous videos. Boosted team velocity and raised the quality bar across PRs.
- Automated CI/CD using GitHub Actions for releases, service status alerts, and in-app OTA updates, cutting manual effort and enabling same-day hotfixes.
- Introduced internal in-app testing tools (log viewer, data reset, data inspectors, and state machine debugger) to accelerate QA and reduce support load.
- Integrated Sentry Replay, Capgo, and analytics platforms to improve UX feedback, debug issues faster, and deliver OTA updates without app store delays.
- Built reusable libraries and front-end patterns to reduce duplication and technical debt, enabling faster iteration while maintaining consistency.
- Collaborated closely with design, product, and QA to scope features, validate delivery feasibility, and adjust priorities when needed.
- Prototyped OpenAI GPT integration with a coaching platform to provide personalized coaching feedback to clinicians.
Technologies: HTML, JavaScript, CSS, HTML5, React, Python, Docker Compose, Tailwind CSS, Ionic, Technical Leadership, PostgreSQL, API Integration, Email Automation, Chatbots, Mobile Development, Front-end, User Experience (UX), User Interface (UI), Full-stack Development, Back-end, Infrastructure as Code (IaC), Scraping, Supabase, Webhooks, REST APIs, Databases, Deployment, OpenAI API, Puppeteer, React Testing Library, Vite, Web Content Accessibility Guidelines (WCAG), Data Visualization, Capacitor, Bluetooth, XState, Playwright, CI/CD Pipelines, OpenAI, Large Language Models (LLMs), React Query, Messaging, Mobile App Development, Encryption, Single-page Applications (SPAs), Solution Architecture, DevOps, Twilio, Front-end Development, NestJS, Minimum Viable Product (MVP), Back-end Development, Data Transformation, Postman, RESTFul APIs, Swagger, Middleware, Customer Relationship Management (CRM), SaaS, Development, ECMAScript (ES6), JSON, Responsive Layout, Figma, Animation, GSAP, Lottie, GreenSock Animation Platform (GSAP), SVG, Artificial Intelligence (AI), Sentry, AI-generated Code, Code Review, Auth, Cloudflare, Analytics, Dashboards, Metabase, System Architecture, Workflow Automation, Data Modeling, Generative Artificial Intelligence (GenAI), Scalability, Scalable Architecture, CRM, Agentic Coding, Software Engineering, Technical Architecture, Multi-tenant Architecture, Server-side Rendering (SSR), NPM, OAuth 2, OAuth, Startups, Healthcare, Agile Software Development, Front-end Architecture, Software Design, Web Applications
Senior Front-end Developer
2015 - 2021
OnTrack Retail
- Built and maintained a high-traffic retail ticketing platform used by five UK rail operators, handling £500,000+ in daily transactions.
- Replaced a slow 3rd-party timetable API with a real-time internal service using MongoDB and Node.js, reducing response time from 10 seconds to sub-second.
- Developed and shipped multiple government-compliant delay compensation portals across Southeastern and GTR under tight deadlines.
- Led a major Webpack system rewrite. Cut CI build times in half, improved developer velocity, and modernized the toolchain.
- Extracted reusable functionality from the monolith to enable faster product spin-up for new rail clients - paving the way for modular growth.
- Created a self-service ticket refund system, improving customer autonomy and reducing support overhead.
- Mentored junior developers remotely - shared domain knowledge, introduced better testing practices, and improved PR quality.
Technologies: Sass, Less, Git, Bash, MongoDB, Jest, Docker, XState, Redux, Webpack, TypeScript, AngularJS, React, Node.js, Test-driven Development (TDD), API Design, JavaScript, Express.js, Redis, Regular Expressions, Webpack 4, Gulp, NGINX, Visual Studio Code (VS Code), Chrome, HTML, SQL, CSS, Debugging, MySQLdb, Handlebars, Accessibility, HTML5, Architecture, Web Development, REST, Google Analytics, Integration, Braintree SDK, PayPal, Web Scraping, Next.js, UI Testing, Algorithms, Full-stack, MySQL, Webpack 3, SSH, Webpack 2, APIs, Capacitor, Mobile Apps, Docker Compose, Amazon Web Services (AWS), API Integration, Azure, Front-end, User Experience (UX), User Interface (UI), Full-stack Development, Back-end, Infrastructure as Code (IaC), Data Scraping, Scraping, Google Tag Manager, REST APIs, Databases, Puppeteer, Material UI, React Testing Library, Vite, Web Content Accessibility Guidelines (WCAG), Single-page Applications (SPAs), Solution Architecture, DevOps, Front-end Development, General Data Protection Regulation (GDPR), NestJS, Minimum Viable Product (MVP), Back-end Development, Headless CMS, Data Transformation, Postman, RESTFul APIs, Swagger, Middleware, Customer Relationship Management (CRM), SaaS, eCommerce, Development, ECMAScript (ES6), JSON, Responsive Layout, Figma, SVG, PayPal API, Sentry, Code Review, Code Auditing, Auth, Dashboards, System Architecture, Workflow Automation, Data Modeling, Database Structure, Scalability, Scalable Architecture, CRM, Software Engineering, Technical Architecture, Server-side Rendering (SSR), NPM, OAuth 2, Microservices, Fintech, Agile Software Development, Front-end Architecture, Software Design, Web Applications
Senior Front-end Developer
2014 - 2015
IG
- Developed an interactive UI for investors to filter thousands of markets using fundamentals and custom criteria optimized for desktop and tablet.
- Refactored server-rendered pages to share front-end templates across views, improving maintainability and reducing duplication.
- Conducted screening and in-person technical interviews for junior and senior developer roles, helping grow and shape the team.
- Initiated weekly tech meetups to foster cross-team learning, improve collaboration, and share engineering best practices.
- Collaborated with iOS and Android teams to debug and fix issues in embedded WebViews, improving cross-platform reliability.
- Built dynamic SVG-based data visualizations using Raphaël, enhancing clarity and usability for high-volume financial data tools.
Technologies: Subversion (SVN), Raphaël, Handlebars, jQuery, JavaScript, Regular Expressions, Gulp, Chrome, HTML, Node.js, CSS, Debugging, Bash, HTML5, Web Development, Integration, Algorithms, Grunt, SSH, Capacitor, Mobile Apps, Content Management Systems (CMS), Cloud Firestore, Data Visualization, D3.js, Front-end Development, NestJS, Data Transformation, Postman, RESTFul APIs, Middleware, Development, JSON, Responsive Layout, Figma, Code Review, NPM, Fintech, Agile Software Development
Front-end Developer
2010 - 2014
BBC
- Developed the front end for custom CMS document repository and filtering.
- Implemented instant and scheduled publishing interfaces according to design.
- Translated business requirements into BDD tests and built monitoring tools to highlight failing tests.
- Supported legacy CMS platform and created migration tools for the new platform.
- Created a visual "segments editor" for media publishers to generate thumbnails and short clips as part of a media publishing pipeline.
- Built a schedule builder to create timetables and assign program metadata to multiple live streams as part of the London 2012 Olympics programming.
- Created and maintained plugins for a custom publishing workflow.
- Built a multi-file upload component capable of handling very large media files, supporting resumes, and reporting back transcode status.
Technologies: Sass, Less, Behat, PHPUnit, Jasmine, Zend Framework, PHP, jQuery, Backbone.js, JavaScript, Perl, Regular Expressions, Gulp, Chrome, HTML, CSS, Debugging, Bash, HTML5, Web Development, REST, Integration, UI Testing, Full-stack, Apache2, Behavior-driven Development (BDD), Grunt, SSH, RSync, APIs, Docker Compose, Content Management Systems (CMS), REST APIs, Databases, Deployment, Solution Architecture, Front-end Development, Minimum Viable Product (MVP), Headless CMS, Data Transformation, Postman, RESTFul APIs, Middleware, Development, JSON, Figma, Code Review, Code Auditing, Agile Software Development
Software Engineer
2002 - 2010
BBC
- Worked with production teams to develop interactive websites using custom tools.
- Designed and developed a desktop CMS application with an Oracle-backed SOAP server for managing, transforming, and publishing vast repositories of XML content.
- Developed an FTP interface to simplify bulk import and export of content.
- Published the organization's first open-source contributions and ported the Document and View framework in wxWidgets to wxPerl.
- Embedded Internet Explorer for in-editor CSS-based preview and full XSLT preview of the content.
- Implemented common editor functions such as in-editor find and replace, cross-repository find and replace, and spell-check.
- Demoed new features on a monthly basis and incorporated user feedback into future releases.
- Completed a Bayesian analysis of 45,000 articles for categorization in the People's War Archive.
- Recovered data from Laserdisc in legacy format for the BBC Domesday Reloaded project.
Technologies: OLE, ODBC, Oracle, XSLT, XML, wxWidgets, CSS, HTML, Perl, Regular Expressions, Debugging, MySQLdb, HTML5, Architecture, Web Development, Web Scraping, UI Testing, Full-stack, Apache2, SSH, RSync, APIs, Content Management Systems (CMS), Email Parsing, Email Automation, Full-stack Development, Back-end, Databases, Solution Architecture, Minimum Viable Product (MVP), Back-end Development, Headless CMS, Data Transformation, RESTFul APIs, Development, Code Review, Code Auditing, Auth, Database Structure, Scalable Architecture, Software Engineering
Assistant CTO
2000 - 2002
Shared Knowledge Systems
- Developed a Perl-based support system for tracking and responding to customer support emails.
- Built a customer control panel in Perl and VBScript so hosting customers could manage their accounts.
- Automated domain registration and service provisioning for customers using VBScript tooling.
Technologies: CTO, Leadership, Perl, Web Development, Workflow Automation
Founder, CEO, Developer
1997 - 2002
The Spider's Web
- Founded and led a software development and web hosting company while at university, later selling it to Shared Knowledge Systems.
- Automated payment processing and web-hosting account provisioning for customers and resellers.
- Developed a WAP service in 1999 for WHOIS lookups and managed dedicated Linux and Windows hosting servers in Docklands.
Technologies: CTO, Leadership, Startups, Web Development, Linux, Workflow Automation & System Integration
Project History
Booking Engine
https://ticket.greatnorthernrail.comA React-based train ticket eCommerce app for planning journeys, purchasing tickets, and processing refunds. The app is used by five train operators, supports tens of thousands of visitors, and has several hundred thousand GBP in transactions on a daily basis.
Delay Repay Compensation
https://delayrepay.southeasternrailway.co.uk/A statically rendered React app for customers to apply for delay repay compensation after experiencing delays to their journey of over 15 minutes. The application is streamlined for returning customers and guides users through the required information. The application logic is modeled with statecharts using XState for improved flexibility and maintainability.
Darwin
A MongoDB-backed API for querying train timetables to find details of trains users traveled on as part of their delay repay claim application. A scheduler pulls the daily timetable XML from Amazon S3 every morning and ingests the data after processing it so it can be queried quickly and efficiently.
Market Screener
https://www.ig.com/uk/markets-stock-screener?country=GB,USA tool for investors to create, save, and edit custom filters for investment opportunities. Written with Vanilla JS, SVG, and a custom in-house framework. It was designed to work well on desktops and tablet devices.
FLiP - Factual and Learning Interactive Publisher
A desktop and server application for editing and publishing large repositories of XML. Written in wxPerl, using wxWidgets for the UI, and interfacing with LDAP for authentication and OLE for embedding Internet Explorer. SciTE. Used SOAP and FTP as primary protocols, and supported multiple repository back ends including filesystem and Oracle.
Arena Strive – Mobile Health App
https://arenalabs.coI led the development of Arena Strive, a cross-platform React and Capacitor app that supports clinicians' stress and recovery. The product guided clinicians through a six-week curriculum of audio, video, and exercises; completing a lesson unlocked the next day's content, while progress tracking, reinforcement, rewards, and gamification supported gradual skill-building and completion. I also designed a Swift and Java Bluetooth plugin for biometric sync with async APIs, queueing, and error handling, and built developer and debug workflows for testing and QA.
Web Portal for Coach-guided Recovery (Arena Strive)
https://arenalabs.coBuilt a companion web dashboard for coaches to manage user journeys and review biometric insights. Integrated with internal APIs and analytics tools. Developed automated test flows using Playwright and deployed to Vercel.
SubScout – AI-powered Subscription Tracker (Prototype)
https://subscout.appDesigned and built an MVP for a privacy-first subscription tracker. The React Native app used Expo with Supabase, PostgreSQL, and PowerSync for local-first, offline-friendly, real-time synchronization. It extracts and classifies subscription data from user-forwarded emails using LLM prompts and regex pipelines, with an emphasis on user value rather than raw usage metrics.
Wellbeing Tracker & Support Bot for Grief Community
Built a custom Discord bot to help community members share how they’re doing and track their well-being over time. Designed with empathy and minimal friction to support emotional expression and solidarity. Also contributed to a related creative art project focused on remembrance and healing.
Access Italy Finance & Operations Workflows
http://accessitaly.comI worked on finance and operations workflows inside Access Italy’s bespoke luxury travel CRM and itinerary platform. The work included multi-currency client payments, commissions, invoice and pro forma handling, supplier payment tracking, payment-time reconciliation, booking creation fixes, partner selection improvements, reminder flows, contact history, and price/cost workflow corrections. I also improved reliability in a brittle inherited system through CI/CD setup, Next.js/Docker runtime fixes, targeted MongoDB indexes, cron safety, data-refresh fixes, and snapshot backfills. The goal was to make complex travel operations and finance data more usable, auditable, and resilient.
Access Italy KPI Analytics Platform
http://accessitaly.comI built an end-to-end KPI analytics platform for Access Italy, a luxury travel business with a bespoke CRM and itinerary/operations system. I designed the KPI architecture, shipped declarative metric definitions, snapshot generation, admin tooling, charts, tooltips, Excel exports, backfills, and reconciliation fixes. The platform grew across 25+ releases and 40+ revenue, P&L, lead lifecycle, hotel production, product, and partner metrics used by sales, operations, product, and finance stakeholders. I also optimized snapshot generation with bulk-write batching and field-projection loaders, reducing database round-trips by up to 50x.
CampGenie Admin & Operations Platform
http://thisiscampgenie.comFor CampGenie, I built and extended internal operations tooling for a non-technical founder managing camps, families, bookings, payments, and marketplace data. The work included admin list and detail pages, booking and transaction drilling, direct program editing, centralized transactional email foundations, Slack operational alerts, notification retry flows, and safer admin access to catalog and payment data. I also extended the marketplace model to support direct and referral partnerships, custom-date mini camps, holiday-shortened schedules, per-session capacity enforcement, and richer family search across camp, program, and location data.
CampGenie Marketplace Stabilization & Launch Readiness
http://thisiscampgenie.comI took over technical ownership of CampGenie, a founder-led marketplace for summer camp discovery and booking, after an agency-built product was too unstable for confident rollout. I assessed the inherited codebase, clarified launch risks, and delivered fixed-scope stabilization work across family search, camp onboarding, booking, Stripe checkout, Supabase/Postgres security, analytics, monitoring, and release workflows. I introduced end-to-end Playwright coverage, local Supabase workflows, RLS hardening, health checks, PostHog/Google Analytics instrumentation, and safer payment/session handling, enabling the founder to move toward launch with much better visibility and confidence.
Skills
Libraries/APIs
Node.js, React, jQuery, REST APIs, OpenAI API, Puppeteer, React Testing Library, Playwright, React Query, Stripe, Auth, Backbone.js, D3.js, GSAP, PayPal API, Stripe API, Handlebars, Raphaël, wxWidgets, ODBC, Slack API
Tools
Git, Webpack 4, Webpack 3, Capacitor, Claude, Claude Code, NPM, Webpack, Gulp, Grunt, Webpack 2, Braintree SDK, Postman, Sentry, NGINX, Subversion (SVN), Behat, Google Analytics, Docker Compose, GitLab CI/CD, Figma, OpenCode, TanStack Router, Expo
Languages
JavaScript, Perl, TypeScript, CSS, HTML, HTML5, ECMAScript (ES6), Less, Sass, XML, Python, Swift, PHP, SQL, Bash, XSLT, Objective-C, Java
Frameworks
Jest, Express.js, Next.js, Tailwind CSS, NestJS, Swagger, AngularJS, React Native, Ionic, Cypress, Material UI, OAuth 2, Chrome, Redux, Zend Framework, Jasmine, PHPUnit, tRPC
Paradigms
Test-driven Development (TDD), REST, Mobile Development, Responsive Layout, Agile Software Development, Behavior-driven Development (BDD), DevOps, Model Context Protocol (MCP), Role-based Access Control (RBAC), Microservices, Offline-first Development, UX Design, E2E Testing, Automated Testing
Platforms
Amazon Web Services (AWS), Vercel, Linux, Docker, Apache2, Android, iOS, Twilio, GreenSock Animation Platform (GSAP), Firefox, Oracle, Visual Studio Code (VS Code), Azure, PostHog
Storage
PostgreSQL, Amazon S3 (AWS S3), Databases, JSON, MySQL, MySQLdb, MongoDB, Cloud Firestore, Database Structure, Redis
Industry Expertise
Healthcare
Other
Regular Expressions, API Design, Debugging, Full-stack, Web Development, Architecture, Web Scraping, APIs, Content Management Systems (CMS), Technical Leadership, Software Architecture, API Integration, Email Automation, Front-end, User Interface (UI), Full-stack Development, Back-end, Data Scraping, Scraping, Data Enrichment, Webhooks, Deployment, Vite, Web Content Accessibility Guidelines (WCAG), Cursor AI, Messaging, Mobile App Development, Single-page Applications (SPAs), Solution Architecture, Front-end Development, Minimum Viable Product (MVP), Back-end Development, Headless CMS, Data Transformation, RESTFul APIs, Middleware, Customer Relationship Management (CRM), SaaS, eCommerce, Development, AI-generated Code, Code Review, Code Auditing, Analytics, Dashboards, System Architecture, Workflow Automation, AI Tools, CRM, Agentic Coding, Software Engineering, Technical Architecture, Multi-tenant Architecture, Server-side Rendering (SSR), Monorepos, Leadership, Authentication, Front-end Architecture, Software Design, Web Applications, UI Testing, Algorithms, RSync, SSH, XState, Accessibility, Integration, PayPal, Large Language Models (LLMs), iPad, ChatGPT API, OpenAI, Bluetooth, Email Parsing, Chatbots, User Experience (UX), Infrastructure as Code (IaC), Supabase, Google Tag Manager, Data Visualization, CI/CD Pipelines, Subscriptions, Encryption, AI Integration, General Data Protection Regulation (GDPR), Lottie, Supabase Auth, SVG, Artificial Intelligence (AI), Cloudflare, Data Modeling, Generative Artificial Intelligence (GenAI), Scalability, Scalable Architecture, Turborepo, OAuth, Startups, Fintech, CTO, Education Technology (Edtech), OLE, Mobile Apps, Videos, Zustand, GitHub Actions, Prototyping, MVP Design, Discord, Discord Bots, Community, Creative Tech, Animation, Metabase, Transactional Emails, Monitoring, Performance, System Improvements, Workflow Automation & System Integration
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