Aleksander Froncz, Developer in Warsaw, Poland
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Aleksander Froncz

Software Developer

Warsaw, Poland

Toptal member since July 27, 2016

Expertise
APIFront EndCSSJavaScriptReact.jsTypeScriptReduxGraphQLReact NativeHTML5Tailwind CSSNext.js
Bio

Aleksander is a highly efficient, experienced, and reliable professional with a diverse skill set in web and mobile development. Over the past decade, he has collaborated on numerous projects with a wide range of clients. Aleksander has a wealth of experience joining mature teams working on applications already in production and building applications from scratch.

Portfolio

2Signals
React, TypeScript, Node.js, HTML5 Web Workers, AWS IoT, MongoDB, Supabase...
Klarna
TypeScript, Node.js, AWS IoT, Amazon DynamoDB, Terraform, Twilio...
SIA (LLC) PATA (Registration No. 40003448619)
React, TypeScript, Mobile Development, Blockchain, Monaco, AI Tools, Claude...

Experience

  • JavaScript - 11 years
  • React - 8 years
  • TypeScript - 6 years
  • GraphQL - 4 years
  • Node.js - 4 years
  • React Native - 3 years
  • Express.js - 3 years
  • ReasonReact - 1 year

Preferred Environment

Git, Visual Studio Code (VS Code), MacOS

The most amazing...

...thing I've developed from the ground up is a huge shopping/review web portal with custom integrations for payments, advanced newsletters, and bookkeeping.

Work Experience

Senior Full-stack Developer and Technical Founder

2023 - PRESENT
2Signals
  • Built a comprehensive algorithmic trading platform from the ground up with 150+ technical indicators.
  • Developed a sophisticated visual strategy builder with multi-timeframe analysis and chart visualization.
  • Implemented Web Workers for parallel processing of complex trading calculations in real time.
  • Architected a React and TypeScript front end with advanced state management and dynamic position sizing.
  • Built an advanced portfolio management system with correlation analysis tools for traders.
  • Designed and developed an integrated educational system with interactive courses and hands-on lessons.
  • Created synchronized chart visualization tools supporting multiple timeframes and asset comparisons.
Technologies: React, TypeScript, Node.js, HTML5 Web Workers, AWS IoT, MongoDB, Supabase, Supabase Auth, Shadcn, AI Tools, Claude, Cursor AI, REST APIs, Amazon Web Services (AWS), CI/CD Pipelines, Claude Code

Senior Full-stack Developer

2025 - 2026
Klarna
  • Built a production-ready phone validation microservice with VOIP detection, blacklist checking, and geofencing using Twilio API.
  • Integrated Signicat for BankID authentication covering Finland and Nordic markets via REST API.
  • Configured AWS infrastructure with DynamoDB, Terraform, and CloudWatch for scalable service deployment.
  • Implemented circuit breaker patterns to improve service resilience across distributed microservices.
  • Contributed to a large-scale TypeScript migration with comprehensive testing frameworks.
  • Delivered back-end authentication services operating at scale across multiple Nordic markets.
Technologies: TypeScript, Node.js, AWS IoT, Amazon DynamoDB, Terraform, Twilio, Amazon CloudWatch, AI Tools, Claude, Cursor AI, REST APIs, Amazon Web Services (AWS), CI/CD Pipelines, Claude Code

React and TypeScript Senior Front-end Developer

2025 - 2025
SIA (LLC) PATA (Registration No. 40003448619)
  • Developed React front-end components for an AI code editor platform integrating Monaco Editor with Sandpack.
  • Built a user interface for AI planning and tool call functionality, enabling users to interact with AI-generated development plans.
  • Implemented front-end integration with AI API to display plans and execute tool calls within the code editor environment.
  • Created responsive TypeScript-based components for the code editor interface supporting AI-enhanced development workflows.
  • Developed user experience for AI-powered code editing, including interface elements for plan visualization and tool call execution.
Technologies: React, TypeScript, Mobile Development, Blockchain, Monaco, AI Tools, Claude, Cursor AI, REST APIs, CI/CD Pipelines

React Native/React Developer

2021 - 2023
Vantage Pass LLC
  • Created an application facilitating connections between models and venues, featuring distinct user interfaces tailored for each. The displayed interface depended on the login credentials used.
  • Managed the deployment to both the App Store and Google Play, resolving and discussing issues raised by the review teams on both platforms.
  • Implemented credits and subscription purchase flows for models and venues, utilizing in-app purchases (IAPs) seamlessly integrated on both platforms. Moreover, added penalty payments via Stripe.
  • Engineered core flows for model-side reservation, check-in, and requirement confirmations. On the venue side, implemented a streamlined requirement review process. Incorporated notifications with deep linking for an enhanced user experience.
  • Developed processes for uploading additional content and purchasing uploaded materials.
  • Incorporated polished animations and transitions to elevate user experience.
  • Implemented an admin panel for data management, approval/review workflows, user activity monitoring, and modification of user information, encompassing status, credits, and subscriptions.
  • Implemented Mailchimp campaigns, bringing the provided design to life.
Technologies: React, TypeScript, AWS Amplify, React Native, Tailwind CSS, AWS AppSync, Bootstrap, IAP SDK, Mixpanel, Analytics, App UX, App UI, Mailchimp, PSD to Mailchimp, HTML, CSS, GraphQL, GitHub, Firefox, Google Chrome, iOS, Android, Startups, Early-stage Startups, Mobile Development, User Interface (UI), Stripe, Mobile, Agile Deployment, Payment Processing, Next.js, Mobile App Development, Mobile Apps, AI Tools, Cursor AI, REST APIs, CI/CD Pipelines

Senior React Developer

2020 - 2021
Dexter (via Toptal)
  • Developed the front end for a decentralized exchange app for Tezos.
  • Restructured the front-end code to mirror the app structure.
  • Implemented given requirements within expected timeframes.
  • Simplified the front-end code of an app by adding a central context with some Bootstrap logic.
  • Helped to create a seamless user experience and solve several UX problems.
  • Took care of Reason ML code quality and front-end good practices.
  • Increased test code coverage by adding unit and snapshot tests to the code.
Technologies: Blockchain, Jest, JavaScript, CSS, React, ReasonReact, Reason ML, Cryptocurrency Exchanges, Decentralized Applications, Front-end, Front-end Development, React Router, Axios, Apollo, APIs, Third-party APIs, GitHub, Firefox, Google Chrome, Startups, Early-stage Startups, User Interface (UI), Chart.js, REST APIs

Senior Front-end Developer

2019 - 2020
TravelWorld (via 360codelab)
  • Used ReasonML and ReScript with the best modern coding patterns.
  • Fixed the project structure and completely refactored the existing code.
  • Implemented nice helpers for a better development experience and reusable components used across the portal.
  • Integrated the front end with GraphQL endpoints for data queries and mutations.
  • Incorporated pixel-perfect responsive pages with an emphasis on great UX.
  • Collaborated with a team of 12 people in an agile methodology.
Technologies: Reason ML, Webpack, HTML5, Agile Software Development, JSX, CSS3, JavaScript, CSS, Material UI, Redux, Express.js, GraphQL, ReasonReact, React, Front-end, Front-end Development, React Router, Axios, Apollo, APIs, Third-party APIs, GitHub, Firefox, Google Chrome, User Interface (UI), Agile Deployment, REST APIs

Lead Full-stack Developer

2019 - 2020
Cultcha
  • Worked with TypeScript, React, React Hooks, React Query, Express, Node.js, and MongoDB.
  • Designed a complex file system of layouts/views for flexible UI creation.
  • Determined requirements and created an implementation plan.
  • Implemented a multilevel permission system as well as nice UX features, such as skeletons.
  • Designed the MongoDB models and REST API structure.
  • Worked in an Agile environment with sprints and routines.
Technologies: TypeScript, Webpack, RESTful Web Services, HTML5, GraphQL, JSX, React Redux, CSS3, JavaScript, CSS, Passport.js, Node.js, Mongoose, Express.js, Recompose, Redux-Saga, Redux, React, WebSockets, Front-end, Front-end Development, React Router, Axios, Apollo, APIs, Third-party APIs, GitHub, Firefox, Google Chrome, API Development, Back-end, Startups, Early-stage Startups, User Interface (UI), Full-stack, Full-stack Development, Back-end Development, Hosting

Lead JavaScript Developer

2019 - 2019
YND
  • Implemented a fully functional e-wallet portal with payment processing capabilities and an intuitive user interface.
  • Managed a monorepo architecture spanning four applications, ensuring consistent code sharing and streamlined deployments.
  • Wrote and maintained unit and integration tests for Redux Sagas, ensuring reliable async data flow across the application.
  • Implemented component-level styling using Styled Components and CSS-in-JS patterns for maintainable, scoped visual design.
  • Collaborated in an Agile development environment with regular sprints, code reviews, and iterative feature delivery cycles.
Technologies: RESTful Web Services, HTML5, Jira, Agile Software Development, JSX, React Redux, CSS3, JavaScript, CSS, Reselect, Recompose, Styled-components, Redux-Saga, Redux, React, Front-end, Front-end Development, React Router, Axios, Apollo, APIs, Third-party APIs, GitHub, Firefox, Google Chrome, User Interface (UI), Stripe, Agile Deployment, Payment Processing

Full-stack JavaScript Developer

2018 - 2018
DPOrganizer
  • Worked in a Scrum environment in coordination with international teams.
  • Developed additional features for a product on the market.
  • Engineered a library of reusable React components, improving development speed and consistency across the application.
  • Utilized Docker for containerized development environments, ensuring consistent builds and smooth deployment pipelines.
  • Gained hands-on experience implementing Flux architecture patterns for predictable, unidirectional application state management.
  • Conducted regular code reviews for team members, providing constructive feedback to maintain code quality and best practices.
Technologies: RESTful Web Services, HTML5, Jira, Agile Software Development, Express.js, JSX, CSS3, CSS, Docker, Webpack, Less, MySQL, Sails.js, Hapi.js, Node.js, Fluxible, React, JavaScript, Front-end, Front-end Development, React Router, Axios, APIs, Third-party APIs, GitHub, Firefox, Google Chrome, API Development, Back-end, User Interface (UI), Agile Deployment, Full-stack, Full-stack Development, Back-end Development

Lead React/Redux Front-end Developer

2018 - 2018
9bits Sp. z o.o.
  • Created an engine to generate complex eCommerce pages based on JSON from a CMS.
  • Architected and established the foundational code structure for a box-based subscription eCommerce project, ensuring scalability and maintainability.
  • Implemented fully responsive designs across multiple breakpoints, ensuring a consistent user experience on desktop, tablet, and mobile devices.
  • Implemented checkout flow for multiple payments and delivery types.
  • Collaborated closely within a cross-functional development team, participating in code reviews, sprint planning, and daily standups.
  • Set up server-side rendering with dynamic page building.
Technologies: Webpack, RESTful Web Services, HTML5, Jira, JSX, React Redux, CSS3, CSS, Java, Spring, JavaScript, Redux, React, Front-end, Front-end Development, React Router, Axios, APIs, Third-party APIs, GitHub, Firefox, Google Chrome, User Interface (UI), Payment Processing

React Front-end Developer

2017 - 2017
Curvetips (via Toptal)
  • Implemented fully responsive designs across multiple breakpoints, ensuring consistent user experience on desktop, tablet, and mobile devices.
  • Refactored legacy React and Redux codebase to improve performance, maintainability, and adherence to modern best practices and patterns.
  • Set up server-side rendering with config fetched from the API.
  • Designed and implemented a complete user authorization flow including login, registration, and protected route handling with JWT tokens.
  • Integrated Google Analytics to track user behavior, set up custom events, and provide actionable insights for product decision-making.
Technologies: Webpack, HTML5, Redux, JSX, CSS3, JavaScript, CSS, Sass, React, Front-end, Front-end Development, APIs, Third-party APIs, GitHub, Firefox, Google Chrome, Startups, Early-stage Startups, User Interface (UI)

Full-stack JavaScript Developer

2017 - 2017
LOVEx (via Toptal)
  • Designed the architecture for the application and built it from scratch with React/Redux/Node/Express/Mongo.
  • Built a cross-platform audio player in React and Redux based on complex sessions with steps.
  • Created a milestones-based system for application analytics (with segment and tag manager).
  • Prepared essential reports and funnels both on the back end and in Mixpanel.
  • Helped create Facebook ad campaigns and plan A/B tests.
  • Implemented a contribution system enabling users to support creators through in-app transactions, integrated with the platform's existing payment infrastructure.
Technologies: Webpack, RESTful Web Services, HTML5, JSX, React Redux, CSS3, JavaScript, CSS, Facebook Ads, Mixpanel, Segment, MongoDB, Express.js, Redux, React, Front-end, Front-end Development, Third-party APIs, GitHub, Firefox, Google Chrome, API Development, Back-end, Startups, Early-stage Startups, User Interface (UI), Stripe, Full-stack, Full-stack Development, Back-end Development, Hosting, Payment Processing

React Front-end Developer

2016 - 2017
RoomDots (via Toptal)
  • Created a responsive, animated, multi-step onboarding process.
  • Developed reusable React components with Redux state management, enabling scalable and maintainable front-end architecture.
  • Implemented responsive UI layouts using HTML5 and CSS3, ensuring consistent cross-browser compatibility across Chrome and Firefox.
  • Collaborated with the product team in an early-stage startup environment, rapidly iterating on features based on user feedback.
Technologies: Webpack, HTML5, JSX, React Redux, CSS3, JavaScript, CSS, Redux, React, Front-end, Front-end Development, Third-party APIs, GitHub, Firefox, Google Chrome, Startups, Early-stage Startups, User Interface (UI)

React Front-end Developer

2016 - 2016
Eventfull (via Toptal)
  • Built the complete front-end architecture for an event platform using React and Webpack, delivering a high-performance, scalable user interface.
  • Contributed to back-end development using Node.js and Express.js, implementing REST API endpoints to support front-end features.
  • Developed a real-time multi-room chat feature using Socket.IO and MongoDB, enabling live communication between event participants.
  • Designed and implemented a multi-level permissions system controlling user access and roles across different sections of the platform.
  • Collaborated on UI/UX design decisions, translating wireframes into functional React components with clean, intuitive interfaces.
  • Implemented a fully responsive design using CSS3 and HTML5, ensuring a seamless user experience across desktop, tablet, and mobile devices.
Technologies: Webpack, HTML5, JSX, React Redux, CSS3, JavaScript, CSS, Socket.IO, MongoDB, Mongoose, Express.js, Node.js, Redux, React, WebSockets, Front-end, Front-end Development, GitHub, Firefox, Google Chrome, Startups, Early-stage Startups, User Interface (UI)

Senior JavaScript Developer

2016 - 2016
Spark Solutions
  • Prepared eCommerce platforms.
  • Created a custom slideshow player with lector and subtitles.
  • Worked in an Agile environment.
Technologies: Webpack, BrainTree, HTML5, jQuery, JSX, React Redux, CSS3, JavaScript, CSS, Spree Commerce, PostgreSQL, Redux, React, Front-end, Front-end Development, GitHub, Firefox, Google Chrome, User Interface (UI)

Senior JavaScript Developer

2015 - 2016
Yaska
  • Led a project.
  • Customized and created reusable modules for React.
  • Supported a portal for brokers for an insurance company.
  • Prepared single-page applications.
  • Created a website for building a company with live webcam viewer module.
Technologies: HTML5, jQuery, JSX, React Redux, CSS3, JavaScript, CSS, KeystoneJS, MongoDB, Mongoose, Express.js, Node.js, Redux, React, Front-end, Front-end Development, GitHub, Firefox, Google Chrome, API Development, Back-end, User Interface (UI)

PHP Developer

2015 - 2015
XStream
  • Prepared migrations.
  • Worked in Scrum teams.
  • Performed code reviews.
  • Built REST APIs.
  • Worked with continuous integration.
Technologies: Agile Software Development, Symphony, API Development, Back-end

JavaScript/ABAP Developer

2014 - 2015
Ipopema BC
  • Developed custom reports for a client in ABAP.
  • Prepared user interfaces with SAPUI5 for OData APIs.
  • Provided helpdesk support to solve current problems.
  • Configured MM, FICO, and BW modules of SAP.
  • Performed code reviews.
Technologies: HTML5, JavaScript, CSS, ABAP, SAPUI5, Front-end, Front-end Development

Web Developer/Co-founder

2012 - 2014
Fishapps
  • Implemented live cam portal solutions.
  • Built large custom portals from the ground up.
  • Created eCommerce platforms.
  • Prepared websites with Joomla CMS.
  • Designed websites.
  • Renewed old-fashioned websites with new technologies.
  • Worked directly with clients.
Technologies: HTML5, CSS3, CSS, HTML, Apache, Joomla, CodeIgniter, jQuery, JavaScript, Front-end, Front-end Development, API Development, Back-end, User Interface (UI), Full-stack Development, Back-end Development, Hosting, Payment Processing

Experience

CFE Polska

I developed a website for a building company that is very interactive, with smooth and complex transitions between pages. I was involved in every step of the process that was developed based on KeystoneJS.

Canopy

Website for a gardening company.

Projekt Wyprawka

Shopping/opinion-forming platform with products for kids.

Willemot NV Broker's Platform

Pretty big project for an insurance company in Gent, Belgium.

New Age Fitness Website

Website for a gym with timetable of activities.

New Age Fitness CRM

Customer relationship management platform for a gym, integration with cash register, and barcode reader for member cards.

Lueli ES

eCommerce platform with interactive product configurator for a Spanish jewelery producer.

Eventfull (via Toptal)

Platform for events management. Preparation of the whole user interface (React + Redux).

Roomdots (via Toptal)

Preparation of onboarding processes in React + Redux.

LOVEx (via Toptal)

https://www.lovex.com
LOVEx is an audio-guided step-based interactive web audio player application built to help people overcome emotional upsets.

Technologies used in the project: React and Redux on the front end and Node.js/Express on the back end.

Curvetips (via Toptal)

Curvetips is an online stylist platform working with products from different eCommerce outlets.

travelWorld (via 360codelab)

travelWorld is a booking portal with a high variety of hotels' offers. The design is really modern and minimalistic, yet very beautiful.

Dexter

Dexter is a decentralized exchange app for Tezos cryptocurrency. I was a front-end developer, preparing the pixel-perfect UIs, caring for the best UX possible, and keeping the code clean and well organized.

Dexter Stats

Statistics platform for decentralized exchange app for Tezos cryptocurrency. I was a front-end developer, I've been preparing the pixel-perfect UIs, taking care of nice the best UX possible, keeping the code clean and well organized.

Education

2011 - 2015

Engineer's Degree in Computer Science

Warsaw University of Technology - Warsaw, Poland

Skills

Libraries/APIs

React Redux, React, React Router, Context API, REST APIs, jQuery, Node.js, API Development, Stripe, Chart.js, D3.js, Sockets, Redux-Saga, Recompose, Reselect, Passport.js, Emotion API, Facebook Open Graph API, Facebook API, Braintree API, Socket.IO, Chai, ReasonReact, AWS Amplify, HTML5 Web Workers

Tools

JSX, Git, GitHub, Atom, Jira, Adobe Photoshop, Pivotal Tracker, Mongoose, Shadcn, Claude, Claude Code, Sequelize, Apache, SAPUI5, Webpack, Facebook Ads Manager, Gulp, Braintree SDK, Adobe Illustrator, Karma, Sublime Text, Chrome Developer Tools, Grunt, Android Studio, Mocha, AWS AppSync, Mailchimp, Advertising Tools & Platforms, Terraform, Amazon CloudWatch

Languages

GraphQL, HTML5, CSS3, CSS, TypeScript, HTML, JavaScript, ECMAScript (ES6), ABAP, Sass, Less, CoffeeScript, Java, Reason ML, Python, SQL

Frameworks

React Native, Redux, Tailwind CSS, Next.js, Express.js, CodeIgniter, Spring, Hapi.js, Jest, Bootstrap, Material UI, Sails.js

Paradigms

Agile Software Development, RESTful Development, REST, Mobile Development, Agile, Agile Product Management, Scrum, Hosting

Platforms

MacOS, Mobile, Joomla, KeystoneJS, Mixpanel, Docker, Blockchain, Ubuntu, Visual Studio Code (VS Code), Ubuntu Linux, Firefox, Google Chrome, Windows, Amazon Web Services (AWS), iOS, Android, AWS IoT, Twilio

Storage

MongoDB, JSON/XML Schemas, SQLite, NoSQL, MySQL, PostgreSQL, Amazon DynamoDB

Industry Expertise

Web Design

Other

Front-end, PSD to HTML, Front-end Development, Axios, Third-party APIs, User Interface (UI), Mobile App Development, Mobile Apps, Cursor AI, Data Architecture, API Integration, WebSockets, PSD Slicing, PSD, RESTful Web Services, Apps, Apollo, APIs, Back-end, Startups, Early-stage Startups, Agile Deployment, Full-stack, Full-stack Development, Back-end Development, Payment Processing, AI Tools, Symphony, Spree Commerce, Segment, Fluxible, Styled-components, Cryptocurrency Exchanges, Decentralized Applications, Facebook Ads, Metadata, Agile Sprints, Schemas, BrainTree, Server-side Rendering (SSR), Server-side, SVG, PSD to Mailchimp, IAP SDK, Analytics, App UX, App UI, Monaco, Supabase, Supabase Auth, CI/CD Pipelines

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