Available for new projects

Senior Frontend Developer
Modernizing Legacy Frontends

I help enterprise teams and ambitious startups turn slow, tangled frontends (specializing in Angular) into clean, modular systems that ship faster and stay fast. Expect measurable performance wins, audit-ready engineering practices, and architecture that scales with your product.

Angular is my specialty. I can also ship React when your stack or roadmap requires it.

Founder @ Scale Sail
  • Angular 21
  • Signals Architecture
  • SSR-Ready Performance
  • Tailwind CSS 4
Book a Call Download Resume View Case Studies

Trusted by engineering leaders at companies like Citi and BNP Paribas.

Engineering
Audit-ready delivery
Performance
Core Web Vitals wins
Product
Ship faster, safer
Karol Modelski
What I Deliver

Bank-Grade Engineering. Startup-Speed Delivery.

For enterprise teams and ambitious startups, I modernize legacy frontends, fix performance bottlenecks, and build modular Angular systems that stay maintainable as your product grows. Clear outcomes, measurable wins, and audit-ready quality.

  • Enterprise Migration & Modernization

    Transform slow, tangled legacy frontends into clean, modular Angular systems. I specialize in Angular migrations (AngularJS/legacy → modern), refactoring strategies that remove technical debt, and scalable architectures that keep delivery moving.

    Angular Migration Expert Codebase Refactoring Nx Monorepo

  • SaaS Product Engineering

    End-to-end delivery for scalable SaaS products. I design maintainable UI architectures, predictable state management, and performance-first frontends that support rapid iteration without regressions.

    Full-Cycle Dev Scalable Architecture Reactive State

  • Architecture Audits & Performance

    Deep-dive audits into your codebase structure and runtime performance. I identify scaling bottlenecks, optimize Core Web Vitals, and turn spaghetti code into feature-based modules with clear ownership.

    40%+Faster load times

  • Feature Delivery

    Turn product requirements into deployed reality. I execute end-to-end feature development—from architectural planning to pixel-perfect UI—ensuring new capabilities launch smoothly and integrate seamlessly with your existing system.

    End-to-End Implementation Complex State Management API Integration

  • Fractional Tech Lead / Mentorship

    Elevate your team's capabilities. I provide code reviews, set up best practices (CI/CD, Testing), and mentor developers on advanced architectural patterns like Signals and RxJS.

    Code Reviews Team Upskilling Process Setup
Work History

Professional Experience

Delivering modernization and performance improvements at scale—from regulated enterprise platforms to high-velocity product teams.

Modern Architecture

Built for Scale

Combining architectural rigor with product velocity.

Download Resume
  • G

    Senior Frontend Developer

    GFT Technologies (Citi)

    Mar 2025 - Present

    Leading architecture for mission-critical trading platforms. Enforcing 100% test coverage policies while migrating legacy micro-frontends to a coherent Nx monorepo. Balancing enterprise compliance with developer velocity.

    Angular 19 Nx Micro-Frontends Regulatory Compliance
  • S

    Frontend Developer

    Silent Eight

    Dec 2023 - Feb 2025

    AI Compliance SaaS. Led the frontend modernization initiative (Angular 14 → 19) and introduced "Zoneless" architecture for 50% performance gains. Worked directly with Product Owners to accelerate feature delivery.

    Angular 19 TanStack Query AI Integration SaaS
  • B

    Software Developer

    BNP Paribas

    Nov 2021 - Nov 2023

    GOonline Platform. Delivered features for 1M+ active users. Managed the complex migration of core banking modules from AngularJS, ensuring zero downtime during high-traffic windows.

    Angular Enterprise Scale Migration Strategies Fintech
  • A

    Javascript Developer

    Amway

    Apr 2019 - Oct 2021

    Global E-commerce. Built high-traffic dashboards for business owners. Optimized data visualization components handling millions of records.

    Angular Data Viz E-commerce
Technical Proficiency

Engineering for Scale & Performance

A comprehensive Frontend toolchain. While Angular is my specialty for enterprise scale, I maintain deep proficiency in React and the broader JavaScript ecosystem to architect the right solution for the problem.

Primary Stack

  • Angular

    6+ years exp.

    Expert

  • React

    1+ years exp.

  • TypeScript

    6+ years exp.

    Expert

  • JavaScript

    6+ years exp.

    Expert

  • RxJS / Signals

    6+ years exp.

    Expert

  • NgRx / Signal Store

    6+ years exp.

    Expert

  • SCSS

    6+ years exp.

    Expert

  • Angular Material

    6+ years exp.

    Expert

Additional Tools

  • React
  • TanStack Query
  • Context API
  • Redux
  • Zustand
  • React Hook Form
  • Styled Components
  • TanStack Table

Detailed Expertise

More than just code.Tech Leadership.

Software is built by people. I foster environments where technical excellence meets product vision.

  • Leadership & Team Mentorship
  • Technical Communication
  • Problem Solving
  • Agile & Collaborative Mindset
  • Adaptability & Continuous Learning
Team collaboration
Get in Touch

Start a Project

If you're dealing with a slow, hard-to-change frontend (or planning a new SaaS build), I can help you ship with confidence. Share your goals and constraints—I'll review your inquiry and if it's a good fit, I'll send you a link to book a discovery call.

Email
karol-modelski@scale-sail.io
Address
Warsaw, Mazowieckie, Poland
Short on time? View my Calendar →