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juancho

Cork, Ireland

JuanchoBarroso

Senior frontend engineer specialising in React, Next.js and TypeScript — 8+ years shipping payments and trading platforms where UX is revenue.

Who I am

I build the frontends money moves through — $300M+ a year in payments, and live trading odds updating thousands of times a second.

Eight-plus years deep in React, TypeScript and Next.js, T-shaped into Node.js and AWS — much of it spent turning legacy code into platforms that ship: AngularJS to Next.js migrated in production without stopping the business, a design system powering five apps, a state-management rewrite de-risked before the migration touched the product. Development gets easier, quality goes up, and users get better solutions sooner.

I treat performance as a feature and the frontend as a first-class source of truth — measured Core Web Vitals work, and client-side telemetry that can reconstruct a single payment's journey. The UI is where users decide to trust you; it deserves the same engineering rigour as the backend.

I work AI-native — repo-wide agents, deliberate context engineering, AI-assisted review — and at NS90 I turned that personal workflow into the team's: standards, shared context, MCP servers, now adopted team-wide. If you're looking for someone who loves to own the product and get deep into the architecture, let's connect.

Highlights

What I do best

I turn legacy code into platforms that ship

Years of experience taking old codebases with bad habits and bringing them up to modern stacks and real standards, without stopping the business. Development gets easier, quality goes up, and users get better solutions sooner.

Now

Real-time trading UI at NS90

Building 3et.com — live odds over WebSockets, complex state, and a Core Web Vitals push that cut LCP from 3.8s to 1.8s.

See it live

Craft

Design systems

Architected a component library as a facade over MUI — one system powering five apps, the UI library swappable underneath.

Range

Full-stack when it counts

REST APIs, auth services and AWS Lambda with hexagonal architecture.

AI-native

I led my team's AI adoption

Repo-wide agents, context engineering and MCP servers are my daily workflow — turned into team-wide standards at NS90.

Stack

Tools of choice

  • React
  • Next.js
  • TypeScript
  • Redux
  • Node.js
  • AWS
  • Tailwind

Based in

Cork, Ireland

Open to product-driven teams.

Say hi

Experience

Apr 2025 — Present

NS90

Senior Frontend Engineer · Betting & trading

1.8sLargest Contentful Paint — down from 3.8s, on a UI streaming thousands of live odds

  • React
  • Redux
  • TypeScript
  • WebSockets
  • Core Web Vitals

Building 3et.com, a real-time trading platform where every millisecond of UI is the product.

  • Led a Core Web Vitals push on a live platform: LCP from 3.8s to 1.8s, layout shift from 0.381 to 0.074, interactions smooth under load with React transitions.
  • Spotted that the platform ran as two domains on two infrastructures, pitched the CDO, and led the frontend re-architecture into one — eliminating every post-login redirect.
  • Helped take identity verification from a 3-day manual support process to a self-serve flow that takes users a couple of minutes — now live in production.
  • Started the team's AI-native engineering workflow — shared context, standards, MCP servers — now adopted across the team; de-risked a Rematch → Redux Toolkit migration with an adapter-layer proof of concept.

Apr 2018 — Mar 2025

MYMOID

Junior → Senior Full-Stack Engineer · Payments

$300M+in transactions yearly through the payment platform — 2–3M payments a month

  • Next.js
  • React
  • TypeScript
  • Node.js
  • AWS Lambda

Seven years on a payments platform moving $300M+ a year — from junior hire to the engineer the frontend decisions ran through.

  • Led the production migration from AngularJS to Next.js without stopping the business — the work that earned the promotion to senior, not the other way around.
  • Rebuilt the payment form end to end: PSD2-compliant 3D Secure, PCI card tokenization, and per-merchant branding on a form that previously couldn't be personalised at all.
  • Architected the design system as a facade over MUI — one component library powering five apps, with the UI library swappable underneath.
  • De facto frontend lead for the final 3 years: hired 2 engineers, mentored 3, established code review and an 80% test-coverage gate, and ran the two-week release cycle.

Oct 2017 — Mar 2018

BAKS GmbH

Junior Frontend Developer · Automotive — First industry role: reverse-engineered a Visual Basic desktop app well enough to rebuild it as an Angular web prototype, on an international German–Spanish–UK team.

That's the short version. The full story lives on LinkedIn (opens in a new tab)

Contact

Let's build something that ships.

Open to senior frontend roles on product-driven teams pushing into AI. Spanish (EU) citizen — no visa sponsorship needed.

me@juanchojbarroso.com
LinkedIn (opens in a new tab)GitHub (opens in a new tab)