README.md

Tomás Vallejos Semino

Frontend Developer · JavaScript · React · Next.js · React Native

Tomás

Frontend Developer with 5+ years of experience building web and mobile applications for companies in Spain and Argentina, from the first design to production. I'm also Systems Manager at ST Group, which gives me an end-to-end view of product, business, and infrastructure.

🚀 Quick start

⚡ summary.md: my whole profile on one page (ideal if you're short on time)
📁 experience/: where I've worked and what I've built
🧩 skills/: my stack
🛠️ projects/: projects (starting with this portfolio)
🎓 about/: education, certifications, languages and hobbies
📬 contact/: let's talk
📄 cv/: download my resume

🛠 Main stack

JavaScript · React · Next.js · React Native · Node.js · Tailwind CSS · Supabase

📬 Contact

tvallejossemino@gmail.com · LinkedIn · GitHub

💡 Tip: open the terminal with Alt+T and type help.

about· 5 files

about-me.md

About me

I'm a Frontend Developer with 5+ years of experience building web and mobile apps with JavaScript, React, Next.js, and React Native for companies in Spain and Argentina, from the first design to production.

Some of the things I've built

A mobile app for Flechabus bus drivers with React Native and Expo, from the Figma design to production: ticket and ID scanning, stop tracking, and offline support.
The first mobile version of a product quotation platform for a software company in Spain.
A service marketplace with real-time chat and reviews, and a portal for a clinical lab to manage medical tests.

Since 2026 I've also been Systems Manager at ST Group, leading infrastructure, cybersecurity, and custom software for 5 corporate clients. This gives me a close view of how technology impacts the business.

What I bring

Clean, maintainable code, a strong focus on UX, ownership from idea to deployment, and clear communication.

certifications.md

Certifications

Fortinet

Fortinet Certified Fundamentals in Cybersecurity

15 Feb 2026 · Valid until 15 Feb 2028 · ID 5581514127TV

Earned during the 1st year of the B.S. in Cybersecurity (UNSO).

View certificate: certificates/fortinet-certified-fundamentals-cybersecurity.pdf · Verify

Introduction to the Threat Landscape 3.0

15 Feb 2026

Fortinet course, taken in Spanish.

View certificate: certificates/fortinet-introduction-to-the-threat-landscape.pdf

Getting Started in Cybersecurity 3.0

2 Feb 2026

Fortinet course, taken in Spanish.

View certificate: certificates/fortinet-getting-started-in-cybersecurity.pdf

Plataforma 5

Full-Stack Development Program · Coding Bootcamp

Aug 2021 · ID EM866280

View certificate: certificates/plataforma5-full-stack-bootcamp.pdf

Front End Introductory Module

2021

View certificate: certificates/plataforma5-intro-frontend.pdf

education.md

Education

B.S. in Cybersecurity

Universidad Nacional Raúl Scalabrini Ortiz (UNSO)

In progress · 2nd year

As part of the first year I earned the Fortinet Certified Fundamentals in Cybersecurity certification (valid until 2028), along with the Getting Started in Cybersecurity and Introduction to the Threat Landscape courses. Details in about/certifications.md.

Full-Stack Development Program

Plataforma 5 · Coding Bootcamp

Mar 2021 – Aug 2021

Intensive full-stack JavaScript program, completed after the Front End Introductory Module. The certificates are in certificates/.

hobbies.md

Hobbies

☕ Specialty coffee

Besides writing code, I make coffee: I'm a certified barista. I took the Barista & Latte Art Course at Puro Lab · Escuela de Baristas (March 2026, 2 months; certificate issued together with Escuela Gourmet Online and Puro Café).

That's why this portfolio's package.json starts with "dev": "coffee && code". Try typing coffee in the terminal.

Barista & Latte Art Course certificate

Certificate: certificates/puro-lab-barista-latte-art.jpg

languages.json

{
  "spanish": "Native",
  "english": "Upper-intermediate (B2)"
}

certificates· 6 files

fortinet-certified-fundamentals-cybersecurity.pdf

Download PDF

fortinet-getting-started-in-cybersecurity.pdf

Download PDF

fortinet-introduction-to-the-threat-landscape.pdf

Download PDF

plataforma5-full-stack-bootcamp.pdf

Download PDF

plataforma5-intro-frontend.pdf

Download PDF

puro-lab-barista-latte-art.jpg

contact· 1 files

contact.js

export const contact = {
  email: "tvallejossemino@gmail.com",
  linkedin: "linkedin.com/in/tomasvallejos",
  github: "github.com/Tvallejoss",
  location: "Buenos Aires, Argentina",
  status: "Open to work · Frontend / React Developer",
};

cv· 2 files

CV_Tomas_Vallejos_Semino_ES.pdf

Download PDF

Resume_Tomas_Vallejos_Semino_EN.pdf

Download PDF

experience· 7 files

01-st-group.md

Systems Manager

ST Group

Jan 2026 – Present · Buenos Aires

React Native
Next.js
PWA
Jira
Jira API

ST Group is an IT consulting firm. I run the systems area for 5 of its corporate clients, including +Vida and Román Servicios: from infrastructure and security to custom software, working alongside the CEOs and managers and leading the helpdesk technicians.

What I did

Lead the helpdesk technicians: planning, prioritizing and following up on their work.
Make systems and software decisions for client companies.
Install and maintain infrastructure: servers, networks and internet, CCTV, backups and software licensing.
Define cybersecurity, backup and incident recovery policies, and help manage contingencies.
Audit and document each client's IT infrastructure.

Custom software

Control and management applications for clients, designed from scratch by me (UX/UI) and built as PWAs or mobile apps with React Native and Next.js.
Jira rollout across several areas of +Vida to automate processes and track metrics.
Apps connected to the Jira API, so teams can manage and track their work from the app.

Highlight

The Jira work at +Vida: process automation and metrics for several areas, plus custom apps built on top of the Jira API. One of the biggest achievements of this role so far.

02-flechabus.md

Frontend & Mobile Developer

Flechabus

Sep 2024 – Present · Freelance

React Native
Expo
JavaScript
Figma

A production mobile app, built from scratch for the bus drivers of the Flechabus group, a group of long-distance bus companies. It replaced the paper list drivers used to check who could board with a digital flow that works offline. The MVP reached production in 3 months.

What I did

Designed the whole app in Figma: logo, color palette and every screen.
Built the frontend from scratch with React Native and Expo, for Android and iOS.
Integrated the existing Flechabus API (the backend was built by the company).
Provide ongoing support, maintenance, and new features in production.

How it works

Drivers sign in with their own account.
They scan the ticket's QR code and, when needed, the passenger's ID card to validate it.
The app checks that the passenger belongs on that bus and shows clear success screens ("Passenger found, can board").
If a code or document can't be read, the passenger can be entered manually.
Each trip comes with its assigned stops: the driver taps "Arrived" and "Departed" at each one, which gives timing control for every route.
Uses the driver's geolocation during the trip.

The challenge: offline and sync

Buses travel through areas with no signal, so the app is offline-first: it validates passengers and records each stop without internet, keeping that data in AsyncStorage. The hardest part was synchronization: making sure no record was ever lost when the connection came back mid-route, so Flechabus servers always end up knowing which passengers boarded at each stop.

Published on Android and iOS for internal use by the Flechabus group.

03-buspack.md

Frontend Developer

Buspack (Flechabus group)

Jul 2023 – Present · Freelance

React
Next.js
Node.js
Docker

Buspack is the parcel and logistics company of the Flechabus group. I support and evolve its main application, where orders and customers are managed, and I built additional tools around it.

What I did

New features and improvements for Buspack's main application (orders, customers and more).
Improved the mobile design of the app.
Fixed reported bugs and keep the platform running in production.
Built an internal web app, integrated with the SAIT locations API, to load and update locations and addresses.

The challenge: modernizing the code

Fixing bugs in a legacy codebase while migrating React class components to modern React with hooks, and bringing outdated dependencies up to date without breaking production.

04-hetmo.md

Frontend Developer

Hetmo Informática

Aug 2023 – Sep 2026 · Full-time · Remote, Spain (Cantabria)

JavaScript
React
Next.js
Vercel

Hetmo builds software to quote any kind of window or door, used by clients across Europe and Latin America, including Obramat. I worked as a frontend developer on its main application and on metrics apps for the factories that manufacture those windows.

What I did

New features and bug fixes in the main quoting application, with JavaScript, React and Next.js.
Led development of the first mobile version of the quoting platform.
Metrics apps for factories: real-time dashboards of daily and monthly production, and daily efficiency checks to see whether the day's production was running behind. Deployed on Vercel.
Also worked on the mobile side of those apps.

The company

Spanish software company with 25+ years in management software and machinery integration for aluminium, PVC and wood joinery manufacturers, present in Spain, Portugal and Latin America.

🔗 hetmo.com

05-vidual.md

Frontend Developer

Vidual

Apr 2023 – Aug 2023 · Freelance · Remote

React
Next.js
Socket.io
Node.js

A web platform for hiring services: people publish the services they offer, and other users find and hire them.

What I did

Built the platform from scratch with React and Next.js.
Publishing and hiring of services, with a user review system.
Real-time chat between users with Socket.io.

The challenge

The real-time chat with Socket.io was the biggest technical challenge of the project.

06-laboratorio-cordoba.md

Frontend Developer

Laboratorio Córdoba

May 2022 – Apr 2023 · Freelance · Remote

React
JavaScript

A client project integrated with the NOBILIS clinical laboratory, which provided the test results. A web platform built so that a company can manage the medical tests of its employees in one place, designed as a product to sell to corporate clients.

What I did

Designed, developed and deployed the web platform from scratch.
View and download results as PDF, and create new tests and requests.
Tables tracking the status of orders and referrals.
Admin view to manage the platform.

07-netglobal.md

Frontend Developer

NetGlobal Solutions

Aug 2021 – Apr 2022 · Full-time · Remote, Argentina

React
PWA
Jest
React Testing Library

My first role as a developer. NetGlobal builds a CRM, and DirecTV was one of its main clients.

What I did

Added unit tests to the CRM with Jest and React Testing Library.
Worked on a PWA built specifically for DirecTV's field units: geolocation to see where each unit is, plus work order tracking and requests.
New features and mobile UX improvements for the CRM.
Fixed bugs and improved legacy React code written with class components.

The company

Software development company with 20+ years building business-process solutions for the insurance, manufacturing and services industries.

🔗 netglobal.tech

projects· 1 files

tvallejoss-portfolio.md

tvallejoss · VS Code–style portfolio

This very site

Next.js
React
JavaScript
Tailwind CSS
Shiki

My portfolio as a VS Code workspace: folders, tabs, a terminal, git history and skills as extensions.

The story

The idea started in September 2022, in my first months as a developer: a React mock-up of the editor window. It stayed at that, a static mock-up.

In 2026 I picked it up again with four more years of experience and turned it into a complete application where every part of the interface actually works.

🔗 Source code · First sketch (2022)

Technical decisions

Content is data. Experience, skills and certifications live in /content. Each role generates its .md file, its Source Control commit and its terminal output, with no duplicated copy.
Server-side highlighting. Shiki colors everything once for all 3 themes; switching themes is pure CSS and the browser never downloads the highlighter.
Fully indexable. All copy is server-rendered, in Spanish and English.
Terminal outside React. The session (history, working directory, scrollback) lives in its own store, so it survives closing the panel.
Shortcuts that don't fight the browser. Ctrl+P prints and Ctrl+W closes the tab, so shortcuts were chosen to avoid clashes and are matched by physical key, so they work on any keyboard layout.
Responsive without flashes. CSS decides which screen to show (desktop, tablet or mobile) before any JavaScript runs.
Accessible. Tree, tabs and palette follow WAI-ARIA patterns, and everything works from the keyboard.

Credits

Design inspired by VS Code (not affiliated with Microsoft).

skills· 6 files

backend.js

export const backend = {
  runtime: ["Node.js"],
  frameworks: ["Express", "NestJS"],
  apis: ["REST APIs"],
  realtime: ["Socket.io"],
};

databases.json

{
  "baas": ["Supabase", "Firebase"],
  "sql": ["PostgreSQL", "MySQL"],
  "nosql": ["MongoDB"]
}

devops.yml

version_control:
  - Git
  - GitHub
containers:
  - Docker
deploy:
  - Vercel
  - AWS
  - Heroku
  - Railway

frontend.js

export const frontend = {
  languages: ["JavaScript", "TypeScript", "HTML5", "CSS3", "Sass"],
  frameworks: ["React", "Next.js"],
  state: ["Redux"],
  styling: ["Tailwind CSS", "Material UI", "CSS Modules"],
  testing: ["Jest", "React Testing Library"],
};

mobile.js

export const mobile = {
  frameworks: ["React Native"],
  tooling: ["Expo", "Expo EAS"],
};

tools.json

{
  "design": ["Figma", "v0"],
  "management": ["Jira", "Notion", "Scrum"],
  "ai": ["Claude Code", "Cursor", "ChatGPT"]
}

.gitignore

# Things that never reach production
node_modules/
.env
.env.local
# Habits I left behind
console.log("here")
console.log("here 2")

package.json

{
  "name": "tvallejoss",
  "version": "5.0.0",
  "description": "Frontend Developer · 5+ years",
  "author": "Tomás Vallejos Semino",
  "scripts": {
    "dev": "coffee && code",
    "hire": "mailto:tvallejossemino@gmail.com"
  },
  "dependencies": {
    "javascript": "^ES2026",
    "react": "^19.2",
    "next": "^16",
    "react-native": "^0.8x",
    "tailwindcss": "^4",
    "node": "^24",
    "supabase": "latest"
  }
}

summary.md

Tomás Vallejos Semino

Frontend Developer · JavaScript · React · Next.js · React Native

The whole profile on one page. 5+ years building web and mobile apps for companies in Spain and Argentina, from the first design to production.

Open to work: Frontend / React Developer, remote or hybrid.

Contact

tvallejossemino@gmail.com
LinkedIn · GitHub
Buenos Aires, Argentina
Download resume: ES · EN

Experience

Systems Manager · ST Group · Jan 2026 – Present

Lead the helpdesk technicians: planning, prioritizing and following up on their work.
Make systems and software decisions for client companies.
Install and maintain infrastructure: servers, networks and internet, CCTV, backups and software licensing.
Define cybersecurity, backup and incident recovery policies, and help manage contingencies.
Audit and document each client's IT infrastructure.

Frontend & Mobile Developer · Flechabus · Sep 2024 – Present

Designed the whole app in Figma: logo, color palette and every screen.
Built the frontend from scratch with React Native and Expo, for Android and iOS.
Integrated the existing Flechabus API (the backend was built by the company).
Provide ongoing support, maintenance, and new features in production.

Frontend Developer · Buspack · Jul 2023 – Present

New features and improvements for Buspack's main application (orders, customers and more).
Improved the mobile design of the app.
Fixed reported bugs and keep the platform running in production.
Built an internal web app, integrated with the SAIT locations API, to load and update locations and addresses.

Frontend Developer · Hetmo Informática · Aug 2023 – Sep 2026

New features and bug fixes in the main quoting application, with JavaScript, React and Next.js.
Led development of the first mobile version of the quoting platform.
Metrics apps for factories: real-time dashboards of daily and monthly production, and daily efficiency checks to see whether the day's production was running behind. Deployed on Vercel.
Also worked on the mobile side of those apps.

Frontend Developer · Vidual · Apr 2023 – Aug 2023

Built the platform from scratch with React and Next.js.
Publishing and hiring of services, with a user review system.
Real-time chat between users with Socket.io.

Frontend Developer · Laboratorio Córdoba · May 2022 – Apr 2023

Designed, developed and deployed the web platform from scratch.
View and download results as PDF, and create new tests and requests.
Tables tracking the status of orders and referrals.
Admin view to manage the platform.

Frontend Developer · NetGlobal Solutions · Aug 2021 – Apr 2022

Added unit tests to the CRM with Jest and React Testing Library.
Worked on a PWA built specifically for DirecTV's field units: geolocation to see where each unit is, plus work order tracking and requests.
New features and mobile UX improvements for the CRM.
Fixed bugs and improved legacy React code written with class components.

Skills

Frontend: JavaScript, TypeScript, HTML5, CSS3, Sass, React, Next.js, Redux, Tailwind CSS, Material UI, CSS Modules, Jest, React Testing Library
Mobile: React Native, Expo, Expo EAS
Backend: Node.js, Express, NestJS, REST APIs, Socket.io
Databases: Supabase, Firebase, PostgreSQL, MySQL, MongoDB
Devops: Git, GitHub, Docker, Vercel, AWS, Heroku, Railway
Tools: Figma, v0, Jira, Notion, Scrum, Claude Code, Cursor, ChatGPT

Education

B.S. in Cybersecurity · Universidad Nacional Raúl Scalabrini Ortiz (UNSO) · In progress, 2nd year
Full-Stack Development Program · Plataforma 5, Coding Bootcamp · Mar 2021 – Aug 2021

Certifications

Fortinet Certified Fundamentals in Cybersecurity · Fortinet · 15 Feb 2026
Introduction to the Threat Landscape 3.0 · Fortinet · 15 Feb 2026
Getting Started in Cybersecurity 3.0 · Fortinet · 2 Feb 2026
Full-Stack Development Program · Coding Bootcamp · Plataforma 5 · Aug 2021
Front End Introductory Module · Plataforma 5 · 2021

Beyond code

☕ Certified barista: Barista & Latte Art Course (Puro Lab · Escuela de Baristas, Mar 2026)

Languages

Spanish (native) · English (upper-intermediate, B2)

welcome.js

/**
 * 👋 Hi! I'm Tomás.
 * This is my portfolio, built as a VS Code workspace:
 * you can browse my career as if you were exploring a repository.
 */
const name = "Tomás Vallejos Semino";
const developer = {
  name,
  role: "Frontend Developer",
  stack: ["JavaScript", "React", "Next.js", "React Native"],
  experience: "5+ years",
  location: "Buenos Aires, Argentina",
  openToWork: true,
};
// 🧭 How to get around:
// 📁 Browse the folders on the left: experience/, skills/, about/
// 📄 .md files open as rendered documents, just like in VS Code
// ⌨️  Ctrl+K finds files · F1 shows all commands
// 💻 Alt+T opens the terminal: try typing "help"
// 📬 Let's talk: contact/contact.js, or grab my resume in cv/
export default developer;