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Haessler León

@haesslerlen

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I build full-stack and mobile fintech, Web3, and e-commerce applications used at scale.

Brazil

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What I'm looking for

I'm seeking a remote Full-Stack or Mobile Developer opportunity where I can build scalable web and mobile products, contribute across frontend and backend systems, and apply my experience in fintech, Web3, AI integrations, and team leadership.

I've built production web and mobile applications for 7Net Corporation, Busi Technology Developments, TurpialDev, and Zumetrics, spanning fintech, Web3, and e-commerce. My work includes backend improvements that lifted performance by 15%, financial modules optimized by 7%, and AI-agent workflow integrations using MCP and n8n.

At Busi, I led development of three mobile applications supporting 30,000+ active users and introduced a Yarn monorepo that reduced code duplication by 40%. I've also led a five-developer team building a real-time financial dashboard for 5,000+ monthly users, delivered cross-chain transactions with a 99.8% success rate, and work across React, React Native, Node.js, Next.js, Nest.js, and native mobile tooling.

Experience

Work history, roles, and key accomplishments

Mobile Developer | Full Stack Developer

7Net Corporation

May 2025 - May 2026 (1 year)

Engineered security and architectural enhancements for backend systems using Nest.js, Sequelize, and PostgreSQL, resulting in a 15% performance boost. Optimized financial application modules by 7% through advanced implementation of React, Angular, and Flutter.

NestJS Sequelize PostgreSQL React Angular Flutter N8n MCP

Mobile Developer | Full Stack Developer

Busi Technology Developments

Oct 2023 - May 2025 (1 year 7 months)

Directed the development of 3+ mobile applications for Busi Boletos using React Native, Expo, and Swift, supporting over 30,000 active users. Implemented a Yarn Monorepo architecture to centralize reusable UI libraries, reducing code duplication by 40% and accelerating feature deployment by 25%.

React Native Expo Swift Yarn Monorepo XCode Android Studio

Web3 Developer | Full Stack | Mobile Developer

TurpialDev

Jul 2021 - Aug 2023 (2 years 1 month)

Developed the DecentralFi dApp frontend using Angular, React, and React Native, integrating secure wallet connections via MetaMask and XDEFI. Achieved 90% test coverage for critical components by implementing comprehensive testing suites with Jest and Cypress.

Angular React React Native MetaMask XDEFI Jest Cypress Material UI Tailwind Thorchain Terra

Full Stack Developer | Tech Lead

Zumetrics Tecnologies

Aug 2020 - Apr 2022 (1 year 8 months)

Managed a team of 5 developers to build a real-time financial dashboard using Next.js, D3.js, and Chart.js, serving 5,000+ monthly users. Architected a GraphQL layer to unify disparate APIs, reducing query latency by 40%.

Next.js D3.Js Chart.Js GraphQL Onion Architecture Jest Mocha Storybook CI CD

Front End Developer

Ovmafot / Ponceleon Devs

Jan 2018 - Aug 2020 (2 years 7 months)

Developed the primary corporate landing page using WordPress, Wix, JavaScript, and CSS. Built the core web application frontend using React, Redux, and Bootstrap, while contributing to Node.js and MongoDB backend services.

WordPress Wix JavaScript CSS React Redux Bootstrap NodeJS MongoDB

Full Stack & Mobile Developer

Haessler León

Full Stack & Mobile Developer with 9+ years of experience building production-grade web and mobile applications across fintech, Web3, and e-commerce. Specializes in React, React Native, Node.js, Next.js, and Nest.js, with hands-on experience leading teams, architecting scalable backend systems, and shipping mobile apps to 30,000+ active users.