Allan Castellanos
Verified Expert in Engineering
Full-stack Software Engineer and Developer
Mexico City, Mexico
Toptal member since July 24, 2026
Allan is a results-driven and highly skilled full-stack software engineer with 8 years of experience in the industry. He is adept at collaborating in multicultural teams globally, with experience contributing to the success of major corporations. Allan will be a great addition to any team.
Portfolio
Experience
- JavaScript - 9 years
- React - 8 years
- Next.js - 7 years
- TypeScript - 6 years
- Node.js - 6 years
- AWS IoT - 6 years
- Java - 5 years
- PostgreSQL - 5 years
Preferred Environment
React, Next.js, TypeScript, Node.js, Python, AWS IoT
The most amazing...
...testing strategy I've implemented has increased codebase coverage from 20% to over 5,285% in 3 months at PayPal, significantly improving reliability.
Work Experience
Senior Software Engineer
Signify Technology Group
- Collaborated continuously on feature enhancements aimed at improving user experience and conversion rates.
- Implemented the unit test testing strategy for the codebase, defining scalable testing practices for the project, increasing coverage of React components, Redux logic, and shared utilities to improve reliability and reduce regressions.
- Developed reusable React and Next.js components with TypeScript and managed complex state using Redux.
- Optimized styling and responsiveness with TailwindCSS, integrating seamlessly with PHP and WordPress-driven back ends.
- Ensured high availability and performance through AWS CDN (CloudFront), DynamoDB, Amplify, Cognito, and GCP services, automating deployments with Jenkins.
- Developed APIs using Node.js and NestJS and documentation using Swagger and versioning.
- Built and delivered new features across React, TypeScript, Node.js, and Python, supporting payment platform capabilities and internal product initiatives.
- Improved long-term maintainability by reducing technical debt, refactoring legacy code paths, and driving cleaner engineering patterns across the codebase.
- Led the implementation of the automated testing strategy, raising coverage to 90% and improving release confidence through stronger unit, integration, and CI validation.
- Supported production reliability through on-call incident response, including live troubleshooting, root cause analysis, and follow-up fixes to prevent regressions.
Senior Software Engineer
PayPal
- Developed key features using React, Next.js, TypeScript, Java, Node.js, and Python, contributing to components used by millions.
- Increased codebase coverage from 20% to 85% within the first 3 months.
- Took ownership of SDLC tasks, including planning, coding, testing, code reviews, and deployment.
- Collaborated with cross-functional teams to align features with business goals.
- Led incident resolution, improved legacy codebases, and optimized CI/CD pipelines with Jenkins.
- Mentored junior engineers and contributed to technical roadmap planning.
- Developed APIs using Node and NestJS, with documentation and versioning using Swagger.
Lead Software Engineer
Sr Pago (Acquired by Konfio)
- Collaborated with the CEO and marketing team directly to craft user-centric experiences for the company's eCommerce, product, and SDK websites.
- Guided a team of engineers, copywriters, and designers as a technical lead, architecting and building digital products from scratch.
- Migrated a complete legacy website to new technologies.
Experience
Clienteo
https://www.clienteo.appBusiness owners get a native iOS/Android app and a web dashboard that share a single back end, while their end clients get automated reminders and a no-login, magic-link status page. It's a full-stack, multi-tenant product with PostgreSQL-enforced tenancy and free-tier limits, scheduled reminder jobs, and 3 AI features built on Claude.
Education
Bachelor's Degree in Information Systems
IPN UPIICSA - Mexico City, Mexico
Skills
Libraries/APIs
React, Node.js, Stripe API, Liquid, Stripe, REST APIs, Google Maps, Vue 3, AWS Amplify, Formik, React Testing Library, Playwright, Chai, Puppeteer
Tools
Figma, Claude Code, Claude, Google Analytics, Amazon Elastic Container Service (ECS), Expo, Amazon CloudFront CDN, Amazon Cognito, Jenkins, GitHub, GitLab, Jira, Slack, Confluence, TeamCity, Vitest, Mocha, Prisma, Terraform, SonarQube
Languages
TypeScript, Python, JavaScript, HTML5, CSS3, CSS, HTML, PHP, Java, SQL, GraphQL, Shopify Liquid, Sass, Less
Frameworks
Next.js, Tailwind CSS, Agentic Frameworks, NestJS, Angular, Jest, React Native, Laravel, Flutter, Express.js, .NET, Django, Redux, Swagger, Bootstrap, MUI CSS, Remix, Cypress, Jasmine
Paradigms
UX Design, Search Engine Optimization (SEO), UI Design, Microservices, Mobile Development, Agile Software Development, Database Design, Role-based Access Control (RBAC), Scrum
Platforms
Vercel, Shopify, Amazon Web Services (AWS), iOS, Mixpanel, Docker, Netlify, WordPress, AWS IoT, Google Cloud Platform (GCP), AWS Lambda, WooCommerce, Magento, Webflow
Storage
PostgreSQL, JSON, MongoDB, Redis, MySQL, Amazon DynamoDB, Amazon S3 (AWS S3)
Other
Full-stack Development, Web Development, Back-end, Fintech, Architecture, Full-stack, Front-end, Front-end Development, SQL Server, Software Architecture, System Architecture, User Interface (UI), Web Applications, User Experience (UX), SEO Tools, SaaS, Communication, Attention to Detail, Code Review, Screeners, Written Communication, Team Leadership, Mobile App Development, Scalability, App Store, Web App Development, Mobile Apps, eCommerce, RESTFul APIs, APIs, Solution Architecture, Cloud, GitHub Actions, Leadership, Technical Leadership, Technical Architecture, Agentic Coding, Large Language Models (LLMs), Supabase, API Integration, Cross-platform App Development, Financial Services, Monorepos, Financial APIs, CI/CD Pipelines, CMS, Web Content Accessibility Guidelines (WCAG), Agentic AI, Artificial Intelligence (AI), FastAPI, MVP Design, Billing, Integration, Cursor AI, Email Automation, SMS, AI Agents, Data Modeling, Speech-to-Text (STT), AI Chatbots, Dashboard Development, Cloudflare, NextAuth.js, Storybook, Enzyme, MSW, OpenAI, Serverless, SES, EC2, Coding, WebSockets, Real-time Tracking
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