Iliyan Kupenov, Developer in Sofia, Bulgaria
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Iliyan Kupenov

Verified Expert  in Engineering

Full-stack Web Developer

Sofia, Bulgaria
Toptal Member Since
February 17, 2022

Iliyan is a senior full-stack engineer with over seven years of experience building high-grade software products. He has worked with large enterprises and startups alike. He's co-founded a small-scale B2B SAAS platform, where he worked completely hands-on. He was part of the founding team that built a product used by over 6 million users every month. At this stage, he is more focused on the front-end and is interested in applying his knowledge to companies building innovative products.


React, Stripe, Stripe API, C#, .NET, .NET Core, Fastify, Node.js, PostgreSQL...
JavaScript, Next.js, TypeScript, Redux, React Query, Datadog, Algolia, PostCSS...
C#, .NET Core, React, NServiceBus, HTML, CSS, SCSS, Jira, Microsoft SQL Server...




Preferred Environment

React, Visual Studio Code (VS Code)

The most amazing...

...product I've worked on is an enterprise-grade CMS that is powering eCommerce and marketing solutions for clients like Aston Martin, WHO, Nestle, and more.

Work Experience

Full-stack Engineer

2021 - PRESENT
  • Implemented payment and subscription management, coupons, invoicing, and billing history functionality using Stripe.
  • Rolled out a full-fledged OAuth authentication solution using IdentityServer, Razor Pages, and Tailwind.
  • Carried out a notification system on both the front and back end using SignalR.
  • Implemented significant parts of the back-end system using .NET Core for the REST APIs and PostgreSQL, along with Entity Framework as a data store.
  • Designed the public website and the private application using Figma. The public website is a static site generated with Gatsby, which uses a headless CMS as a data store. The data from the CMS is retrieved using GraphQL.
  • Added a comprehensive chat system featuring attachments, emojis, and statuses on both the front and back end.
  • Integrated external tracking and analytical tools, as well as feature flagging, on the front end.
  • Hosted, deployed, and monitored the infrastructure of all the projects and services, using Render and Google Cloud for the hosting.
Technologies: React, Stripe, Stripe API, C#, .NET, .NET Core, Fastify, Node.js, PostgreSQL, RabbitMQ, Tailwind CSS, Identity Server, GitHub, Docker, Docker Compose, Windows PowerShell, Gatsby, Prismic, Webflow, Webflow CMS, Razor Pages, React Hook Form, Chakra UI, SignalR, Sentry, Framer Motion, Figma, Slate, NGINX, Notion, Netlify, Heroku, SendGrid, Twilio, SendGrid API, Twilio API, HubSpot CRM, MassTransit, Styled-components, Front-end Development, Back-end Development, Git, Front-end, Design, User Interface (UI), TypeScript, Mixpanel, FullStory, Flagsmith, Next.js, Render, Google Cloud

Front-end Engineer

2022 - 2022
  • Improved team productivity by implementing foundational and reusable core components and enforcing coding standards and conventions.
  • Integrated a site-wide A/B testing solution that enabled the marketing and product team to increase conversions and efficiency through the organic and paid acquisition channels.
  • Reduced regressions and increased the team's confidence when deploying code to production by integrating end-to-end tests into the pipeline.
  • Implemented some of the highest-traffic pages and features, averaging around four million unique visits each month.
Technologies: JavaScript, Next.js, TypeScript, Redux, React Query, Datadog, Algolia, PostCSS, CSS, Taplytics, Notion, Jira, Git, GitHub, React, Front-end Development, Amazon Web Services (AWS), Terraform

Full-stack Engineer

2019 - 2021
  • Implemented components for an internal UI library—similar to Material UI and Ant Design—using React.
  • Worked heavily with .NET Core for creating REST APIs. SQL Server was used as a database, along with Entity Framework.
  • Owned the design of the initial solution for a greenfield project. The solution was modeled with DDD and CQRS in mind.
  • Worked on establishing best practices and their enforcement in the codebase.
Technologies: C#, .NET Core, React, NServiceBus, HTML, CSS, SCSS, Jira, Microsoft SQL Server, Entity Framework Core, IIS, GitHub, TypeScript, Front-end Development, Back-end Development, Git, Front-end

Full-stack Engineer

2016 - 2019
  • Worked on the core team of Sitefinity, a heavily customizable enterprise-grade CMS.
  • Integrated a large-scale front-end migration from WebForms to Angular.
  • Took the initiative to improve some internal processes by creating an internal tool that made it easier to benchmark the existing solution and visualize bottlenecks and potential errors.
Technologies: .NET, C#, JavaScript, AngularJS, Angular, Azure DevOps Services, HTML, CSS, Entity Framework, Microsoft SQL Server, IIS, TypeScript, GitHub, Front-end Development, Back-end Development, Git, Front-end

I researched, designed, and front-to-back developed an application that lets clients attract new leads using a multi-channel approach. Some of the most challenging, unrelated to business logic tasks were implementing a subscription pricing model and a custom full-fledged STS system.


A Next.js-based application with a Python back end that was in the process of being torn down into microservices using NestJS. I built the core architecture of the project. I worked on most currently live features: authentication, search, product and brand pages, media library, user reviews, uploads, and UGC interactions.

I extensively worked on Sitefinity, an enterprise content management system. It's a vast and very complex product consisting of millions of lines of code.

I was responsible for improving the product's performance and reliability and developing new complex features. The solution was mainly written in .NET Webforms and .NET MVC, using AngularJS on the front end. Later on, we started migrating the front end to Angular 2+, which I worked on exclusively for several months.

Internal MDM

MDM stands for master data module. It is a core service used to manage, synchronize, and centralize data across different yet related products across an organization. The data stored there is a single source of truth, which means the service must be highly reliable.

Med Talent Solutions

Med Talent Solutions is a recruitment agency specializing in the healthcare industry.

My job was to create a brand identity appropriate for their target customers, design a logo, and implement the whole solution using Webflow.


An Android and IOS application that lets users discover nearby restaurants, bars, and other points of interest. The project's back end was implemented using Express.js, along with MongoDB. The authentication was handled using Passport.js.

Veracode Application Security



Software Academy Graduate

Telerik Academy


Entity Framework, React, React Hook Form, React Query, Stripe, Stripe API, SendGrid API, Twilio API, Node.js, jQuery, NServiceBus, SignalR, Google APIs, Passport.js


Visual Studio, GitHub, Notion, PostCSS, Azure DevOps Services, Jira, Sentry, Figma, SendGrid, Git, RabbitMQ, Docker Compose, NGINX, FullStory, Terraform


.NET, .NET Core, Tailwind CSS, Next.js, Entity Framework Core, Angular, AngularJS, Fastify, Windows PowerShell, Razor Pages, Express.js, MassTransit, Redux


C#, JavaScript, HTML, CSS, TypeScript, SCSS, Java, Swift


Webflow, Webflow CMS, Netlify, Visual Studio Code (VS Code), Twilio, Taplytics, Docker, Heroku, Algolia, Mixpanel, Amazon Web Services (AWS)


Microsoft SQL Server, MongoDB, PostgreSQL, Datadog, Google Cloud


Prismic, Chakra UI, Front-end Development, Back-end Development, Styled-components, Front-end, Web Security, Gatsby, Framer Motion, IIS, Identity Server, Slate, HubSpot CRM, Design, User Interface (UI), Flagsmith, Render

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