Viktor Fonic, Developer in Buenos Aires, Argentina
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Viktor Fonic

Verified Expert  in Engineering

Shopify Customizations Developer

Buenos Aires, Argentina
Toptal Member Since
August 19, 2013

With a master's degree in computer science and over a decade of experience in full-stack development and eCommerce, Viktor has the knowledge and expertise to solve complex problems and build fast, scalable applications working in any role, from individual developer to team leader.


Ruby on Rails (RoR), JavaScript, TypeScript, GraphQL, Hasura, PostgreSQL...
Radio Depot
Shopify, Liquid, JavaScript, HTML, CSS
Shopify, JavaScript, HTML, CSS




Preferred Environment

MacOS, Visual Studio Code (VS Code), GitHub

The most amazing... I've developed is a pharmaceuticals app used to diagnose patients based on their symptoms and medical history.

Work Experience

Project Tech Lead | Full-stack Developer

2020 - PRESENT
  • Guided the client from zero to one from day one of the project. The client was so happy that he offered me the position of CTO.
  • Expanded the original Shopify webshop project into a marketplace.
  • Wrote various automation scripts for communication between company staff, vendors, and customers.
Technologies: Ruby on Rails (RoR), JavaScript, TypeScript, GraphQL, Hasura, PostgreSQL, DigitalOcean, Heroku, Shopify, Shopify API, Shopify Plus, Liquid

Shopify Expert | Full-stack developer

2020 - 2021
Radio Depot
  • Developed effective site structure and product selection.
  • Optimized page speed loading times to improve user experience.
  • Worked on creating a unique customer portal with customer preferences saved for faster repetition of orders and configuration of bought products.
Technologies: Shopify, Liquid, JavaScript, HTML, CSS

Shopify Expert | Full-stack Developer

2020 - 2020
  • Researched color-blindness issues to develop a competitive color-blindness test.
  • Developed color-blindness test following the most credible method for testing color-blindness.
  • Organized several email campaigns for potential customers who fill-in the color-blindness test.
Technologies: Shopify, JavaScript, HTML, CSS

Shopify App Developer

2017 - 2019
AllNet Apps
  • Built a common, shared codebase framework to reuse across all apps.
  • Since the launch, both apps have had exclusively 5-star reviews.
  • Continued working on the shared framework to build additional apps rapidly.
Technologies: JavaScript, Shopify API, Ruby on Rails (RoR), Ruby, Shopify, Shopify Customizations, HTML, Liquid, Shopify Theme, Custom Shopify Apps, Next.js, Tailwind CSS, Shopify Design, eCommerce, GraphQL

Full-stack Software Developer

2017 - 2017
  • Implemented long tasks running in the background.
  • Created recurring events schedules and management.
  • Increased database performance.
  • Built drag-n-drop management of homepage blocks.
  • Implemented reusable search components and used them to search and filter multiple types of records.
Technologies: MongoDB, Meteor, React, HTML, GraphQL, Figma

Shopify Developer

2017 - 2017
TopGlas GmbH
  • Completely rewrote the shop's main selling point, Konfigurator, to use a more modern state machine pattern (similar to Redux).
  • Developed a page composer tool that transforms Shopify store CMS functionality into a drag-n-drop interface for building-block style construction of shop pages.
  • Optimized JS and CSS delivery, which reduced page size and increased page download speeds by ~50%.
  • Improved "Add to cart" speed by 6x.
  • Implemented custom user click tracking.
  • Implemented user account pages where the user can manage orders, addresses, and other account information.
  • Performed SEO audits.
Technologies: Liquid, Shopify, JavaScript, SCSS, HTML, Shopify Customizations, Shopify Theme, Custom Shopify Apps, Shopify Plus, Tailwind CSS, Shopify Design, eCommerce, GraphQL, Figma

Shopify Developer

2016 - 2016
Sitar s.r.l.
  • Built a new Shopify experience from the ground up.
  • Migrated existing shop data from the old PrestaShop installation.
  • Integrated a subscriptions app with "unsupported" QuickView - made it work.
Technologies: Liquid, Shopify, JavaScript, SCSS, HTML, Shopify Customizations, Shopify Theme, Shopify Design, eCommerce

Full-stack Ruby on Rails Developer

2015 - 2016
  • Rebuilt the sign-up flow into a single-page multi-step React form that saw a 5x increase in conversion.
  • Introduced React and Redux technologies to the company.
  • Wrote an open-source SEO plugin for Ruby on Rails apps.
  • Introduced a Shopify theme development template for faster theme development.
  • Built several custom Ruby on Rails apps from the ground up.
Technologies: PostgreSQL, Heroku, RSpec, Redux, React, Ruby on Rails (RoR), Ruby, Shopify, Shopify Customizations, HTML, Liquid, Shopify Theme, Shopify Plus, Shopify Design, eCommerce

Software Engineer

2014 - 2015
Microsoft Ireland R&D
  • Integrated a TypeScript testing framework into the build system to improve development reliability.
  • Implemented a website redesign that increased the number of installs by over 5% (+20,000 users).
  • Worked on the organization of a company-wide hackathon (120,000 employees, more than 100 hackathon projects, worldwide event).
  • Nominated as employee of the year after the first ten months of employment.
Technologies: jQuery, JavaScript, TypeScript, C#, HTML

Full-stack Ruby on Rails Developer

2013 - 2013
Music Delta
  • Reduced the number of database queries by 80%.
  • Implemented a sustainable authorization system.
  • Fixed security issues and other various bugs.
  • Completely rewrote the administration interface, improving speed, usability, and security.
  • Added application monitoring.
Technologies: Ruby on Rails (RoR), HTML

Full-stack Ruby on Rails Developer

2012 - 2013
Do the Life
  • Developed a highly scalable solution for generating user schedules with a heuristic algorithm.
  • Integrated the Ruby on Rails application with a Java worker to improve the algorithm's performance.
  • Implemented a natural language parser, allowing users to describe events in natural language.
  • Implemented a tutorial/website tour for first-time login and introduction to new features.
  • Handled cross-time zone event synchronization.
  • Implemented a "Coming Soon" landing page with user sign-up.
  • Implemented a cron process to handle recurring jobs with periodic execution.
Technologies: Ruby on Rails (RoR), HTML

Software Developer

2010 - 2013
Sitar s.r.l.
  • Developed several custom-made pharmaceutical applications to perform diagnosis predictions.
  • Created a specific user-management application for managing user licenses.
  • Gained experience working with a foreign client.
  • Implemented a completely automatic update system with no need for user action when new data or application versions were made available.
  • Created specific database management application for pharmacists to insert data with a single click.
  • Built a dynamic GUI with vector graphics to function appropriately across all screen resolutions.
Technologies: Java

Software Developer

2011 - 2011
Netgen d.o.o.
  • Created an API to integrate the ShoutEm platform with eZ Publish CMS.
  • Worked with eZ Publish CMS internals for better tailoring of content to ShoutEm users.
  • Gained significant experience with the eZ Publish CMS and ShoutEm API.
Technologies: JavaScript, PHP, HTML

Do The Life

Do The Life is a Ruby on Rails virtual secretary web application.

GitHub Account
A collection of code samples and contributions to open-source projects.

Liceknjiga is a Ruby on Rails social network for sharing photos with privacy, likes, comments, and friends functionality. For more information, see

A TODO application which demonstrates how one might connect AngularJS with a Ruby on Rails back-end for authentication and usage. Additionally, includes an API for mobile application access.

A simple Twitter clone written in Ruby on Rails.

Water Logic
Solution for pouring water game for web, cross-compiled to JS from C++ with Emscripten.


2011 - 2013

Master's Degree in Computer Science

University of Zagreb - Zagreb, Croatia

2007 - 2011

Bachelor's Degree in Software Engineering

University of Zagreb - Zagreb, Croatia

2009 - 2010

Bachelor's Degree in Software Engineering

University of Maribor - Maribor, Slovenia


Liquid, React, Shopify API, Chai, jQuery


RSpec, Mocha, Shopify Plus, Figma, Sublime Text, Git, GitHub


ZURB Foundation, Jasmine, Redux, Bootstrap, Ruby on Rails (RoR), Next.js, Tailwind CSS


HTML, JavaScript, Sass, GraphQL, PHP, Java, C#, TypeScript, Ruby, SCSS, CSS


Agile Software Development


Meteor, Shopify, Heroku, WordPress, MacOS, Visual Studio Code (VS Code), DigitalOcean


MongoDB, PostgreSQL, Hasura


Apps, Shopify Customizations, Shopify Theme, Custom Shopify Apps, Shopify Design, eCommerce

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