Saša Šijak, Developer in Belgrade, Serbia
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Saša Šijak

Verified Expert  in Engineering

Full-stack Developer

Belgrade, Serbia
Toptal Member Since
January 3, 2017

Saša has more than ten years of professional software engineering experience under his belt. He also has strong analytical skills, a wide range of computer expertise, and a history of successful project outcomes. All in all, Saša is organized, attentive-to-details, and a meticulous self-starter who's able to organize and prioritize multiple tasks and stay calm and productive under pressure.


Pwnalot LLC
Next.js, Firebase, Web Analytics, React, Styled-components, Stripe...
DigitalOcean, PostgreSQL, Spring Boot, CSS, HTML, Docker, Cordova, MongoDB...
Geek (Sole Proprietorship)
CSS, HTML, Gradle, Kotlin, Spring, Java, Vue, Gatsby, GraphQL, React




Preferred Environment

Slack, Jenkins, YouTrack, Jira, Git, Linux, IntelliJ IDEA

The most amazing...

...project I've done was working with a team that made a distributed search engine which was bought by Facebook.

Work Experience

Full-stack Engineer

2020 - PRESENT
Pwnalot LLC
  • Developed a website to sell beautiful and responsive UI blocks made with ChakraUI and Framer Motion with code export to NextJS or GatsbyJS:
  • Created a website with a purpose of a digital will. Created switches, how often should service checkup on you and if you become unavailable it sends your predefined messages.
  • Handed all sides of the business: registration, taxes, marketing, sales, project implementation, and more.
Technologies: Next.js, Firebase, Web Analytics, React, Styled-components, Stripe, Coinbase API, Mailgun, Twilio API, Mixpanel, Google Tag Manager, Facebook Pixel, Cloud Firestore, Firebase Web SDK

Freelance Full-stack Engineer

2017 - PRESENT
  • Implemented public-facing websites for the most popular app which connects classrooms with parents and engage kids Classdojo:,,
  • Developed mobile apps for one of the largest car manufacturers.
  • Implemented a website in React/Gatsby that was featured on Adidas's homepage and is a heavily marketed campaign for sports coaches to teach them how to approach and keep girls in sport (
  • Worked in a team that created an engagement platform designed for education (Quadwrangle). My responsibilities were a front end in AngularJS, back-end API in Play framework, and creating Docker-based deployments.
  • Implemented a web app from scratch for a small business in a timber processing domain. Previously they organized and tracked their business by using large Excel tables and files, shared through email and Dropbox.
  • Built a website for a school with GatsbyJS and backed by a headless CMS Contentful (
Technologies: DigitalOcean, PostgreSQL, Spring Boot, CSS, HTML, Docker, Cordova, MongoDB, Linux, SQL, TypeScript, JavaScript, Vue, React, Kotlin, Java, Next.js, Gatsby, Contentful

Owner | Software Engineer

2016 - PRESENT
Geek (Sole Proprietorship)
  • Created the website from scratch using GatsbyJS, React, GraphQL, Netlify CMS, and hosted it on Netlify; also implemented the SEO and marketing campaigns.
  • Developed a multiplayer and multiplatform soccer game.
  • Built a multiplayer turn-based browser game implemented using Vue.js and Firebase platform.
  • Created an open-source Kotlin-based startup template.
Technologies: CSS, HTML, Gradle, Kotlin, Spring, Java, Vue, Gatsby, GraphQL, React

Software Engineer

2013 - 2019
  • Pushed for and led the implementation of a new land-based casino system that replaced a legacy one—resulting in new business opportunities and better architecture. Used in more than 700 hundred betting shops and six countries.
  • Hacked a proprietary communication protocol for gambling machines on my own; after which we could make our own solution pushing out third parties, cutting on expenses, and opening the doors for exciting possibilities.
  • Implemented the jackpot animation and notification system. It replaced a third-party solution and enabled us to push features much faster. This app is used in every casino and it boosted profits and improved player's experience.
  • Strove continuously to improve the business and system—going beyond basic requirements and pushing the boundaries of the old enterprise system and company. Provided many proposals and solutions to more than just technical problems.
  • Implemented a betting machines/terminals project where machines are placed into betting shops or cafes. It involved integrating several hardware products, make an API, deploy the system to several hundred machines, server and machine administration.
  • Implemented a messaging system which allows our managers to interact with players and customers, and that also sends automated messages based on some configurable rules.
Technologies: CSS, HTML, Docker, Server Administration, Linux, Redis, ActiveMQ, Oracle, Spring, Vue, AngularJS, Java

Software Engineer

2017 - 2017
  • Implemented new-and-supported old features for Quadwrangle (engagement platform targeting universities).
  • Developed a new deployment system using Docker.
  • Integrated several different APIs.
Technologies: CSS, HTML, Cordova, Linux Administration, AngularJS, MongoDB, Docker, Play Framework, Java

Software Engineer | Team Lead

2012 - 2013
  • Designed the back-end architecture for a scalable, high-performing online casino system.
  • Led the team which implemented the back end from scratch. The back end was then used by various clients and tens of thousands of players making big profits.
  • Implemented various gaming providers and communicated with external providers.
Technologies: Redis, Linux, ZeroMQ, MySQL, Python

Software Engineer

2008 - 2011
  • Improved the app performance by profiling and optimizing a Java virtual machine.
  • Created the first version of the Wowd iOS app.
  • Handled QA and testing.
Technologies: Linux, Java

Associate Lecturer

2009 - 2010
Union University, School of Computing
  • Organized lectures for the operating systems class.
  • Implemented various operating system parts with students in a simulator—improving their understanding and knowledge.
  • Earned one of the best student evaluations given during my time there.
Technologies: Operating Systems, Java

Software Test Engineer

2008 - 2008
  • Built a custom testing tool for Asseco banking software.
  • Designed and wrote various test cases.
  • Organized the Asseco programming challenges in the university.
Technologies: Microsoft SQL Server, C#

Classdojo Websites
Implemented high traffic public-facing websites for the most popular app which connects classrooms with parents and engage kids: Classdojo.

The website was previously built with older technology and was in a need of a complete rewrite. I've implemented the website with React, GatsbyJS, and Styled System.

Besides the main websites, I implemented other public-facing sites and

Website for Adidas
I implemented a website that was featured on Adidas' homepage and is a heavily marketed campaign for sports coaches to teach them how to approach and keep girls in sport.

As the sole developer, I built the website using React/Gatsby along with implementing the custom styling and design.

Online Casino Management System

I designed and implemented the back-end for the online casino management system. This system was later used on many websites and had tens of thousands of users. It was very flexible and offered some crucial features that competitors did not have. It made a good profit for the clients and is still used today on many websites.

Technologies: Python, ZeroMQ, MySQL, SQLAlchemy, Redis, Linux

React UI Boutique
Developed a website to sell beautiful and responsive UI blocks made with ChakraUI and Framer Motion with code export to NextJS or GatsbyJS.

Built hundreds of ready-to-use blocks and drag and drop builder to quickly build pages and preview them.

I built it from scratch and did everything from developing it to marketing, sales, deployments, and support.

Live Man Switch
Created a website that serves a purpose of a digital will and as a protection of digital assets like crypto assets.

It is a digital version of a physical switch that is designed to be activated or deactivated if the human operator becomes incapacitated, such as through death, loss of consciousness or being bodily removed from control.

User can create switches, and configure how often should service check-up on them, over which channel (email or SMS) and if they become unavailable it sends their predefined messages.

ECFS School Website
Public-facing website for a school build with GatsbyJS and backed by a headless CMS Contentful.

I did not build the site from scratch. I inherited the project at some point because the client was not happy with the previous agency and I am now the sole developer on this website for more than a year.

Land-based Casino Management System

I led the design and implementation of a land-based casino management system which was used in the largest betting operator in Southeastern Europe. It is used in several countries to manage more than 500 betting shops and more than 3,000 gambling machines in real-time. It is used to manage everything from machine information to jackpots and everything going-on in real-time. It was presented with large success at the London ICE Totally Gaming fair.

Technologies: Java, Spring Framework, AngularJS, Oracle Database, Redis, Linux, ActiveMQ

Customer Messaging System

I implemented a messaging system; this included both the back and front end along with an integration with the rest of the system. This enabled our managers to interact with players and customers. It also sends automated messages to customers when relevant events occur.

Technologies: Java, Spring, Vue.js, ActiveMQ, Oracle, Linux

Betting Machine and Terminal

I implemented a large part of the betting machine/terminal project. These machines are placed in betting shops or cafes. It involved working with the hardware approval process, integrating several hardware products, making an API and integration with the rest of the system, deployment system to several hundred machines on random locations, server and machine administration, and so on.

Technologies: Java, Spring, Vue.js, ActiveMQ, SQLite, Oracle, Linux

Jackpot System with Roulette

With a colleague, we implemented a first-of-its-kind jackpot system used with roulette in land-based casinos in our business region of interest.

Technologies: Java, Spring, ActiveMQ, Oracle, Linux

Amazing Multiplayer Soccer Game

I worked with a team to create a multiplayer turn-based soccer game which was played by tens of thousands of players and attracted investors.

Technologies: Java, JavaScript, Vert.x

Hacked Proprietery Protocol

I hacked the proprietary protocol for communication with gambling machines by sniffing the traffic with modified serial cables and traffic analysis. Later, I wrote the server which implemented that protocol and which is successfully in use today. | Online Presence
I set up the website, Facebook page, Instagram account, the brand's whole online presence, SEO and marketing campaigns for a personal trainer's business. At first, the website was created with WordPress because of the very high deadline pressure and later I rewrote it in GatsbyJS (a React and GraphQL-based static site generator) and hosted it on Netlify. There is also a protected part of the website which is using Firebase Auth and Firestore.

Technologies: React, GatsbyJS, Firebase, WordPress, Contentful, Netlify

Flat (Apartment) Ad Scraper

I needed to find a flat (apartment) to rent and 90% of the ads on the internet were from the really bad agencies—I spent a month trying to manually find a flat.

To solve this, I wrote a program that scrapes websites with flat ads every few hours, filters out all the agency and fishy ads, and showed me all good ones in the part of the city and other parameters that I was interested in.

In the end, I found a flat in one day. I used this program again two years later and had the same result—I also found a flat in one day. Then I decided to share it with friends.

Technologies: Java

Fun Factory Apps on Android

I tested various earning methods on Google Play (Google Market before the switch), marketing strategies, and app positioning in the search results. The app earns a very good dollar return per hour due to the applied techniques.

Technologies: Java, Android


Java, HTML, SQL, JavaScript, CSS, Sass, YAML, C#, HTML5, Python, TypeScript, Kotlin, GraphQL


Spring, Spring Boot, Spring 5, Tailwind CSS, Next.js, JPA, Play Framework, Flask, AngularJS, Bootstrap, Ktor, Redux


React, Vue, Vuex, MobX, ZeroMQ, Stripe, Coinbase API, Twilio API, Firebase Web SDK, Socket.IO, JOOQ, SQLAlchemy, Google Cloud API, Emotion Library, Google Maps, Google Geocoding API


JetBrains, Terminal, Slack, Git, ActiveMQ, Apache Maven, IntelliJ IDEA, Facebook Pixel, Google Analytics, MailerLite, Figma, Docker Hub, Vagrant, YouTrack, Jenkins, Subversion (SVN), Gradle, PostCSS, JSX


Netlify, Linux, Contentful, Oracle, DigitalOcean, Mailgun, Mixpanel, Vercel, Algolia, Firebase, Kubernetes, Docker, Amazon EC2, Amazon Web Services (AWS)


Redis, H2, PostgreSQL, Cloud Firestore, Firebase Realtime Database, Spring Data, Microsoft SQL Server, Google Cloud, Memcached, Oracle 12c, MySQL, MongoDB, MySQL/MariaDB


Single-page Applications (SPA), Full-stack, Web App Deployment, Creative Problem Solving, Software Design, Gatsby, Front-end, User Interface (UI), Casinos & Gaming, APIs, Software Architecture, Server Administration, Web Development, Browsers, Data Structures, Operating Systems, Linux Administration, Cordova, Web Analytics, Google Tag Manager, Design Systems, Chakra UI, Framer Motion, Message Queues, Analytics, Administration, Cloud, Styled-components


Scrum, Design Patterns, REST, Service-oriented Architecture (SOA), Object-oriented Programming (OOP), Continuous Integration (CI), DevOps, Test-driven Development (TDD), Functional Programming, On-page SEO, Testing, Hosting

2006 - 2010

Bachelor's Degree in Computer Science

Union University, School of Computing - Belgrade, Serbia


Google Associate Cloud Engineer


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