Klas Mellbourn
Verified Expert in Engineering
Full-stack Developer
Stockholm, Sweden
Toptal member since October 14, 2022
A full-stack developer, Klas has worked professionally since 1994. For the last six years, he has worked at Klarna, where he has participated in developing their app from the start and is ranked third on the list of most commits to the Klarna app monorepo out of over 500 developers and three million lines of code in TypeScript. Klas has worked with React, React Native, and Node.js at Klarna and was one of the people who introduced TypeScript and GraphQL. He loves the challenge of freelancing.
Portfolio
Experience
- JavaScript - 13 years
- Git - 11 years
- TypeScript - 10 years
- React - 6 years
- React Native - 6 years
- Redux - 6 years
- Node.js - 6 years
- GraphQL - 4 years
Preferred Environment
TypeScript, React Native, Front-end
The most amazing...
...thing I've done is being ranked third on the list of most commits to a monorepo with over 500 developers and seven million lines of code for a Klarna app.
Work Experience
Senior Full-stack Developer
Klarna
- Worked as part of the team developing Klarna Checkout, a globally deployed high-traffic payment application handling millions of transactions.
- Worked as part of a team developing the Klarna app with 150+ million users across iOS, Android, and web. Developed full-stack solutions using React, TypeScript, and Node.js in performance-critical systems.
- Used AI-assisted development workflows extensively.
Architect and Senior Developer
FirstVet
- Architected the first React Native-based FirstVet app from the ground up. It is an internationally leading provider of app-based pet care.
- Initiated and led the development of the app. Created the design and template for the app.
- Orchestrated multiple teams in the modular development of this large-scale app.
Senior Full-stack Developer
Klarna
- Developed a Klarna checkout as a web developer, app developer, and Node.js developer. The checkout is the web application of Klarna, which handles end-user payments for millions of users and thousands simultaneously worldwide.
- Developed a Klarna app, which is the end user app with 150 million users on iOS, Android, and the web.
- Introduced new technologies like TypeScript and GraphQL. I advocated for these, built prototypes, and developed the final implementation.
Web Architect | Developer
H&M
- Created an SPA from scratch and led the development of this. Was the first developer in a team that grew to 40 developers.
- Introduced Angular, TypeScript, Node.js and Git. Had to teach and spread knowledge of these technologies. Held many courses in Git.
- Built a continuous deployment environment, including automated unit tests, integration tests, and code coverage of both JavaScript and .NET using Node.js, TeamCity, and Octopus.
Experience
Klarna App
http://app.klarna.comFirstVet
https://apps.apple.com/us/app/firstvet/id1349241101RaySearch Laboratories
https://www.raysearchlabs.com/Education
Master of Science Degree in Engineering Physics
KTH Royal Institute of Technology - Stockholm, Sweden
Certifications
Unsupervised Learning, Recommenders, Reinforcement Learning
Stanford University
Advanced Learning Algorithms
Stanford University
Supervised Machine Learning: Regression and Classification
Stanford University
MCSA: Web Applications
Microsoft
Skills
Libraries/APIs
React, Node.js, RxJS, Reactive Extensions, jQuery, React Query
Tools
Git, GitHub, Helm, Bitbucket, Jenkins, NPM, Webpack, Lerna, Zsh, Splunk, Nx, CircleCI, Redux Toolkit, RTK Query, Fastlane, Yeoman, Bower, Grunt, Gulp, Karma, Visual Studio, TeamCity, ReSharper, NuGet, ChatGPT, Claude, Claude Code, Figma, Cucumber
Languages
TypeScript, JavaScript, GraphQL, Ruby, SCSS, CSS, HTML, C#, Sass, Less, ECMAScript (ES6)
Frameworks
React Native, Redux, Jest, Tailwind CSS, Windows Presentation Foundation (WPF), .NET, AngularJS, Protractor, Bootstrap, ASP.NET Core, ASP.NET MVC, ASP.NET Web API, Windows PowerShell
Paradigms
Test-driven Development (TDD), Automation, Mob Programming, Kanban, App Development, Model View Presenter (MVP), Dependency Injection, REST, Scrum, Pair Programming
Platforms
Visual Studio Code (VS Code), Docker, Kubernetes, Amazon Web Services (AWS), iOS, Android, MacOS, Web, Apache Kafka, Opsgenie
Storage
Redis
Other
Web App Development, Klarna, APIs, AI-generated Code, Full-stack Development, eCommerce, Mobile First, Architecture, Front-end, Computer Science, Full-stack, Most.js, Storybook, Scripting, Remote Work, Web Development, Chrome Extensions, Octopus Deploy, Windows Communication Foundation (WCF), Async/Await, MyGet, CI/CD Pipelines, Cursor AI, AI Copilots, Galen Framework, iPhone, Machine Learning, Artificial Intelligence (AI), Neural Networks, AI Agents, AI Integration
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