Klas Mellbourn, Developer in Stockholm, Sweden
Klas is available for hire
Hire Klas

Klas Mellbourn

Bio

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

Klarna
TypeScript, JavaScript, React, React Native, Node.js, Cursor AI...
FirstVet
React Native, TypeScript, Nx, Lerna, GitHub, CircleCI, Tailwind CSS, Redux...
Klarna
TypeScript, JavaScript, React, React Native, Front-end, Full-stack, Node.js...

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

2024 - PRESENT
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.
Technologies: TypeScript, JavaScript, React, React Native, Node.js, Cursor AI, Visual Studio Code (VS Code), AI Copilots, ChatGPT, Claude, Claude Code, Redux, React Query, REST, GraphQL, Amazon Web Services (AWS), Figma, Jest, Storybook, Git, GitHub, Jenkins, RxJS, Most.js, ECMAScript (ES6), Kubernetes, Helm, Docker, Apache Kafka, Ruby, Galen Framework, Cucumber, NPM, Webpack, Lerna, SCSS, Test-driven Development (TDD), Zsh, Scripting, Splunk, Opsgenie, Kanban, Scrum, iOS, iPhone, MacOS, Android, Remote Work, Pair Programming, HTML, Full-stack Development, Mobile First, AI Agents, AI Integration

Architect and Senior Developer

2022 - 2024
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.
Technologies: React Native, TypeScript, Nx, Lerna, GitHub, CircleCI, Tailwind CSS, Redux, Redux Toolkit, RTK Query, Storybook, Docker, APIs, eCommerce, AI-generated Code, Automation, Full-stack Development, Mobile First, Architecture

Senior Full-stack Developer

2016 - 2022
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.
Technologies: TypeScript, JavaScript, React, React Native, Front-end, Full-stack, Node.js, RxJS, Most.js, Redux, Kubernetes, Helm, GraphQL, Mob Programming, Git, GitHub, Bitbucket, Jenkins, Visual Studio Code (VS Code), Redis, Docker, Amazon Web Services (AWS), Storybook, Jest, Ruby, NPM, Webpack, Lerna, SCSS, Test-driven Development (TDD), iOS, Android, MacOS, Scripting, Zsh, Splunk, Kanban, Remote Work, Chrome Extensions, Klarna, APIs, eCommerce, AI-generated Code, Automation, Full-stack Development, Mobile First

Web Architect | Developer

2013 - 2016
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.
Technologies: AngularJS, JavaScript, TypeScript, Node.js, NPM, Webpack, Yeoman, Bower, Grunt, Gulp, Karma, Protractor, Reactive Extensions, RxJS, jQuery, HTML, CSS, Sass, Less, Bootstrap, .NET, ASP.NET Core, ASP.NET MVC, ASP.NET Web API, Visual Studio, Git, TeamCity, Octopus Deploy, Test-driven Development (TDD), Windows Communication Foundation (WCF), ReSharper, Async/Await, NuGet, MyGet, Windows PowerShell, CI/CD Pipelines, Automation, Full-stack Development

Experience

Klarna App

http://app.klarna.com
An end user app with 150 million iOS, Android, and web users. I participated in the development of their app from the start, and I am currently in third place on the list of most commits to the app monorepo out of over 500 developers and three million lines of TypeScript code.

FirstVet

https://apps.apple.com/us/app/firstvet/id1349241101
A React Native-based app for pet owners to get online access to pet care, both information and virtual meetings with real veterinarians. I architected a technical solution for this app, including a choice of technologies, architecture, design patterns, test automation, continuous integration, and continuous deployment. Also, I orchestrated the cooperation of multiple teams so that they could work together on the large-scale development of this app. Finally, I actively participated in developing significant parts of the app.

RaySearch Laboratories

https://www.raysearchlabs.com/
An advanced cancer treatment system that analyzes the three-dimensional structure of organs in humans to optimize radiation treatment. I developed the graphical user interface (GUI) in a team that regularly delivered quality-controlled medical software to international customers.

Education

1989 - 1994

Master of Science Degree in Engineering Physics

KTH Royal Institute of Technology - Stockholm, Sweden

Certifications

MARCH 2024 - PRESENT

Unsupervised Learning, Recommenders, Reinforcement Learning

Stanford University

MARCH 2024 - PRESENT

Advanced Learning Algorithms

Stanford University

FEBRUARY 2024 - PRESENT

Supervised Machine Learning: Regression and Classification

Stanford University

SEPTEMBER 2016 - PRESENT

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

Collaboration That Works

How to Work with Toptal

Toptal matches you directly with global industry experts from our network in hours—not weeks or months.

1

Share your needs

Discuss your requirements and refine your scope in a call with a Toptal domain expert.
2

Choose your talent

Get a short list of expertly matched talent within 24 hours to review, interview, and choose from.
3

Start your risk-free talent trial

Work with your chosen talent on a trial basis for up to two weeks. Pay only if you decide to hire them.

Top talent is in high demand.

Start hiring