CV / Resume

Jörn Zaefferer
Sofware Engineer

With over 13 years of building web applications, I’m an expert for leading development teams that create and operate content on the web.

I am based in Köln (Cologne), Germany; I’m good at working remotely.
Contact me via E-Mail.

Below you find lists of skills, work and other experience. I’ve also written down some expectations.


  • Semantic & accessible HTML. Modern CSS & JavaScript.
  • Operating Node.js services in Docker containers
  • Technical leadership with business needs in mind.
  • Conception, development, testing, and operation of web apps.
  • Strong understanding of design, user experience, and usability.
  • Conscious of accessibility, internationalisation and performance.
  • Engineering recruitment.
  • Fluent in English and German, a little Bulgarian

Professional Experience

Boxine GmbH, Düsseldorf
Senior Software Developer, Frontend Tech Lead
April 2020 – today
Currently working on meinetonies. GmbH, Köln
Head of Development
May 2015 – March 2020
What started as a frontend engineering position turned into the Head of Development, where I was responsible for all software development efforts. This still included the React-based single-page web application for composing, deploying, maintaining and monitoring Docker containers. My role was expanded to the backend that integrates all container hosting, user management and subscription services. On top came various additional services, for example for automatically creating and renewing customer SSL certificates using Let’s Encrypt. This was rounded off with customer (via Intercom) and operations support.

Freelance consulting developer
Consulting for over 20 clients, including SoundCloud, Software Freedom Conservancy, jQuery Foundation, adesso AG, Wimdu, eBay, AOK, CloudControl, AutoScout24, Denkwerk, Pixelpark, Sevenval
Dec 2010 – Apr 2015
The end of my time as a Java developer and the start of my freelance career. The first year kicked of with projects for SoundCloud (building their then-new mobile site) and the Software Freedom Conservancy (working on a grid component for jQuery UI). A common theme for these and future projects were single-page applications (SPA), back when very few libraries and frameworks existed to build entire applications in the browser. Backbone and requirejs were early tools of choice.

maxence integration technologies GmbH, Köln
Senior Consultant
Feb 2007 – Nov 2010
Developing web and desktop applications in Java for clients in banking and insurance sectors. First dev lead position for an attempt at a “replace Excel” in-house product.

TimeToAct GmbH, Köln
Apr 2006 – Jan 2007
After finishing my study at the bib International College in Bergisch Gladbach, this was the start of my professional career, mostly developing web applications in Java. JavaScript mattered very little in these projects, yet in that time I started using and contributing to jQuery in my free time.

Volunteering and extraprofessional experience

Apr 2019 – today
Making the city more bicycle friendly.

JavaScript User Group Köln
2011 – 2014
My personal highlight was the meetup in December 2012, when Felix Geisendörfer did a live demo of nodecopters, including a big quad drone. Probably the biggest audience we ever had.

Open-Source JavaScript libraries
Developer, project manager, team lead
2006 – today
I was one of the first contributors to jQuery and started developing various jQuery Plugins early on. Some were later turned into components of jQuery UI, where I was dev lead for several years. The jQuery Validation plugin is still very popular these days, running one 1m+ websites. I created QUnit, a JS testing library and shaped Globalize, a library for dealing with internalization, which in turn paved the way for distributing CLDR to JavaScript developers.

jQuery Foundation
Board of Directors
2009 – 2015
What started as a tiny open-source JavaScript library became one of the most popular libraries on the web. In 2009 the jQuery Foundation was formed, originally as a project of the Software Freedom Conservancy (SFC). When the board of directors was formed, I got to be on the original board and served there until 2015, including the transition to a standalone Foundation, independent of the SFC.


Beyond software development, I like to be involved with technical architecture, recruitment, company culture, and mentoring. Therefore I am particularly looking for a role as a technical team lead.


An open and diverse environment is critical to me. In my experience, autonomous people and cross functional-teams working through consensus and helping each other, bring the best results.

Work-life balance

I am ok with putting in a few extra hours on rare occasions, or getting paged at night when a critical system is failing. But regular over-time is not something I do, and must be addressed if recurring. I value flexible working hours and remote work from home.

Shared Vision

I work and contribute more effectively when the company shares as much of their long-term intentions as possible. It prevents misunderstandings, leads to better technical decisions, and encourages a trusting work environment.