CV / Resume

Jörn Zaefferer
Sofware Engineer

With over 17 years of building frontend applications, I’m an expert for leading development teams that create and operate content on the web and mobile.

I am based in Köln (Cologne), Germany; I work from home.
Contact me via E-Mail.

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


  • building web and mobile applications with JavaScript (web, React-Native)
  • optimizing applications for performance, accessibility, usability and internalization
  • operating and observing services (nodejs, Docker, Sentry)
  • designing and optimizing team processes for fast iterations, flow and continuous delivery (Kanban; Team Topologies)
  • publishing and managing open-source projects on GitHub
  • strong technical writing (documentation, ADRs)
  • recruiting engineers for agile product teams
  • Fluent in English and German, some Bulgarian

Professional Experience

tonies GmbH, Düsseldorf
Staff Engineer
April 2023 – today
These days I’m working on the mytonies-app (iOS, Android), built with React Native. We’re doing continuous delivery with weekly releases and heavy use of feature flags (via ConfigCat). We use Bitrise for running our Jest and Detox tests and deploying builds to the App and Play stores.

Engineering Manager
January 2021 – March 2023
Technical leadership and manager for a group of engineers, first for the team working on mytonies-web (see below), later the mytonies app (see above). A big part of this was recruiting, where I also helped document and refine our process.

Senior Software Developer, Frontend Tech Lead
April 2020 – December 2020
At first I worked on mytonies-web, where tonies customers can set up and customize their Toniebox, upload content to their Creative Tonies and shop for more digital content, a feature I helped launch in the summer of 2020, along with the market launch in the US. Part of this was also replacing LaunchDarkly, who’s business model and architecture no longer matched our needs, with ConfigCat, which has proofed to be an excellent choice for a feature flag service in the following years. 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 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 and operations support.

Freelance consulting developer
Consulting for over 20 clients, including SoundCloud, Software Freedom Conservancy, jQuery Foundation, adesso, 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 off 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.

Between 2011 and 2015 I spent half my time contracting with the jQuery Foundation, working on open source projects such as jQuery UI, QUnit and Globalize (more details under ‘Open-Source JavaScript libraries’ below). I twice participated in Google’s Summer of Code, mentoring students working on projects related to QUnit.

maxence integration technologies GmbH, Köln
Senior Consultant
Feb 2007 – Nov 2010
Developing web and desktop applications in Java for clients in banking and insurance. My 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, and expanded that until 2011 (see above).

Volunteering and extraprofessional experience

Väter in Köln – Vätercafe
January 2023 – today

I’m volunteering for the organization of brunches for dads and their kids, usually groups of 4 to 8 dads with one or two kids each. Saturday morning is more accessible for working dads than those comparable meetups during the week.

Apr 2019 – today
Making the city more bicycle friendly. The most important format is the “Runder Tisch Radverkehr”, where I’m consulting with others from ADFC, directly to both the local district politicians (Bezirksvertretung Kalk) and the municipality (Verwaltung, Büro des Radverkehrsbeauftragten).

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 – 2016
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 popular these days, running on one 1m+ websites. I created QUnit, a JavaScript 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 (now the OpenJS 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, mentoring and coaching. I love pair programming, to share knowledge (in both directions), skip separate code review steps and collect data on developer tooling that needs improvements. I am looking for a role as a senior/staff engineer or engineering manager.


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 okay 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 want to primarily 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.