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JC. Jonathan Claessens

Open to full stack roles & custom web projects for clients

Full stack engineering that ships and scales.

Full stack engineer specializing in React and Java/Spring Boot. I build fast, dependable web products — whether you need a custom website for your business or a complex full-stack web application.

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Portrait of Jonathan Claessens

Impact

Results I can point to

  • −90%Internal management time savedBuilt a custom React admin panel on a Spring Boot backend.
  • 2,000+Clients on the product I work on
  • 150+Features shipped, 60+ issues resolved
  • −90%Storage costs, after moving to cloud buckets
  • 20+Features on the Peppol e-invoicing integration
  • −70%Backend processing time, optimised Node.js

Experience

Where I've built things

  1. Oct 2023 – Apr 2026Ghent, Belgium

    Full Stack Software Developer

    Admisol · Accounting software company

    • Owned the Peppol e-invoicing integration and delivered 20+ features across backend and frontend, supporting teammates through onboarding.
    • Built a React admin panel on a Spring Boot backend that cut internal management time by 90%.
    • Moved document storage to cloud buckets, lowering storage costs by ~90% and improving processes for several teams.
    • Upgraded Java versions and dependencies across 7+ modules, reducing technical debt and improving stability.
    • Delivered 150+ features and resolved 60+ user-reported issues for 2,000+ clients.
    • Stayed productive in a distributed setup, including a 3-month remote period in Barcelona.
    • Java
    • Spring Boot
    • React
    • TypeScript
    • REST APIs
    • Peppol

  2. Feb – Jun 2023Barcelona, Spain

    Full Stack Developer Intern

    Mail platform company

    • Migrated a legacy AngularJS mailer application to React: 50% faster load time and less maintenance overhead.
    • Optimised the Node.js backend, cutting processing time by 70%.
    • React
    • AngularJS
    • Node.js
    • JavaScript

  3. Nov 2022 – Jan 2023Hasselt, Belgium

    Full Stack Developer, IT project

    Student team project

    • Built a webshop with Next.js and Spring Boot as part of a student team, with stable end-to-end functionality.
    • Integrated the Adyen payment platform for secure, reliable payments.
    • Next.js
    • Spring Boot
    • Java
    • Adyen

  4. Feb – Mar 2021Ehlerange, Luxembourg

    Java Developer Intern

    Software company

    • Implemented new features and updated existing functionality in Java applications, based on user needs.
    • Gained hands-on experience with enterprise development practices and team workflows.
    • Java

Skills

Tools I work with

Select a skill to highlight the roles where I used it.

Backend

  • SQL
  • PostgreSQL

Frontend

  • Angular

Tooling & process

  • Git
  • Docker
  • Agile/Scrum

Selected work

Case studies

Integration

Peppol e-invoicing integration

  • Java
  • Spring Boot
Problem
Peppol e-invoicing became mandatory by a fixed deadline, and the platform did not fully support it yet. Missing the deadline would have meant legal trouble for the company and its clients.
Technical solution
I took ownership of the integration and worked on both backend and frontend: Spring Boot services plus the legacy frontend, which was also Java-based. I delivered 20+ features and helped teammates get up to speed.
Business impact
The platform met the legal e-invoicing requirement on time, keeping the company and its clients compliant and out of legal trouble. 20+ features shipped.

Private codebase, no public demo

SaaS Admin

React admin panel

  • React
  • Spring Boot
Problem
Routine changes to internal data were slow and tedious, and often meant crafting manual API calls.
Technical solution
I designed and built an admin panel on my own: a React frontend on top of a Spring Boot backend, so those changes became a few clicks.
Business impact
Internal management time dropped by 90%: what used to take manual API calls now takes a few clicks, and the team adopted the tool happily.

Private codebase, no public demo

Migration

AngularJS to React migration

  • React
  • Node.js
Problem
A client-facing mailer application ran on legacy AngularJS, which was slow and costly to maintain.
Technical solution
I migrated it to React, built 6+ reusable components for a consistent UI, and optimised the Node.js backend behind it.
Business impact
Load time improved by 50% and backend processing time by 70%, with less maintenance effort for the team. Clients were happy with the faster, fresher application.

Private codebase, no public demo

Background

Education & languages

Education

  • Professional Bachelor of Applied Computer Science

    PXL, Hasselt, Belgium · Sep 2020 – Jul 2023

    Additional academic experience at TU Delft and UHasselt.

  • Computer Science, secondary school

    Ensorinstituut, Ostend, Belgium

Languages

  • English (Fluent)
  • Dutch (Native)
  • French (Working)
  • Spanish (Basic)

Contact

Let's work together

Hiring for a full stack role, or need a website built? Send me a short message and I'll get back to you.

jonathanclaessens34@gmail.com