Senior Developer

This role is being posted on the Critical Mass job board on behalf of Tribal Worldwide, part of Omnicom Group's DDB Worldwide. The successful candidate will join Tribal Worldwide as an employee.

Tribal is a creative force that blends strategy, design, and technology, and we are on the hunt for a talented and experienced Senior Developer who is equal parts craftsperson, collaborator, and tech trailblazer to elevate our London-based team! As a Senior Front-End Developer (SFED), you’ll help shape digital experiences from bold marketing sites to interactive web applications, briging both creative flair and technical excellence to everything you touch and the code you produce.

In this role, you will work with a team of creative individuals while also collaborating closely with your Tech Lead to provide insightful solutions and accurate estimates for your tasks. You’ll immerse yourself in a diverse array of front-end technologies, including REACT, HTML5, CSS, and JavaScript, and you’ll be right at home with content management systems.

Our new hires & employees are the future of our organization, and we want to set you up for long-term success. In an effort to do so, we expect our team to work from an office a minimum of 3 days a week.

You will:

• Develop web applications that follow the latest web standards and conform to leading-edge front-end technologies


• Work closely with the Creative, Business, Technology, and QA teams to ensure the solution meets the requirements and performance targets (mobile and desktop).


• Own the development of web applications and ensure they follow the overall technical architecture and design established by the technical lead


• Implement user and business requirements, which are composed of multiple development activities


• Maintain up-to-date and complete documentation on projects as dictated by the capability’s guidelines and best practices


• Debug or troubleshoot complex issues in existing web applications


• Proactively provide feedback on policies and procedures when an opportunity for improvement exists


• Proactively provide guidance and support to other members in fulfilling their tasks and improving their performance and overall technical skills.

You have: 

• 5+ years of professional front-end development experience with expert-level React and JavaScript/TypeScript proficiency


• Strong understanding of modern CSS (CSS Modules, styled-components) and solid grasp of web fundamentals (semantic HTML, responsive design, performance optimization)


• Experience with modern build tools and bundlers (Vite, Webpack, etc.), proficiency with Git and CI/CD workflows (GitHub Actions/Workflows)


• Testing experience (unit tests, integration tests, e2e tests with tools like Jest, React Testing Library, Cypress/Playwright)


• Accessibility best practices (WCAG, ARIA) and SEO fundamentals


• Experience integrating frontend with external APIs and backend services


• Familiarity with data formats (JSON, REST, GraphQL), knowledge of authentication and secure data handling


• Experience with Supabase or similar backend-as-a-service platforms


• State management experience (XState, Zustand, or similar libraries)


• Component library or design system work


• AWS or similar cloud platform familiarity


• Experience mentoring junior developers

You may have:

• Experience in Python


• Proficiency with LLMs, Pydantic, LangChain/LangGraph or similar orchestration tools


• Familiarity with AEM

Critical Mass is an equal opportunity employer. 

Critical Mass uses artificial intelligence in our recruitment process to enhance job postings, filter keywords during the review of prospective candidates, and, in some cases, transcribe interviews with our recruiters. Human review remains central to the process, and all hiring decisions are ultimately made by our team.

The Critical Mass Talent Acquisition team will only communicate from email addresses that use the URLs criticalmass.com, omc.com and us.greenhouse-mail.io. We will not use apps such as Facebook Messenger, WhatsApp, or Google Hangouts for communicating with you. We will never ask you to send us money, technology, or anything else to work for our company. If you believe you are the victim of a scam, please review your local government consumer protections guidance and reach out to them directly.

If U.S. based: https://www.consumer.ftc.gov/articles/job-scams#avoid
If Canada based: https://www.canada.ca/en/services/finance/consumer-affairs.html
If U.K. based: https://www.gov.uk/consumer-protection-rights
If Costa Rica based: https://www.consumo.go.cr/educacion_consumidor/consejos_practicos.aspx