Senior Developer in London

As a key member of our development team, you will independently design and implement solutions for JIRA tickets, rigorously test your work across environments, and actively contribute to stand‐ups and day‐to‐day delivery. You'll support troubleshooting efforts, review and approve pull requests, and drive code quality through enhancement and refactoring ideas shared with our Tech Leads. You will also lead smaller projects with support from technical leadership and contribute to interviewing, onboarding, and mentoring to help grow the wider team. If you're ready to lead front‐end development on our existing projects, elevate code quality, and help grow the team's skills, we'd love to hear from you.

Our new hires and 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:

• Lead the design, development, and maintenance of web applications using HTML, CSS, and JavaScript (ES6+).
• Own front‐end architecture and implementation decisions, ensuring code is scalable, maintainable, and consistent across projects.
• Turn UI/UX designs and wireframes into accessible, responsive, and cross‐browser compatible web interfaces.
• Collaborate closely with designers, back‐end developers, project managers, and other stakeholders to shape requirements and deliver high‐quality features.
• Conduct thorough code reviews, providing constructive feedback and helping to maintain coding standards and best practices.
• Mentor and support intermediate and junior developers, guiding them on technical approaches, patterns, and problem‐solving.
• Proactively identify and resolve performance issues, improving load times, responsiveness, and overall usability.
• Debug and troubleshoot complex front‐end issues.
• Promote and maintain clean, well‐documented code, and ensure project documentation is complete and up to date in line with platform guidelines.
• Stay current with front‐end technologies and best practices - and help evaluate and introduce new tools/approaches where they add value.

You have:

• 5+ years of professional front‐end development experience, including work on complex or large‐scale web applications.
• Expert‐level proficiency in vanilla JavaScript (ES6+).
• Strong, hands‐on experience with HTML5 and CSS3, including pre‐processors such as SASS/SCSS.
• Deep understanding of responsive design, cross‐browser compatibility, and mobile‐first development.
• Proven experience designing or working within scalable front‐end architectures and patterns (modular components, separation of concerns, reuse).
• Solid experience with Git and modern collaborative workflows (pull requests, branching strategies, code reviews).
• Demonstrated ability to take ownership of features from concept through to deployment and support.
• Excellent communication and collaboration skills, with the ability to work effectively across disciplines.
• Experience guiding or mentoring other developers and contributing to a positive, learning‐focused team culture.

You may have:

• Experience using jQuery for DOM manipulation and event handling, particularly in maintaining or modernizing existing codebases.
• Strong experience integrating with REST APIs, including error handling, state management, and data flow on the front end.
• Familiarity with SEO‐related front‐end practices (semantic HTML, performance, metadata, structured content).
• Hands‐on experience with front‐end build tools such as Webpack, Gulp, Grunt, or similar (configuration, optimization, troubleshooting).
• Knowledge of front‐end performance optimization techniques, such as bundling, code splitting, minification, and asset optimization.

What we offer:

• Global maternity and parental leave.
• Competitive benefits packages.
• Vacation, compassionate leave, personal/sick days, and flex days.
• Access to online services for families and new parents.
• Early Dismissal Fridays (off at 3:00 PM local time every Friday).
• 13 affinity groups.
• Internal learning and development programs.
• Enterprise‐wide employee discounts.

Critical Mass is an equal opportunity employer.

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.