Senior Frontend Engineer, Activation and Engagement Web (Growth)

Block is one company built from many blocks, all united by the same purpose of economic empowerment. The blocks that form our foundational teams — People, Finance, Counsel, Hardware, Information Security, Platform Infrastructure Engineering, and more — provide support and guidance at the corporate level. They work across business groups and around the globe, spanning time zones and disciplines to develop inclusive People policies, forecast finances, give legal counsel, safeguard systems, nurture new initiatives, and more. Every challenge creates possibilities, and we need different perspectives to see them all. Bring yours to Block.

The Role

The Activation and Engagement Web team is a critical part of Square's Growth Product organization, with a mission to help sellers start, run, and grow their businesses with Square. As part of the Growth strategy to Simplify, Automate, and Graduate seller experiences, we build and maintain the frontend experiences and supporting infrastructure that guide sellers from signup through their first payment and into ongoing product engagement.

We enable both end-to-end seller journeys and the platform capabilities that empower other teams to build exceptional web experiences—through shared components, frameworks, and extensible architecture. Our focus is on automating complex workflows and removing friction from key seller moments, with a direct impact on Square’s growth initiatives.

We’re looking for a Senior Frontend Engineer (L6) to help us scale our platform, deliver high-impact features, and mentor fellow engineers. You’ll contribute to and evolve the technical vision for the Activation Web platform, defining best practices within the Activation Web team and collaborating with adjacent Growth squads. This role blends team leadership with hands-on execution to shape the systems and experiences that power millions of seller interactions.

You Will

• Design and implement user-facing features and frontend architectures at scale primarily using React and Next.js, and occasionally using Ember and Svelte.
• Build and maintain performant, accessible, and reliable web applications.
• Own the full feature lifecycle - from requirements gathering through implementation, testing, deployment, and monitoring.
• Contribute to shared component libraries and frameworks that accelerate development across Growth teams.
• Collaborate with backend, mobile, and design teams to prototype and iterate on new seller experiences.
• Lead team-level architecture improvements (e.g., Ember deprecation), championing performance, maintainability, and accessibility best practices.
• Design and build scalable product infrastructure and platform capabilities that support both our team and broader engineering teams, with a focus on solving problems at the platform level beyond individual user-facing features.
• Participate in incident response and root-cause analysis for production issues.
• Mentor and coach mid-level and junior engineers through code reviews and pair-programming.
• Foster a culture of technical excellence, inclusion, and high-velocity execution.
• Explore and apply emerging technologies - including AI-assisted tooling to improve engineering workflows and seller experience.

You Have

• 8+ years of professional experience with JavaScript/TypeScript and modern web frameworks (React, Next.js).
• Hands-on experience building and operating large-scale web applications with dozens of active contributors.
• Proven ability to lead large projects from start to finish, with strong collaboration, communication, and ownership.
• Deep expertise in performance tuning, accessibility compliance, and frontend observability (monitoring, SLOs, SEV response).
• Proven track record of owning production services and driving operational excellence (SLOs, monitoring, SEV response).
• Strong component-driven development skills and familiarity with design-system tooling (e.g., Storybook).
• Excellent communication and collaboration skills, with experience working on cross-functional teams in a fast-paced environment.
• Familiarity with cloud infrastructure (AWS, Kubernetes) and CI/CD pipelines for frontend deployments.
• Curiosity about emerging technologies and a practical mindset for applying them in system design and workflow improvement.

Even Better

• Experience in growth or activation engineering: onboarding flows, A/B testing, and conversion funnel instrumentation.
• Data engineering and analytics proficiency, including advanced SQL querying, statistical analysis, and the product intuition to turn insights into impactful features.
• History of owning and evolv