Senior JavaScript / TypeScript Developer

Job Title: Senior JavaScript / TypeScript Developer – UI (AI Interfaces) & Excel Add-in Location: London - (Hybrid) Type: Full-time About Us: Cornspring is a dynamic start-up developing Family Office services through a unique combination of innovative technology and data management excellence. Our platform leverages state-of-the-art generative AI and large language models to provide unparalleled insights and efficiency to Family Office investment and accounting data. We pride ourselves on fostering a collaborative and inclusive work environment where we prize creativity, excellence in engineering, a powerful customer focus and attention to problem-solving through great design. Job Description: We are seeking an experienced Front-end Developer to join our team, focusing on building our Excel Add-in solutions and web portal platform. You will be responsible for developing sophisticated data manipulation interfaces using TypeScript within the Microsoft Office Add-in ecosystem while also creating intuitive web applications where clients interact with their data using AI. This role requires deep understanding of data visualization, REST API integration, and responsive design principles. You will work closely with our back-end team to ensure seamless integration with our AWS services while delivering exceptional user experiences in both Excel and our React-based web portal. The Role We’re looking for a Senior JavaScript / TypeScript Developer who thrives at the intersection of elegant UI development and advanced Excel Add-in engineering. You’ll lead development on two core user-facing components: Our AI-driven web platform, built using JavaScript, React and TypeScript Our Excel Add-in, deployed internally within Microsoft 365, integrating directly with our platform APIs This role is perfect for someone who enjoys building robust user experiences that bridge traditional productivity tools (like Excel) with modern, responsive web apps—and who is excited by the opportunity to work with AI and financial data. Key Responsibilities Design, build, and maintain high-performance Excel Add-ins using TypeScript and Office.js, tailored for internal Microsoft 365 deployment Develop and scale web interfaces our AI solutions, data visualization, and reporting Collaborate with backend engineers to connect APIs, manage state, and streamline data integration Ensure excellent UX in both browser and Excel environments, with intuitive, performant, and consistent interfaces Contribute to architectural decisions, component libraries, and development best practices Support cross-team coordination around security, testing, deployment, and CI/CD pipelines What We're Looking For Professional front-end development experience with JavaScript, React and TypeScript Strong expertise in modern UI development practices Proven experience building and deploying Office Add-ins, especially within Microsoft 365 for internal enterprise use Deep knowledge of the Excel JavaScript API, object models, and data handling within workbook constraints Comfortable integrating with REST APIs, handling authentication flows (e.g., OAuth), and managing async data interactions Strong understanding of front-end performance optimization, state management, and cross-environment compatibility Experience working in Agile teams, with a proactive and ownership-driven mindset Bonus Points For Experience with data-intensive applications or financial tools Familiarity with data visualization libraries (e.g., D3.js, Chart.js, Recharts) Knowledge of Microsoft Graph API and M365 deployment configurations Exposure to AWS services (Lambda, API Gateway, Cognito, etc.) Understanding of AI and LLM patterns for chat, collaboration or search interfaces What We Offer Work with cutting-edge AI and data technologies. Collaborate with a small, high-calibre, mission-driven team Opportunities for professional growth and development. Flexible work hours and remote work options. Engaging and challenging projects that make a real impact to our sector. Fast-paced, deeply impactful work with real users and rapid iterations Important Application Criteria To ensure we make the best use of your time and ours, please review the following before submitting your application. This role is suited to candidates who: Are based in the UK and able to work effectively with a London-based team Have the unrestricted right to work in the UK – note that we are currently not able to offer visa sponsorship Bring proven experience in both modern UI development and Excel Add-in engineering in data-centric or enterprise environments Operate confidently at a senior level, capable of contributing to architectural decisions and working autonomously in a fast-moving setting Candidates passing initial review will be asked to undertake a skills assessment We value thoughtful, relevant applications and will prioritise those that clearly align with the requirements outlined above. If this sounds like the right fit for your background and ambitions, we’d be delighted to hear from you. How to Apply Send us your CV and (optionally) GitHub/portfolio, along with a short note on your experience with Excel Add-ins, UI development, and what excites you about this role.