Middle+/Senior Full-stack Developer (Node.js, backend-focused) | Brainer

Hi! We are Boosta, a holding tech company that creates, develops, and invests in digital businesses with global potential. Today, we collaborate with over 800 specialists from different parts of the world, and millions of users use our products and services. We are currently looking for a Middle+/Senior Full-stack Developer to join our project Brainer , which is building a comprehensive online platform for measuring, training, and systematically developing human cognitive abilities. Brainer is a digital space for those who want to think faster, deeper, and more effectively. It brings together: brain quizzes; brain games; puzzles; courses and educational modules. What will be in your focus: Work approximately in a 70% backend / 30% frontend ratio. Take significant ownership of the product’s backend development and independently handle related frontend tasks. Design and develop core server-side logic, including users, authorization, access management, products, reports, and other product modules. Evolve the architecture with regard to the growth of users, traffic, data, and integrations. Ensure the reliability of payment logic and correct handling of repeated requests, errors, and edge cases. Design and optimize database operations, identify bottlenecks, and improve system performance. Develop the product’s event-driven and analytics components to ensure that Product and Marketing teams receive accurate and reliable data. Independently implement product scenarios on the frontend in integration with APIs. Collaborate with the Product team on technical solutions by assessing risks and complexity and suggesting ways to validate product hypotheses. Improve the product’s engineering quality through code reviews, monitoring, security, integration quality, and systematic technical debt management. Participate in architectural decisions that will shape the product’s development over the next 1–2 years. You are our perfect match if: 3+ years of commercial backend development experience with Node.js and practical experience with NestJS. Strong knowledge of JavaScript and TypeScript. Good understanding of fundamental programming and software design principles, SOLID, and GRASP, with the ability to apply them in practice. Practical experience with relational databases, preferably PostgreSQL, including schema design, indexes, transactions, and query optimization. Experience designing and developing REST APIs, authorization, JWT, sessions, cookies, and secure handling of user data. Understanding of reliable server-side systems, including error handling, retries, concurrent operations, scaling, and bottleneck identification. Practical experience with React and Next.js, sufficient to independently implement the frontend part of product scenarios. Understanding of product analytics and experience integrating third-party services. Experience using AI tools in development, with mandatory verification and understanding of the generated code. Will be a plus: Experience working with high traffic, a significant number of users, or high-load systems. Experience with payments, subscriptions, billing, and recurring payments. Practical experience with caching, queues, and background task processing. Experience integrating AI APIs or developing generative AI-based functionality. Experience with CI/CD, Docker, monitoring, or cloud infrastructure. Experience launching a web product or a significant part of it from scratch. Experience working with B2C products with a high volume of paid traffic. The stages you’ll go through: Stage 1. Pre-screen. Stage 2. Technical interview. Stage 3. Code review exercise. Stage 4. Bar interview. Stage 5. Reference checks. Stage 6. Job Offer! For comfortable work, we offer: Support for your self-realization and career growth: both vertical and horizontal development, opportunities to work across different niches, and compensation for external courses and conferences. Flexible working hours (you can start your workday anytime between 8:00 and 11:00 AM Kyiv time). An office in Kyiv, coworking spaces in Lviv or Warsaw, or the option to work remotely — the choice is yours. 28 working days of paid vacation per year, up to 30 days of sick leave with medical confirmation, plus all public holidays observed. Care for your health: medical insurance, a Mental Health program, and compensation for therapy sessions. Ukrainian Victory Support program and other meaningful social initiatives. Gifts for special occasions in your life and career. Quarterly team buildings (online or offline). Equipment compensation during blackouts. Maximum support in challenging life situations. If you’re driven by the challenge of building outstanding products, send us your CV — let’s get to know each other!