This speech is a history of moving the frontend of one of the largest and the most loaded services in Russia — Yandex Search to ReactJS and TypeScript. We didn’t talk about the change in architecture and our motivation before. Today ReactJS successfully works on Yandex services and we have something interesting to share. We’ve done a lot of work: we adapted our methodology for using with ReactJS, resolved problems with the rendering speed, made partial hydrating, created methodology for doing hundreds of A/B experiments with React online. I'll tell you why we decided not to use most of our tools and libraries, and why did we choose Open Source as a foundation for our services. How we implemented this and how we are living in two runtimes.