Explore a conference talk from JSConf Iceland 2016 detailing Pinterest's transition from a custom Backbone/Django framework to React. Learn about the challenges faced during the migration process, including performance bottlenecks, code bloat, and scaling issues. Discover the strategies employed to overcome these obstacles, such as measuring individual features, implementing a "Backbone lite" solution, and adopting declarative states. Gain insights into the mistakes made during the transition, including the lack of a top-down strategy and coupling data fetching with rendering. Understand the true value of React in terms of testability and the importance of iteratively updating semantics when pivoting to a new JavaScript framework. Dive into the complexities of managing a large-scale web application with nearly 500,000 lines of code and a team of 164 web developers.