Building a complex web application front-end, such as the one at Heap, involves integrating various technologies and principles to enhance productivity, testability, and extensibility. Initially, Heap utilized CoffeeScript and Backbone but transitioned to TypeScript and React for their benefits in type safety and development speed. MobX is used for state management, with domain stores handling specific data models, while dependencies are managed through a root store to clarify relationships and improve testing. The separation of container and presentational components in React aids in organizing the code, making it easier to maintain and extend, with MobX's reactive properties ensuring components update automatically. Dependency injection further enhances testability by allowing components to receive specific store instances, avoiding hidden dependencies. This architecture supports adding new features and adapting existing ones without significant refactoring, demonstrating how MobX and thoughtful design principles can create a robust, flexible codebase.