The Modern Application Stack – Part 4: Building a Client UI Using Angular 2 (formerly AngularJS) & TypeScript. This article demonstrates how to build a client-side user interface for the Mongopop application using Angular 2 and Typescript, focusing on building components and services that interact with the back-end REST API implemented in Part 3: Building a REST API with Express.js. The application uses Observables from the RxJS reactive libraries to simplify asynchronous function calls and provides a consistent pattern of files and directories for developers.