We used the ArcGIS JS API's CDN to access the API and tried to understand the map and Esri geometry modules. We tried to better understand extents and spatial references by brushing up our knowledge of coordinate geometry. We now know that an extent is just a minimum bounding rectangle that could be defined using two coordinates, and a spatial reference system is akin to the coordinate axes on a graph. We tried to look at some of the amazing modules that the dojo toolkit provides, which we must consider using in our code. ArcGIS Server exposes its GIS data and other resources as a REST API, that is, it is available as a URL. You also learned that a developer must always consult the Service Catalog before starting to consume any service through the API. we laid down our preferences in the way of development environment for working through projects in this book. The next chapter deals with the different types of layer used in the API and the ideal context where each type is used. We will also be introduced to some of the most commonly used in-built widgets provided by Esri, and we will use them in our application.