Print7.2 Querying
One of the most common operations performed in GIS is the query. The Esri JS API makes it possible for developers to query layers in a number of different ways. For one, you can set a layer’s definition expression so that it displays just a subset of its data. You can also query a layer to get a count of features meeting the criteria, to get the IDs of the features, or to get the features (attributes & geometries) themselves.