Objects

Now that we've gone through some basic JavaScript concepts we'll tackle the most important concept in this section. In order to effectively program mapping applications with the ArcGIS API for JavaScript, you need to have a good fundamental understanding of objects, so this is a critical concept that you need to grasp before moving forward.

The ArcGIS API for JavaScript makes extensive use of objects. We'll cover the details of this programming library in detail, but for now we'll focus on the high-level concepts. Objects are complex structures capable of aggregating multiple data values and actions into a single structure. This differs greatly from our primitive data types such as numbers, strings, and Booleans which can hold only a single value.

Objects are composed of both data and actions. Data, in the form of properties, contains information about an object. For example, with a Map object found in the ArcGIS Server JavaScript API, there are a number of properties including the map extent, graphics associated with a map, the height and width of the map, layers associated with the map, and so on. These properties contain information about the object.

Objects also have actions which we typically call methods, but we can also group constructors and events into this category. Methods are actions that a map can perform such as adding a layer, setting the map extent, or getting the map scale.

Constructors are special purpose functions that are used to create new instances of an object. With some objects it is also possible to pass parameters into the constructor to give more control over the object that is created. The following code example shows how a constructor is used to create a new instance of a Map object. You can tell that this method is a constructor because of the use of the new keyword which I've highlighted. The new keyword followed by the name of the object and any parameters used to control the new object defines the constructor for the object. In this particular case, we've created a new Map object and stored it in a variable called map. Three parameters are passed into the constructor to control various aspects of the Map object including the basemap, center of the map, and the zoom scale level:

var map = new Map("mapDiv", {  
  basemap: "streets", 
  center:[-117.148, 32.706], //long, lat 
  zoom: 12 
}); 

Events are actions that take place on the object and are triggered by the end user or the application. This would include events such as clicking on the map, panning the map, or a layer being added to the map.

Properties and methods are accessed via dot notation wherein the object instance name is separated from the property or method by a dot. For instance, to access the current map extent you would enter map.extent in your code. The following examples demonstrate how to access an object's properties:

var theExtent = map.extent; 
var graphics = map.graphics; 

The same is the case with methods except that methods have parentheses at the end of the method name. Data can be passed into a method through the use of parameters. In the following first line of code, we're passing a variable called pt into the map.centerAt(pt) method:

map.centerAt(pt); 
map.panRight();