Several JavaScript API components depend on a server side component to be fully functional. For example, the FeatureService calls a server in order to query spatial data that isn't directly available in the browser. However, due to security restrictions that are put in place by all modern browsers, it is not possible to call any arbitrary server on the web.
The JavaScript API requires a proxy in order to work around those security restrictions. The following sections describe why this is necessary and how a proxy can be used for your own application development.