Register named callbacks and call them with arguments

 Dispatching is a convenient mechanism for separating concerns with
 loosely-coupled code: register named callbacks and then call them with
 arbitrary arguments. A variety of D3 components, such as d3-request, use this
 mechanism to emit events to listeners. Think of this like Node’s EventEmitter,
 except every listener has a well-defined name so it’s easy to remove or
 replace them.
 .
 Node.js is an event-based server-side JavaScript engine.