lib/events.js
function on(emitter, eventName, handler, context) {
  handler = context ? handler.bind(context) : handler;
  emitter.addEventListener(eventName, handler);
  return handler;
}
function on(emitter, eventName, handler, context) {
  handler = context ? handler.bind(context) : handler;
  emitter.addEventListener(eventName, handler);
  return handler;
}
function on ( emitter , eventName , handler , context ) { handler = context ? handler . bind ( context ) : handler ; emitter . addEventListener ( eventName , handler ) ; return handler ; }
Register to receive events.
Register to receive events .