How the create an event handler similar to Bangle.on("step"...........)

There are some examples. I recommend that you download/clone the BangleApps repo, https://github.com/espruino/BangleApps. And use a decent IDE/text editor that can search through all the projects in there.
I searched for keyword "emit" and found an example.

https://github.com/espruino/BangleApps/blob/master/apps/jbells/jbells.js

From briefly looking at the examples, its enough to call

var myobj = new Object();
myobj.on("customevent",callback)

and later

myobj.emit("customevent");

The "Object" class is a compatible EventEmitter by default.