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

This is a working example I have come up with with your help, maybe it's of any use to others.

/*

Example of an Espruino Object that fires an event when the button on pin D17 is pressed.

*/

var P8 = {
  init : function(){
    setWatch(function(){
      P8.emit("button","D17");
    },D17 ,{ repeat:true,debounce:25 }
    );
  }
}; // end of P8 Object

// ============================= test of P8 object  ===============

P8.init();


P8.on('button', function(btn){ print("Button",btn, "press detected");});

Output of the script when the button is pressed:
Button D17 press detected

In this example the init function needs to be called at start of the main script. Not sure if there is a way to move this init into the Object, but this works for me.