Serial.on() issues...

I think this is actually exactly the same issue as you'd have with setTimeout - this isn't set to the right thing.

To solve it you'd define a variable in your function's scope and then use that:

function EasyVR(ser,onCm,onTo) {
  this.ser = ser;
  this.onCommand=onCm;
  this.onTimeout=onTo;
  var evr = this;             // <------------------------------
  this.ser.on('data',function(){evr.onWatch(); }); // <--------------
  this.vrstate=-1;
  this.stsr='o';
  this.rcvv="";
} ;
EasyVR.prototype.onWatch=function(data) {
    console.log(this);
};

Possibly this should actually be set to the serial object the event was on though - I'd need to figure out what NodeJS does. Even if that was the case, just calling this.ser.on('data',this.onWatch); wouldn't do what you'd want.

There's some stuff on StackOverflow about it, and a neat solution would be to use bind - although I'm afraid that's not implemented yet :(