handling of this strange(?)

trying this, I would expect an errormessage for incrementing a.
In Espruino, a is incremented twice, and in log there is 1 3 5 ....21 23
But by typing console.log(b.a) thje answer is 0, always.
I'm confused, so is the problem on my side or is it Espruino ?

function test(){
  var interval;
  this.a = 0;
  interval = setInterval(function(){
      a++;
      this.a++;console.log(a,this.a);
    },2000);
}
var b = new test();