Scope when calling a method from another method in the same object in JavaScript

Question

var Obj = function(par) {
  this.par = par;
};

Obj.prototype.addPar = function(num) {
  return num + this.par;
};

Obj.prototype.add3more = function(fun, num) {
  return fun(num) + 3;
};

Obj.prototype.callAdd3more = function(num) {
  return this.add3more(this.addPar, num);
};

var obj = new Obj(2);
console.log(obj.callAdd3more(1));

I would expect the code above to print out 5, but it prints out NaN. Somehow, addPar seems to get called out of the scope of the object obj, because when it gets called this.par is undefined. I cannot understand why or how to correct the code. Many thanks for your help.