Javascript Number submultitractivide(n)

Number.prototype.submultitractivide = function(n){
  var operator = ['+','-','*','/'];
  var i = Math.floor(Math.random()*operator.length);
  var j = Math.floor(Math.random()*operator.length);
  var x = operator[i];
  var y = operator[j];
  return (this)+x+n+y+n;
};