1 obj1.valueOf
   2 obj1 is 1
   3 obj1.valueOf
   4 11
   5 obj2.valueOf
   6 obj2 is 2
   7 obj2.valueOf
   8 24
   9 obj1.valueOf
  10 obj2.valueOf
  11 3
  12 obj3.toString
  13 world12
  14 obj3.toString
  15 hello world
  16 obj1.valueOf
  17 obj3.toString
  18 1world
  19 obj2.valueOf
  20 obj3.toString
  21 2world
  22 obj3.toString
  23 obj3.toString
  24 worldworld