Weird object creation

gistfile1.js

function p(t) {
  for (var i in t) console.log(t[i]);
}

var obj1 = Object.create(null);
obj1[1] = "2";
obj1[3] = "4";
var obj2 = Object.create(null);
obj2.__proto__ = obj1;

p(obj1);
p(obj2);