运行程序，解读this指向---case6

function Parent() {
  this.a = 1;
  this.b = [1, 2, this.a];
  this.c = { ckey: 5 };
  this.show = function () {
    console.log(this.a , this.b , this.c.ckey);
  }
}
function Child() {
  this.a = 2;
  this.change = function () {
    this.b.push(this.a);
    this.a = this.b.length;
    this.c.ckey = this.a++;
  }
}
Child.prototype = new Parent();
var parent = new Parent();
var child1 = new Child();
var child2 = new Child();
child1.a = 11;
child2.a = 12;

parent.show(); // 1 [1,2,1] 5
child1.show(); // 11 [1,2,1] 5
child2.show(); // 12 [1,2,1] 5
child1.change();
child2.change();
parent.show(); // 1 [1,2,1] 5
child1.show(); // 5 [1,2,1,11,12] 5
child2.show(); // 6 [1,2,1,11,12] 5