<script>

    var x = 0

    Object.defineProperty(this, 'a', {
      get() {
        x++
        console.log('get');
        return x
      }
    })
    console.log(a === 1 && a === 2 && a === 3);

    var obj = {

    }

    Object.defineProperty(obj, 'name', {
      get() {
        console.log('get');
      }
    })

    name     //无打印
    obj.name  //打印 'get'

    //结论：Object.defineProperty(obj, 'name')会创建name属性加入到obj,并且监听obj.name;并不能监听name

    var obj = {
      name: 1
    }

    console.log(obj.name);  //1

    Object.defineProperty(obj, 'name', {
      get() {
        console.log('get');  //'get'
      }
    })

    console.log(obj.name); //undefined
//打印依次为1,'get',undefined    最后一行代码'console.log(obj.name)'读取obj.name被get劫持并且get无返回值,因此undefined

    var obj = {
      name: 1
    }
    console.log(obj.name);  //1

    Object.defineProperty(obj, 'name', {
      get() {
        console.log('get'); //'get'
        return 10
      }
    })

    console.log(obj.name); //10     最后一行代码'console.log(obj.name)'读取obj.name被get劫持并且get返回值为10替代了obj.name


  </script>