<script>
    function foo(){
       var name=‘ruby‘;
       alert(this.name)  //undefined
    }
    foo();
    console.log(this);  //window
</script>
window中没name这个属性，因此找不到

<script>
   var name=‘jerd‘;
    function foo(){
       var name=‘ruby‘;
       alert(this.name)  //jerd
    }
    foo();
    console.log(this);  //window
</script>

var person = {
    run: function () {
    console.log(this)  //指向person这个对象
    }
}
person.run()

<script>
    var name=‘jerd‘;
    var people= {
    user:"ruby",
    fn:function(){
        console.log(this.user); //this指people这个对象
    }
    };
    window.people.fn()  //ruby
</script>

<script>
    var people = {
    name:‘jerd‘,
    obj:{
        fn:function(){
            alert(this.name);
        }
    }
};
people.obj.fn(); //undefined
此时this指向对象b。b中无a属性，因此值为undefined
</script>

<script>
    var people = {
    name:‘jerd‘,
    obj:{
        fn:function(){
            alert(this); //this指向windonw
        }
    }
};
var obj1=people.obj.fn;
obj1();
</script>
this永远指向的是最后调用它的对象，函数fn是被对象obj1所引用，
但是在将fn赋值给变量obj1的时候并没有执行所以最终指向的是window

var btn = document.querySelector("button")
btn.onclick = function () {
    console.log(this) // btn
}

 function Person(){
      console.log(this)//window
  }
  Person()

function Person(){
      console.log(this)//people
  }
  var people = new Person()

<script>
    var name=‘jerd‘;
    function Foo(){
       this.name=‘ruby‘;
       this.func=function () {
           var name="zhao";
           alert(this.name)
       }
    }
    var obj= new Foo();
     obj.func()
</script>
#ruby

<script>
    var name=‘jerd‘;
    function Foo(){
       this.name=‘ruby‘;
       this.func=function () {
           console.log(this)  //Foo
           (function () {
               alert(this.name) //window
           })()
       }
    }
    var obj= new Foo();
     obj.func()
</script>