this指向练习题

自己去分析写解释
 // 练习题1
        var b = {
            a: 23,
            c: 3,
            d: {
                a: 78,
                e: {
                    a: 100,
                    f: function () {
                        console.log(this.a);
                    }
                }
            }
        }
        var fn = b.d.e.f;
        fn(); // undefined
        b.d.e.f(); // 100

        // 练习题2  修改$对象里的代码，使得以下代码运行正常
        var $ = {
            fn:function(){
                console.log(1);     
                return this;        //返回值缺失
            },
            fn2:function(){
                console.log(2);        
                
            }
        }
        $.fn().fn2();       

        // // 练习题3
        var myObject = {
            foo: "bar",
            func : function(){
                var self = this;
                console.log(this.foo);       //bar   
                console.log(self.foo);       //bar

                (function(){
                    console.log(this.foo);      //undefined
                    console.log(this);          //window

                    console.log(self.foo);   //bar
                }());
                
                
            }
        };
        myObject.func();

        // // 练习题4
        var x = 3;
        var y = 4;
        var obj = {
            x : 1,
            y : 6,
            getX : function(){
                var x = 5;
                return function(){
                    return this.x;      
                }();
            },
            getY: function(){
                var y = 7;
                return this.y;          
            }
        }
        console.log(obj.getX());   //3    
        console.log(obj.getY());   //6  
        
        

        // // 练习题5
        function a(xx) {
            this.x = xx; // window.x = 6
            return this;
        }
        var x = a(5);  // x = window
        var y = a(6);  // y = window   
        console.log(x.x);  // window.x      undefined
        console.log(y.x);  // window.x      6