一道关于JavaScript中this的题目

var num = 10;
var obj = {
    num:8,
    inner: {
        num: 6,
        print: function () {
            console.log(this.num);
        }
    }
}
num = 888;
obj.inner.print(); // 6
var fn = obj.inner.print;
fn(); //888
(obj.inner.print)(); //6
(obj.inner.print = obj.inner.print)(); //888 这个点没有太理解