var v = "hello";
    function f() {
        alert(v);
        var v = "byebye";
        //函数作用域内的变量v【遮盖】了上层作用域变量v
    }
    /* function f() {
        var v;//声明提升
        alert(v);//此函数与上述等价
        v="byebye";
    } */
    f();//undefined
    alert(v);//hello

    var v = "hello";
    if (true) {
        alert(v);//hello
        var v = "byebye";
    }
    alert(v);//byebye