javascript 小代码

if(!("a" in window)){
    var a =1;
}
alert(a);
//undefined

var a = 1,b=function a (x){
    x && a (--x);
};
alert(a);
//1

function a(x){
    return x*2;
}
var a ;
alert(a);
//函数a

function b (x, y, a) {
    arguments[2] = 10;
    alert (a);
}
b(1, 2, 3);
//10

function a () {
    alert (this);
}
a.call (null);
//[object Window]