变量的作用域

var a = 1;
b = 2;
 
function show() {
    alert('a :' + a); //undefined
    alert('b :' + b); //2
    var a = 3;
    b = 4;
    alert('a :' + a); //3
}
show();
alert('a- :' + a); //1
alert('b- :' + b); //4