//作用域鏈
 var a = 1;
 function test() {
 var b =2;
 return a;
 }
 alert(test());//彈出1；
 alert(b);//不能獲取b
//scope chain
 var a = 1;
 function test() {
 var b = 2;
 function test1() {
 var c = 3;
 alert(a);
 alert(b);
 alert(c);
 }
 test1();
 }
 test();//彈出1，彈出2，彈出3；

//詞法作用域；
 function f1() {
 var a = 12;
 return f2();
 }
 function f2() {
 return a;
 }
 alert(f1());//并不能獲取a，a在f2()中并未定義；
function f1() {
 var a = 1;
 return f2();
 }
 function f2() {
 var b = 3;
 alert(b);
 return a;
 }
 alert(f1());//彈出3，a在f2()中未定義

function f1() {
 var a = 1;
 return f2();
 }
 function f2() {
 var b = 3;
 alert(b);
 return a;
 }
 alert(f1());//彈出3，a在f2()中未定義,undefined
 var a=55;
 alert(f1());//彈出3，彈出55

//通過閉包突破全局作用域鏈
 function f() {
 var a = "sun";
 return function () {
 return a;
 }
 }
 var test = f();
 alert(test());//彈出sun
var n;
function f() {
 var a = "sun";
 n = function () {
 return a;
 }
}
f();
alert(n());//彈出sun
 function f(param) {
 var n =function () {
 return param;
 };
 param++;
 return n;
 }
 var test = f(45);
 alert(test());//彈出46；