很难理解JavaScript函数级别范围

Question

我听不太懂JavaScript功能级别范围，为C#程序员看起来有线给我，我会尝试通过代码来解释它：

CODE＃1

//Problem 
//if same named variable (as in global scope) is used inside function scope, 
//then variable defined inside function will be used,global one will be shadowed 
var a = 123; 
function func() { 
    alert(a); //returns undefined,why not just return 123 ? 
    //how come js knew that there is variable 'a' will be defined and used in 
    //this function scope ,js is interpreter based ? 
    var a = 1; //a is defined inside function 
    alert(a); //returns 1 
} 
func(); 

CODE＃2

//when a variable(inside function) not named as same as the global, 
//then it can be used inside function,and global variable is not shadowed 
var k = 123; 
function func2() { 
    alert(k); //returns 123 ,why not 'undefined' 
    var c = 1; 
    alert(c); //returns 1 
} 
func2(); 

所以我的问题是

1. 在CODE＃1为什么第一次a是undefined，为什么不它只是返回123？如何 来js知道有变量'a'将被定义和使用在 这个功能范围内，js是基于解释器？

2. in 代码＃2为什么不是k是'undefined'？

http://jsfiddle.net/Nu2Vu/

Answer 1

CODE＃1

Hoisting原因所有变量d这些问题将被带到范围的顶部，但它将任务留在原来的位置。当一个变量的引用首先在当前范围内查找时，如果它找不到该变量，它将继续查找范围链，直到找到该变量为止。 该代码解释是这样的：

var a = 123; // global var named a declared, assigned a value 
function func() { 
    var a; // the declaration of the local var a gets 
     // hoisted to the top of the scope, but the 
     // assignment is left below, so at the point 
     // it is initialized with a value of `undefined` 
    alert(a); // looks for a local var named a, finds it but 
       // it currently has a value of `undefined` 

    a = 1; // now the value is assigned to the local a 
    alert(a); // returns 1 
} 
func(); 

CODE＃2

此代码的行为确实因为closure的方式。 闭包的基本定义是JavaScript函数不仅可以访问在其自己范围内定义的变量，还可以访问可用于其父范围的变量。

var k = 123; // declares and assigns a value to global k 
function func2() { 
    alert(k); // looks for a local var named k, doesn't find it, 
       // looks in its parent scope (the global scope in 
       // this case) finds k, returns its value of 123 
    var c = 1; 
    alert(c); //returns 1 
} 
func2(); 

Answer 2

第一码等于这样的：

var a = 123; 
function func() { 
    var a; //Undefined! 
    alert(a); 
    a = 1; 
    alert(a); 
} 

这解释了它很好： https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Statements/var