作用域解析及作用域鏈

預解析遇到重名，只留一個。如果與函數重名，就只留下函數。

 console.log(a);// function a(){console.log(4)}
    var a = 1;
    console.log(a); // 1
    function a(){
        console.log(2);
    }
    console.log(a);// 1
    var a = 3;
    console.log(a); //3
    function a() {
        console.log(4);
    }
    console.log(a());//報錯

自上而下，由內而外

 <script type="text/javascript">
      alert(a);//報錯
  </script>
   <script type="text/javascript">
      var a = 1;
  </script>
--------------------------------------------------
 <script type="text/javascript">
      var a = 1; 
  </script>
   <script type="text/javascript">
     alert(a);//1
  </script>

var a = 1;
function test() {
    alert(a);
    var a = 2;
}
alert(a);//1
test();// undefined

var a = 1;
function test() {
    alert(a);
    a = 2;
}
test();// 1
alert(a);// 2

var a = 1;
function test() {
    alert(a);
    a = 2;
}
alert(a);// 1
test();// 1

var a = 1;
function test(a) {
    alert(a);
    a = 2;
}
test();// undefined
alert(a);// 1