JavaScript中匿名函數

在JavaScript中定義函數方式有
    1.  function f( x ){
             return x*x;
         };
         f(x);

    2.  (
               function( x ) {
                     return x*x;
               }
          ) ( x );

    3.  (
              function( x ) {
                  return x*x;
              } ( x )
         );

 其中2和3是定義匿名函數的方式。方式可以理解成
        var f = function( x ) { return x*x; };
        f( x )；
這三中方式的關係如下
  ( function( x ){ return x*x; } ) ( x ); == ( function( x ) { return x*x; } ( x ) ); == function f( x ) { return x*x; }; f(x);
但是他們還是有區別的，
首先，對於像第二和第三種形式，其它的函數和代碼是不可能調用所定義的函數的，所以把這樣的函數稱為匿名函數或者函數直接量。
其次，第二和第三種形式執行的函數，中間變數不會汙染到全域命名空間，你可以把中間的代碼看作純粹的子程序呼叫。
當然使用後面兩種形式的函數定義可以很容易的實現閉包。

例子
<script type="text/javascript">
/*
  http://jibbering.com/faq/faq_notes/closures.html（Dnew.CN注）
  A global variable - getImgInPositionedDivHtml - is declared and
  assigned the value of an inner function expression returned from
  a one-time call to an outer function expression.

  That inner function returns a string of HTML that represents an
  absolutely positioned DIV wrapped round an IMG element, such that
  all of the variable attribute values are provided as parameters
  to the function call:-
*/
var getImgInPositionedDivHtml = (function(){
   /* The - buffAr - Array is assigned to a local variable of the
      outer function expression. It is only created once and that one
      instance of the array is available to the inner function so that
      it can be used on each execution of that inner function.

      Empty strings are used as placeholders for the date that is to
      be inserted into the Array by the inner function:-
   */
   var buffAr = [
       '<div id="',
       '',   //index 1, DIV ID attribute
       '" style="position:absolute;top:',
       '',   //index 3, DIV top position
       'px;left:',
       '',   //index 5, DIV left position
       'px;width:',
       '',   //index 7, DIV width
       'px;height:',
       '',   //index 9, DIV height
       'px;overflow:hidden;/"><img src=/"',
       '',   //index 11, IMG URL
       '/" width=/"',
       '',   //index 13, IMG width
       '/" height=/"',
       '',   //index 15, IMG height
       '/" alt=/"',
       '',   //index 17, IMG alt text
       '/"><//div>'
   ];
   /* Return the inner function object that is the result of the
      evaluation of a function expression. It is this inner function
      object that will be executed on each call to -
      getImgInPositionedDivHtml( ... ) -:-
   */
   return (function(url, id, width, height, top, left, altText){
               /* Assign the various parameters to the corresponding
                  locations in the buffer array:-
               */
               buffAr[1] = id;
               buffAr[3] = top;
               buffAr[5] = left;
               buffAr[13] = (buffAr[7] = width);
               buffAr[15] = (buffAr[9] = height);
               buffAr[11] = url;
               buffAr[17] = altText;
               /* Return the string created by joining each element in the
                  array using an empty string (which is the same as just
                  joining the elements together):-
               */
               return buffAr.join('');
           }); //:End of inner function expression.
})();
/*^^- :The inline execution of the outer function expression. */
</script>