map: function( callback ) {
    return this.pushStack( jQuery.map(this, function( elem, i ) {
      return callback.call( elem, i, elem );
    }));
  },

//将原数组中每个元素加 4 转换为一个新数组。
//jQuery 代码:
$.map( [0,1,2], function(n){
 return n + 4;
});
//结果:
[4, 5, 6]

//原数组中大于 0 的元素加 1 ，否则删除。
//jQuery 代码:
$.map( [0,1,2], function(n){
 return n > 0 ? n + 1 : null;
});
//结果:
[2, 3]

//原数组中每个元素扩展为一个包含其本身和其值加 1 的数组，并转换为一个新数组
//jQuery 代码:
$.map( [0,1,2], function(n){
 return [ n, n + 1 ];
});
//结果:
[0, 1, 1, 2, 2, 3]

var items = [1,2,3,4]; 
$.each(items, function() { 
alert('this is ' + this); 
}); 
var newItems = $.map(items, function(i) { 
return i + 1; 
}); 
// newItems is [2,3,4,5]
//使用each时，改变的还是原来的items数组，而使用map时，不改变items，只是新建一个新的数组。

var items = [0,1,2,3,4,5,6,7,8,9]; 
var itemsLessThanEqualFive = $.map(items, function(i) { 
// removes all items > 5 
if (i > 5) 
　　return null; 
　　return i; 
}); 
// itemsLessThanEqualFive = [0,1,2,3,4,5]

// arg is for internal usage only
  map: function( elems, callback, arg ) {
    var value, key, ret = [],
      i = 0,
      length = elems.length,
      // jquery objects are treated as arrays
      isArray = elems instanceof jQuery || length !== undefined && typeof length === "number" && ( ( length > 0 && elems[ 0 ] && elems[ length -1 ] ) || length === 0 || jQuery.isArray( elems ) ) ;

    // Go through the array, translating each of the items to their
    if ( isArray ) {
      for ( ; i < length; i++ ) {
        value = callback( elems[ i ], i, arg );

        if ( value != null ) {
          ret[ ret.length ] = value;
        }
      }

    // Go through every key on the object,
    } else {
      for ( key in elems ) {
        value = callback( elems[ key ], key, arg );

        if ( value != null ) {
          ret[ ret.length ] = value;
        }
      }
    }

    // Flatten any nested arrays
    return ret.concat.apply( [], ret );
  },

// Go through the array, translating each of the items to their
    if ( isArray ) {
      for ( ; i < length; i++ ) {
        value = callback( elems[ i ], i, arg );

        if ( value != null ) {
          ret[ ret.length ] = value;
        }
      }

// Flatten any nested arrays
    return ret.concat.apply( [], ret );

$.map( [0,1,2], function(n){
 return [ n, n + 1 ];
});