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<title>JavaScrip高级教程</title>
 <strong>第九页：<font size="3">获取难以索引的对象</font></strong> 
      <p><font size="3">　</font></p>
      <table border="0" width="447" cellpadding="0" cellspacing="0">
        <tr> 
          <td width="447"><font size="3">文档对象模型（</font><font face="helvetica, arial, sans-serif" size="3">DOM)</font><font size="3">的问题之一是</font><font face="helvetica, arial, sans-serif" size="3">:</font><font size="3">有时你要获取你要求的<br>
            对象简直就是痛苦。例如，这里有一个函数询问用户要变换<br>
            哪个图象</font><font face="helvetica, arial, sans-serif" size="3">:</font><font size="3"><a href="#" onClick="simpleSwap(); return false;">变换哪个图象</a></font></td>
        </tr>
      </table>
      <p><font size="3">　</font></p>
      <p align="center"><font size="3"><br>
        <img src="http://static.wired.com/webmonkey/98/29/stuff3a/parrot.gif" name="parrot" width="32" height="32"> 
        <img src="http://static.wired.com/webmonkey/98/29/stuff3a/cheese.gif" name="cheese" width="32" height="32"> 
        </font></p>
      <p><font face="verdana, arial, geneva, sans-serif" size="3"></font><font size="3">你可以用下面这个函数：</font></p>
      <blockquote> 
        <pre>
      <font face="verdana, arial, geneva, sans-serif" size="3"></font><font size="3">
</font><font face="verdana, arial, geneva, sans-serif" size="2">function swapOne()
{
	var the_image = prompt(&quot;change parrot or cheese&quot;,&quot;&quot;);
	var the_image_object;

	if (the_image == &quot;parrot&quot;)
	{
		the_image_object = window.document.parrot;
	} 
  else {
		the_image_object = window.document.cheese;
	}

	the_image_object.src = &quot;ant.gif&quot;;
}
</font></pre>
      </blockquote>
      <p> <font face="verdana, arial, geneva, sans-serif" size="3"></font><font size="3">连同这些</font><font face="verdana, arial, geneva, sans-serif" size="3">image</font><font size="3">标记</font><font face="verdana, arial, geneva, sans-serif" size="3">: 
        </font></p>
      <font face="verdana, arial, geneva, sans-serif" size="3"> 
      <blockquote> 
        <pre>
&lt;img src=&quot;stuff3a/parrot.gif&quot; name=&quot;parrot&quot;&gt;
&lt;img src=&quot;stuff3a/cheese.gif&quot; name=&quot;cheese&quot;&gt;
</pre>
      </blockquote>
      </font> 
      <p><font face="verdana, arial, geneva, sans-serif" size="3"></font><font size="3">请注意象这样的几行语句：</font></p>
      <blockquote> 
        <pre>
      <font face="verdana, arial, geneva, sans-serif" size="3"></font><font size="3">
</font><font face="verdana, arial, geneva, sans-serif" size="3">the_image_object = window.document.parrot;
</font></pre>
      </blockquote>
      <table border="0" width="461" cellpadding="0" cellspacing="0">
        <tr> 
          <td width="461"> <font face="verdana, arial, geneva, sans-serif" size="3"> 
            <font face="helvetica, arial, sans-serif" size="2"></font></font><font size="3"> 
            <script language="JavaScript">


<!-- hide the script


function simpleSwap()

{

	var the_image = prompt("change parrot or cheese","");

	var the_image_name = "window.document." + the_image;

	var the_image_object = eval(the_image_name);

	the_image_object.src = "http://static.wired.com/webmonkey/98/29/stuff3a/ant.gif";

}





// --> end hide </script>
            它把一个鹦鹉图象对象敷给了一个变量。虽然看起来有点<br>
            儿奇怪，它在语法上却毫无问题。但当你有</font><font face="verdana, arial, geneva, sans-serif" size="3">100</font><font size="3">个而不是两个<br>
            图象时怎么办？你只好写上一大堆的 </font><font face="verdana, arial, geneva, sans-serif" size="3"><tt>if-then-else</tt></font><font size="3">语句，要是<br>
            能象这样就好了：</font></td>
        </tr>
      </table>
      <p><font size="3">　</font></p>
      <blockquote> 
        <pre>
      <font face="verdana, arial, geneva, sans-serif" size="3"></font><font size="3">
</font><font face="verdana, arial, geneva, sans-serif" size="2">function swapTwo()
{
	var the_image = prompt(&quot;change parrot or cheese&quot;,&quot;&quot;);
	window.document.the_image.src = &quot;ant.gif&quot;;
}</font>
</pre>
      </blockquote>
      <table border="0" width="465" cellpadding="0" cellspacing="0">
        <tr> 
          <td width="465"> <font face="verdana, arial, geneva, sans-serif" size="3"> 
            <font face="helvetica, arial, sans-serif" size="2"></font></font><font size="3">不幸的是</font><font face="verdana, arial, geneva, sans-serif" size="3">, 
            JavaScript</font><font size="3">将会寻找名字叫 </font><font face="verdana, arial, geneva, sans-serif" size="3"><tt>the_image</tt></font><font size="3">而不是你所希<br>
            望的</font><font face="verdana, arial, geneva, sans-serif" size="3">&quot;cheese&quot;</font><font size="3">或者</font><font face="verdana, arial, geneva, sans-serif" size="3">&quot;parrot&quot;</font><font size="3">的图象，于是你得到了错误信息：”<br>
            没听说过一个名为</font><font face="verdana, arial, geneva, sans-serif" size="3"><tt>the_image</tt></font><font size="3">的对象”。</font></td>
        </tr>
      </table>
      <p><font size="3">　</font></p>
      <p> <font face="verdana, arial, geneva, sans-serif" size="3"></font><font size="3">还好，</font><font face="verdana, arial, geneva, sans-serif" size="3">eval</font><font size="3">能够帮你得到你想要的对象。</font></p>
      <pre>
    <font face="verdana, arial, geneva, sans-serif" size="3"></font><font size="3">
</font><font face="verdana, arial, geneva, sans-serif" size="2">function simpleSwap()
{
	var the_image = prompt(&quot;change parrot or cheese&quot;,&quot;&quot;);
	var the_image_name = &quot;window.document.&quot; + the_image;
	var the_image_object = eval(the_image_name);
	the_image_object.src = &quot;ant.gif&quot;;
}
</font></pre>
      <table border="0" width="456" cellpadding="0" cellspacing="0">
        <tr> 
          <td width="556"> <font face="verdana, arial, geneva, sans-serif" size="3"> 
            <font face="helvetica, arial, sans-serif" size="2"></font></font><font size="3">如果用户在提示框里填入</font><font face="verdana, arial, geneva, sans-serif" size="3">&quot;parrot&quot;</font><font size="3">，在第二行里创建了一个字<br>
            符串即</font><font face="verdana, arial, geneva, sans-serif" size="3"><tt>window.document.parrot</tt>. 
            </font><font size="3">然后包含了</font><font face="verdana, arial, geneva, sans-serif" size="3">eval</font><font size="3">的第三<br>
            行意思是： </font><font face="verdana, arial, geneva, sans-serif" size="3">&quot;</font><font size="3">给我对象</font><font face="verdana, arial, geneva, sans-serif" size="3"><tt>window.document.parrot</tt>&quot; 
            - </font><font size="3">也就是<br>
            你要的那个图象对象。一旦你获取了这个图象对象，你可以把<br>
            它的</font><font face="verdana, arial, geneva, sans-serif" size="3">src</font><font size="3">属性设为</font><font face="verdana, arial, geneva, sans-serif" size="3"><tt>ant.gif</tt>. 
            </font><font size="3">有点害怕？用不着。其实这相当有<br>
            用，人们也经常使用它。</font> 
            <p><font face="verdana, arial, geneva, sans-serif" size="3"></font><font size="3">如果你不喜欢</font><font face="verdana, arial, geneva, sans-serif" size="3"><tt>eval()</tt>, 
              </font><font size="3">有另一种获取难以索引的对象的途径。<a href="day4_10.html">&gt;&gt;</a></font>
          </td>
        </tr>
      </table>
      <p align="left"><font face="宋体" size="3" color="#000000"><strong>JavaScript高级教程</strong></font><font color="#FF0000" face="宋体" size="3"><br>
        </font><font size="3"><font color="#FF3300">第一页</font> <a href="day4_1.html"><font face="verdana, arial, geneva, sans-serif">JavaScript</font>高级教程<font face="verdana, arial, geneva, sans-serif">- 
        </font>第<font face="verdana, arial, geneva, sans-serif">4</font>天</a> 
        <br>
        <font color="#FF3300">第二页 </font><a href="day4_2.html">图象映射与<font face="verdana, arial, geneva, sans-serif">JavaScript</font></a><font face="verdana, arial, geneva, sans-serif"><br>
        </font><font color="#FF3300">第三页 </font><a href="day4_3.html">预装图象 - 是什么<font face="verdana, arial, geneva, sans-serif">?</font></a><font face="verdana, arial, geneva, sans-serif"><br>
        </font><font color="#FF3300">第四页 </font><a href="day4_4.html">预装图象 - 怎么做?</a><font face="verdana, arial, geneva, sans-serif"><br>
        </font><font color="#FF3300">第五页 </font><a href="day4_5.html">对象的优点</a><br>
        <font color="#FF3300">第六页 </font><a href="day4_6.html">创建你自己的对象</a><br>
        <font color="#FF3300">第七页 </font><a href="day4_7.html">你的面向对象的虚拟宠物</a><br>
        <font color="#FF3300">第八页 </font><a href="day4_8.html">计算字符串</a><br>
        <font color="#FF3300">第九页 </font>获取难以索引的对象<br>
        <font color="#FF3300">第十页 </font><a href="day4_10.html">另一种获取难以索引的对象的手段</a><br>
        <font color="#FF3300">第十一页 </font><a href="day4_11.html">第四天课程复习</a></font></p>
      <p>[<a href="day1_1.html">第1课</a>][<a href="day2_1.html">第2课</a>][<a href="day3_1.html">第3课</a>][第4课][<a href="day5_1.html">第5课</a>]</p>
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