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<title>JavaScrip高级教程</title>
 <strong>第八页：<font size="3">计算字符串</font></strong> 
      <p><font face="verdana, arial, geneva, sans-serif" size="2"></font><font size="2"> 
        <script language="JavaScript">

<!-- hide me



function doSillyEval()

{



	var the_unevaled_answer = "2 + 3";

	var the_evaled_answer = eval("2 + 3");

	alert("the un-evaled answer is " + the_unevaled_answer + " and the evaled answer is " + the_evaled_answer);

} 



// show me -->

</script>
        </font></p>
      <table width="447">
        <tr> 
          <td width="443"> <font face="verdana, arial, geneva, sans-serif" size="2"></font><font face="helvetica, arial, sans-serif" size="3">JavaScript</font><font size="3">有许多小窍门来使编程更加容易。其中之一就是<br>
            </font><font face="verdana, arial" size="3"><tt>eval()</tt></font><font size="3">函数，这个函数可以把一个字符串当作一个</font><font face="verdana, arial" size="3">JavaScript</font><br>
            <font size="3">表达式一样去执行它。实际上我在<a href="../course/day5_1.html">基础教程第五日</a>里曾讲过<br>
            一点有关</font><font face="verdana, arial" size="3">eval</font><font size="3">函数，这里举个小例子重温一下：</font></td>
        </tr>
      </table>
      <blockquote> 
                <pre><big>
</big>var the_unevaled_answer = &quot;2 + 3&quot;;
var the_evaled_answer = eval(&quot;2 + 3&quot;);
alert(&quot;the un-evaled answer is &quot; + the_unevaled_answer + &quot; and <br>the evaled answer is &quot; + the_evaled_answer);
</pre>
      </blockquote>
      <table width="449">
        <tr> 
          <td width="445"><font size="3">如果你</font><a href="#" onClick="doSillyEval(); return false;"><font size="3">运行这段</font><font face="verdana, arial" size="3">eval</font><font size="3">程序</font></a><font face="verdana, arial" size="3">, 
            </font><font size="3">你将会看到在</font><font face="verdana, arial" size="3">JavaScript</font><font size="3">里字<br>
            符串</font><font face="verdana, arial" size="3">&quot;2 + 3&quot;</font><font size="3">实际上被执行了。所以当你把</font><font face="verdana, arial" size="3"><tt>the_evaled_answer</tt></font><br>
            <font size="3">的值设成 </font><font face="verdana, arial" size="3"><tt>eval(&quot;2 
            + 3&quot;)</tt></font><font size="3">时</font><font face="verdana, arial" size="3">, 
            JavaScript</font><font size="3">将会明白并把</font><font face="verdana, arial" size="3">2</font><font size="3">和</font><font face="verdana, arial" size="3">3</font><br>
            <font size="3">的和返回给</font><font face="verdana, arial" size="3"><tt>the_evaled_answer</tt></font><font size="3">。</font> 
            <p><font face="verdana, arial" size="3"></font><font size="3">这个看起来似乎有点傻，其实可以做出很有趣的事。比如使<br>
              用</font><font face="verdana, arial" size="3">eval</font><font size="3">你可以根据用户的输入直接创建函数。这可以使程序<br>
              根据时间或用户输入的不同而使程序本身发生变化，通过举<br>
              一反三，你可以获得惊人的效果。在实际中，</font><font face="verdana, arial" size="3">eval</font><font size="3">很少被<br>
              用到，但也许你见过有人使用</font><font face="verdana, arial" size="3">eval</font><font size="3">来获取难以索引的对象。<a href="day4_9.html">&gt;&gt;</a></font>
          </td>
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      <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>计算字符串<br>
        <font color="#FF3300">第九页 </font><a href="day4_9.html">获取难以索引的对象</a><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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