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<td bgcolor="#ffff99" width="451">JavaScript教程 - 第二课</td>
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<td bgcolor="#FF6600" width="451"><a href="mailto:thau@wired.com">Thau</a></td>
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<td width="458" valign="top" align="left"><strong><small><small><br>
</small></small><font face="宋体" size="3">第七页：if-then语句的例子</font></strong>
<p><font face="宋体" size="3">如果你在即时对话框中键入<tt>yes</tt>，你将在看到本页其他部分前收<br>
到一个亲切的问候。若敲入别的则没有。</font></p>
<p><font face="宋体" size="3">这里是该语句的核心：</font></p>
<font COLOR="#000000"><p>var monkey_love = prompt("你喜欢网猴吗？","敲入是<br>
或否。");</p>
</font><p><font face="宋体" size="3">if (monkey_love == "是")<br>
<br>
{<br>
<br>
</font><font COLOR="#000000">alert("谢谢！很高兴您能来这儿！请往下读吧！");</font></p>
<p><font face="宋体" size="3"><br>
}<br>
<br>
第一行你见过。它唤起一个对话框并将用户的反馈调入变<br>
量<tt>monkey_love</tt>中。但第二行就有些不同：它有个条件，即如果<br>
变量<tt>monkey_love</tt>等于值"是"
，则运行花括号中的语句。若它<br>
等于其他值，则不运行。</font></p>
<p><font face="宋体"><font size="3">注意该条件中的两个等于标记，这是人们容易搞混的地方之<br>
一。如果你只用一个标记，实际上是告诉JavaScript测试是<br>
否<tt>monkey_love</font><font size="4">等于</font></tt><font size="3"> "是"。幸运的是，多数浏览器在你运行这<br>
些语句时会识别这些错误并警告你。但最好现在开始就注意别<br>
犯这种错误。</font></font></p>
<p><font face="宋体" size="3">其他重要的条件是：</font></p>
<p><font face="宋体" size="3">(variable_1 > variable_2) is true if variable_1
is greater than variable_2<br>
<br>
(variable_1 < variable_2) is true if variable_1 is less than variable_2<br>
<br>
(variable_2 <= variable_2) is true if variable_1 is less than or equal to
variable_2<br>
<br>
(variable_1 != variable_2) is true if variable_1 does <b>not</b> equal variable_2<br>
<br>
有两个方法可使你的条件更合理：</font></p>
<p><font face="宋体" size="3">在运行花括号中的语句前如果你想要两件事为“是”，可这<br>
样做：</font></p>
<p><font face="宋体" size="3"><br>
if ((variable_1 > 18) && (variable_1 < 21)) <br>
<br>
{ <br>
<br>
document.writeln("variable_1 can vote, but can't drink.");<br>
<br>
}<br>
<br>
</font></p>
<p><font face="宋体" size="3">注意这里的两个“&&”在JavaScript中这是“与”的意思。也<br>
注意整个子句有两个部分，&&须在圆括号中。</font></p>
<p><font face="宋体" size="3">若想两件事之一为真，这样做：</font></p>
<p><font face="宋体" size="3"><br>
if ((variable_1 == "bananas") || (variable_1 == "JavaScript")) <br>
<br>
{ <br>
<br>
document.writeln("The monkey is happy because it has " +
variable_1);<br>
<br>
}<br>
<br>
回到<a href="day2_8.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day2_8.html">if-then 练习中来!</a></font><font face="宋体" size="2"> </font></p>
<p align="left"><font face="宋体" size="3" color="#000000"><strong>JavaScript教程</strong></font><font
color="#FF0000" face="宋体" size="3"><br>
</font><font face="宋体" size="3"><font color="#FF0000">第一页</font>：<a
href="day2_7.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day2_7.html">第二天课程简介</a><br>
<font color="#FF0000">第二页</font>：<a href="day2_2.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day2_2.html">变量介绍</a><br>
<font color="#FF0000">第三页</font>：<a href="day2_3.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day2_3.html">首个变量例子的程序主体</a><br>
<font color="#FF0000">第四页</font>：<a href="day2_4.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day2_4.html">字符串的魔力</a><br>
<font color="#FF0000">第五页</font>：<a href="day2_5.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day2_5.html">变量练习</a><br>
<font color="#FF0000">第六页</font>：<a href="day2_6.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day2_6.html">if-then子句</a><br>
<font color="#FF0000">第七页</font>：if-then语句的例子<br>
<font color="#FF0000">第八页</font>：<a href="day2_8.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day2_8.html">if-then练习</a><br>
<font color="#FF0000">第九页</font>：<a href="day2_9.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day2_9.html">链结事件</a><br>
<font color="#FF0000">第十页</font>：<a href="day2_10.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day2_10.html">图片交换</a><br>
<font color="#FF0000">第十一页</font>：<a href="day2_11.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day2_11.html">练习2</a><br>
<font color="#FF0000">第十二页</font>：<a href="day2_12.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day2_12.html">复习</a></font></p>
<p align="left"><font face="宋体" size="3">[<a href="index.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/index.html">第1课</a>][第2课][<a
href="day3_1.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day3_1.html">第3课</a>][<a href="day4_1.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day4_1.html">第4课</a>][<a href="day5_1.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day5_1.html">第5课</a>]</font></p>
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