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<title>Webmonkey: javascript: Thau's JavaScript Tutorial: Day 1</title>
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<td WIDTH="68%" VALIGN="MIDDLE"><font FACE="宋體" LANG="ZH-CN" SIZE="4" COLOR="#ff0000"></font><b><font
FACE="System,黑體" LANG="ZH-CN" COLOR="#ff0000">第</font><font FACE="宋體"
LANG="ZH-CN" COLOR="#ff0000">7</font><font FACE="System,黑體" LANG="ZH-CN"
COLOR="#ff0000">頁</font><font FACE="宋體" LANG="ZH-CN" COLOR="#ff0000">: </font><font
FACE="System,黑體" LANG="ZH-CN" COLOR="#ff0000">一個簡單的</font><font
FACE="宋體" LANG="ZH-CN" COLOR="#ff0000"> if-then </font><font FACE="System,黑體"
LANG="ZH-CN" COLOR="#ff0000">語句的例子</font></b><font FACE="System,黑體"
LANG="ZH-CN"><br>
<!-- BYLINE (commented out for PageX) --><!-- <font size=1 color="#000000" face="arial, helvetica, sans-serif">by <a href="mailto:thau@wired.com" title="Send mail to Thau!">Thau!</a></font>--> </font><font FACE="宋體" LANG="ZH-CN"><br>
<!-- BYLINE --> </font><font FACE="System,黑體" LANG="ZH-CN" SIZE="2">作者：</font><a
HREF="mailto:thau@wired.com"><font FACE="宋體" LANG="ZH-CN" SIZE="2">Thau!</font></a><font
FACE="宋體" LANG="ZH-CN"> <!-- SEE ALSO LINKS --></font></td>
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<td WIDTH="68%" VALIGN="TOP" BGCOLOR="#ffffff"><font FACE="System,黑體" LANG="ZH-CN"></font></td>
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<td WIDTH="68%" VALIGN="MIDDLE" BGCOLOR="#ffffff"><font FACE="System,黑體" LANG="ZH-CN"></font><font
FACE="System,黑體" LANG="ZH-CN" SIZE="2">如果你在即時對話框中鍵入</font><tt>yes</tt><font
FACE="System,黑體" LANG="ZH-CN" SIZE="2">，你將在看到本頁<br>
其他部分前收到一個親切的問候。若敲入別的則<br>
沒有。<p>這里是該語句的核心：</font></td>
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<td WIDTH="68%" VALIGN="TOP" BGCOLOR="#ffffff"><pre>
var monkey_love = prompt("Do you love the monkey?","Type yes or no");
if (monkey_love == "yes")
{
alert("Welcome! I'm so glad you came! Please, read on!");
}
</pre>
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<td WIDTH="68%" VALIGN="MIDDLE" BGCOLOR="#ffffff"><font FACE="System,黑體" LANG="ZH-CN"></font><font
FACE="System,黑體" LANG="ZH-CN" SIZE="2">第一行你見過。它喚起一個對話框并將用戶的反饋調入<br>
變量</font><tt>monkey_love</tt><font FACE="System,黑體" LANG="ZH-CN" SIZE="2">中。但第二行就有些不同：它有個條<br>
件，即如果變量</font><tt>monkey_love</tt><font FACE="System,黑體" LANG="ZH-CN"
SIZE="2">等于值</font><font FACE="宋體" LANG="ZH-CN" SIZE="2">"yes" </font><font
FACE="System,黑體" LANG="ZH-CN" SIZE="2">，則運行花<br>
括號中的語句。若它等于其他值，則不運行。<p>注意該條件中的兩個等于標記，這是人們容易搞混的<br>
地方之一。如果你只用一個標記，實際上是告訴<br>
</font><font FACE="宋體" LANG="ZH-CN" SIZE="2">JavaScript</font><font
FACE="System,黑體" LANG="ZH-CN" SIZE="2">測試是否</font><tt>monkey_love<font
FACE="System,黑體" LANG="ZH-CN">等于</font></tt><font FACE="宋體" LANG="ZH-CN"
SIZE="2"> "yes"</font><font FACE="System,黑體" LANG="ZH-CN" SIZE="2">。<br>
幸運的是，多數瀏覽器在你運行這些語句時會識別<br>
這些錯誤并警告你。但最好現在開始就注意別犯這<br>
種錯誤</p>
<p>其他重要的條件是：</font></td>
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<td WIDTH="68%" VALIGN="TOP" BGCOLOR="#ffffff"><pre>
(variable_1 > variable_2) is true if variable_1 is greater than variable_2
(variable_1 < variable_2) is true if variable_1 is less than variable_2
(variable_2 <= variable_2) is true if variable_1 is less than or equal to variable_2
(variable_1 != variable_2) is true if variable_1 does <b>not</b> equal variable_2
</pre>
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<td WIDTH="68%" VALIGN="MIDDLE" BGCOLOR="#ffffff"><font FACE="System,黑體" LANG="ZH-CN"></font><font
FACE="System,黑體" LANG="ZH-CN" SIZE="2">有兩個方法可使你的條件更合理：<p>在運行花括號中的語句前如果你想要兩件事為“是”，<br>
可這樣做：</font></td>
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<td WIDTH="68%" VALIGN="TOP" BGCOLOR="#ffffff"><pre>
if ((variable_1 > 18) && (variable_1 < 21))
{
document.writeln("variable_1 can vote, but can't drink.");
}
</pre>
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<td WIDTH="68%" VALIGN="MIDDLE" BGCOLOR="#ffffff"><font FACE="System,黑體" LANG="ZH-CN"></font><font
FACE="System,黑體" LANG="ZH-CN" SIZE="2">注意這里的兩個“</font><font
FACE="宋體" LANG="ZH-CN" SIZE="2">&&</font><font FACE="System,黑體"
LANG="ZH-CN" SIZE="2">”在</font><font FACE="宋體" LANG="ZH-CN" SIZE="2">JavaScript</font><font
FACE="System,黑體" LANG="ZH-CN" SIZE="2">中這是“與”的<br>
意思。也注意整個子句有兩個部分，</font><font FACE="宋體" LANG="ZH-CN"
SIZE="2">&&</font><font FACE="System,黑體" LANG="ZH-CN" SIZE="2">須在圓括號中。<p>若想兩件事之一為真，這樣做：</font></td>
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<td WIDTH="68%" VALIGN="TOP" BGCOLOR="#ffffff"><pre>
if ((variable_1 == "bananas") || (variable_1 == "JavaScript"))
{
document.writeln("The monkey is happy because it has " + variable_1);
}
</pre>
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<td WIDTH="68%" VALIGN="MIDDLE" BGCOLOR="#ffffff"><font FACE="System,黑體" LANG="ZH-CN"></font><font
FACE="System,黑體" LANG="ZH-CN" SIZE="2">回到</font><a HREF="tutorial2_page8.html"><font
FACE="宋體" LANG="ZH-CN" SIZE="2">if-then </font><font FACE="System,黑體" LANG="ZH-CN"
SIZE="2">練習中來</font><font FACE="宋體" LANG="ZH-CN" SIZE="2">!</a> </font></td>
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<td WIDTH="68%" VALIGN="TOP" BGCOLOR="#ffffff"><font FACE="System,黑體" LANG="ZH-CN">第</font><font
FACE="宋體" LANG="ZH-CN">1</font><font FACE="System,黑體" LANG="ZH-CN">頁：<a
HREF="tutorial2.html">第二天課程簡介</a><br>
第</font><font FACE="宋體" LANG="ZH-CN">2</font><font FACE="System,黑體" LANG="ZH-CN">頁</font><font
FACE="宋體" LANG="ZH-CN">: </font><font FACE="System,黑體" LANG="ZH-CN"><a
HREF="tutorial2_page2.html">變量介紹</a><br>
第</font><font FACE="宋體" LANG="ZH-CN">3</font><font FACE="System,黑體" LANG="ZH-CN">頁</font><font
FACE="宋體" LANG="ZH-CN">:</font><font FACE="System,黑體" LANG="ZH-CN"><a
HREF="tutorial2_page3.html">首個變量例子的程序主體</a><br>
第</font><font FACE="宋體" LANG="ZH-CN">4</font><font FACE="System,黑體" LANG="ZH-CN">頁</font><font
FACE="宋體" LANG="ZH-CN">: </font><font FACE="System,黑體" LANG="ZH-CN"><a
HREF="tutorial2_page4.html">字符串的魔力</a><br>
第</font><font FACE="宋體" LANG="ZH-CN">5</font><font FACE="System,黑體" LANG="ZH-CN">頁</font><font
FACE="宋體" LANG="ZH-CN">: </font><font FACE="System,黑體" LANG="ZH-CN"><a
HREF="tutorial2_page5.html">變量練習</a><br>
第</font><font FACE="宋體" LANG="ZH-CN">6</font><font FACE="System,黑體" LANG="ZH-CN">頁</font><font
FACE="宋體" LANG="ZH-CN">: <a HREF="tutorial2_page6.html">if-then </font><font
FACE="System,黑體" LANG="ZH-CN">子句</a><br>
第</font><font FACE="宋體" LANG="ZH-CN">7</font><font FACE="System,黑體" LANG="ZH-CN">頁</font><font
FACE="宋體" LANG="ZH-CN">: <a HREF="tutorial2_page7.html">if-then </font><font
FACE="System,黑體" LANG="ZH-CN">語句的例子</a><br>
第</font><font FACE="宋體" LANG="ZH-CN">8</font><font FACE="System,黑體" LANG="ZH-CN">頁</font><font
FACE="宋體" LANG="ZH-CN">: <a HREF="tutorial2_page8.html">if-then </font><font
FACE="System,黑體" LANG="ZH-CN">練習</a><br>
第</font><font FACE="宋體" LANG="ZH-CN">9</font><font FACE="System,黑體" LANG="ZH-CN">頁</font><font
FACE="宋體" LANG="ZH-CN">: </font><font FACE="System,黑體" LANG="ZH-CN"><a
HREF="tutorial2_page9.html">鏈結事件</a><br>
第</font><font FACE="宋體" LANG="ZH-CN">10</font><font FACE="System,黑體"
LANG="ZH-CN">頁</font><font FACE="宋體" LANG="ZH-CN">: </font><font
FACE="System,黑體" LANG="ZH-CN"><a HREF="tutorial2_page10.html">圖片交換</a><br>
第</font><font FACE="宋體" LANG="ZH-CN">11</font><font FACE="System,黑體"
LANG="ZH-CN">頁</font><font FACE="宋體" LANG="ZH-CN">: </font><a
HREF="tutorial2_page11.html"><font FACE="System,黑體" LANG="ZH-CN">練習</font><font
FACE="宋體" LANG="ZH-CN"> 2</a><br>
</font><font FACE="System,黑體" LANG="ZH-CN">第</font><font FACE="宋體" LANG="ZH-CN">12</font><font
FACE="System,黑體" LANG="ZH-CN">頁</font><font FACE="宋體" LANG="ZH-CN">: </font><font
FACE="System,黑體" LANG="ZH-CN"><a HREF="tutorial2_page12.html">復習</a><br>
</font></td>
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