在JavaScript 中，逻辑NOT运算符与C和Java中的逻辑 NOT 运算符相同，都由感叹号（!）表示。与逻辑 OR 和逻辑 AND 运算符不同的是，逻辑 NOT 运算符返回的一定是 Boolean 值。
逻辑 NOT 运算符的行为如下：
- 如果运算数是对象，返回 false
- 如果运算数是数字 0，返回 true
- 如果运算数是 0 以外的任何数字，返回 false
- 如果运算数是 null，返回 true
- 如果运算数是 NaN，返回 true
- 如果运算数是 undefined，发生错误
测试脚本如下：
方法把尝试字符串abc转换成整数，因为abc不是数字，因此是转不了的，所以返回的结果就是NaN
/
writeln() 方法与 write() 方法几乎一样，差别仅在于是前者将在所提供的任何字符串后添加一个换行符。在HTML中，这通常只会在后面产生一个空格；
不过如果使用了
- 和
<span class="pln"> </span><span class="pun">标识，这个换行符会被解释，且在浏览器中<</span><span class="pln">a href</span><span class="pun">=</span><span class="str">"/tag/xianshi/"</span><span class="pln"> target</span><span class="pun">=</span><span class="str">"_blank"</span><span class="pln"> </span><span class="kwd">class</span><span class="pun">=</span><span class="str">"keywords"</span><span class="pun">>显示<</span><span class="str">/a>。<br /</span><span class="pun">></span> <span class="pun"><</span><span class="str">/em>/</span><span class="pun"><</span><span class="pln">br </span><span class="pun">/></span> <span class="pln">document</span><span class="pun">.</span><span class="pln">writeln</span><span class="pun">(</span><span class="str">"<XMP>"</span><span class="pun">);<</span><span class="pln">br </span><span class="pun">/></span> <span class="pln">document</span><span class="pun">.</span><span class="pln">writeln</span><span class="pun">(</span><span class="str">"oNaN=parseInt(\"abc\")返回的结果是"</span><span class="pun">+</span><span class="pln">oNaN</span><span class="pun">);<</span><span class="pln">br </span><span class="pun">/></span> <span class="pln">document</span><span class="pun">.</span><span class="pln">writeln</span><span class="pun">(</span><span class="str">"bool类型的数false与!运算符运算之后的结果是："</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> </span><span class="pun">(!</span><span class="pln">bFalse</span><span class="pun">));<</span><span class="pln">br </span><span class="pun">/></span> <span class="pln">document</span><span class="pun">.</span><span class="pln">writeln</span><span class="pun">(</span><span class="str">"字符串sRed与!运算符运算之后的结果是： "</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> </span><span class="pun">(!</span><span class="pln">sRed</span><span class="pun">));<</span><span class="pln">br </span><span class="pun">/></span> <span class="pln">document</span><span class="pun">.</span><span class="pln">writeln</span><span class="pun">(</span><span class="str">"数字0与!运算符运算之后的结果是："</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> </span><span class="pun">(!</span><span class="pln">iZero</span><span class="pun">));</span><span class="com">//如果运算数是数字 0，返回 true<br /></span> <span class="pln">document</span><span class="pun">.</span><span class="pln">writeln</span><span class="pun">(</span><span class="str">"数字345与!运算符运算之后的结果是："</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> </span><span class="pun">(!</span><span class="pln">iThreeFourFive</span><span class="pun">));</span><span class="com">//如果运算数是 0 以外的任何数字，返回 false<br /></span> <span class="pln">document</span><span class="pun">.</span><span class="pln">writeln</span><span class="pun">(</span><span class="str">"对象oObject与!运算符运算之后的结果是："</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> </span><span class="pun">(!</span><span class="pln">oObject</span><span class="pun">));</span><span class="com">//如果运算数是对象，返回 false<br /></span> <span class="pln">document</span><span class="pun">.</span><span class="pln">writeln</span><span class="pun">(</span><span class="str">"NaN与!运算符运算之后的结果是："</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> </span><span class="pun">(!</span><span class="pln">oNaN</span><span class="pun">));</span><span class="com">//如果运算数是NaN，返回 true<br /></span> <span class="pln">document</span><span class="pun">.</span><span class="pln">writeln</span><span class="pun">(</span><span class="str">"null与!运算符运算之后的结果是："</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> </span><span class="pun">(!</span><span class="pln">oNull</span><span class="pun">));</span><span class="com">//如果运算数是 null，返回 true<br /></span> <span class="pln">document</span><span class="pun">.</span><span class="pln">writeln</span><span class="pun">(</span><span class="str">"undifined与!运算符运算之后的结果是："</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> </span><span class="pun">(!</span><span class="pln">oUndefined</span><span class="pun">));<</span><span class="pln">br </span><span class="pun">/></span> <span class="com">//document.writeln("未定义的字符串sBule与!运算符运算之后的结果是：" + (!sBule));//sBule前面没有定义，也就是sBule运算数是 undefined，因此这里发生<a href="/tag/cuowu/" target="_blank" class="keywords">错误</a><br /></span> <span class="pln">document</span><span class="pun">.</span><span class="pln">writeln</span><span class="pun">(</span><span class="str">"</span> ");
运行结果：
判断JavaScript变量的Boolean 值时，也可以使用逻辑NOT运算符。这样做需要在一行代码中使用两个 NOT 运算符。无论运算数是什么类型，第一个NOT运算符返回 Boolean值，第二个NOT将对该Boolean值取反，从而给出变量真正的Boolean值。使用not运算符判断JavaScript变量的Boolean值是一个非常有用的技巧，只要知道了变量的boolean值，那么当使用变量进行&&或者||运算时，就可以很快知道运算的结果了。
测试脚本如下：
方法把尝试字符串abc转换成整数，因为abc不是数字，因此是转不了的，所以返回的结果就是NaN
/*
判断JavaScript 变量的 Boolean 值时，也可以使用逻辑 NOT 运算符。这样做需要在一行代码中使用两个 NOT 运算符。
无论运算数是什么类型，第一个 NOT 运算符返回 Boolean 值。第二个 NOT 将对该 Boolean 值取反，从而给出变量真正的 Boolean 值。
*/
document.write("
");
- ");
- document.writeln("布尔数false 的逻辑值是 " + (!!bFalse));
- document.writeln("字符串sRed 的逻辑值是 " + (!!sRed));
- document.writeln("数字0 的逻辑值是 " + (!!iZero));
- document.writeln("数字345 的逻辑值是 " + (!!iThreeFourFive));
- document.writeln("对象Object 的逻辑值是 " + (!!oObject));
- document.writeln("NaN的逻辑值是 ：" + (!!oNaN));
- document.writeln("null 的逻辑值是 " + (!!oNull));
- document.writeln("undefined 的逻辑值是 " + (!!oUndefined));
- document.write("
