uTools-Manuals/docs/javascript/Reference/Global_Objects/isNaN.html

<article id="wikiArticle">
<div></div>
<p><code><strong>isNaN()</strong></code> 函数用来确定一个值是否为<a href="Reference/Global_Objects/NaN" title="全局属性 NaN 的值表示不是一个数字（Not-A-Number）。"><code>NaN</code></a> 。注：<code>isNaN</code>函数内包含一些非常有趣的<a href="#Description">规则</a>；你也可以通过ECMAScript 2015/ES6 中定义的<a href="Reference/Global_Objects/Number/isNaN" title="Number.isNaN() 方法确定传递的值是否为 NaN和其类型是 Number。它是原始的全局isNaN()的更强大的版本。"><code>Number.isNaN()</code></a>或者 可以使用<code><a href="/en-US/docs/Web/JavaScript/Reference/Operators/typeof">typeof</a></code> 来判断该值是否为一个非数字。</p>
<h2 id="Syntax" name="Syntax">语法</h2>
<pre><code class="language-javascript"><code>isNaN(<em>testValue</em>)</code></code></pre>
<h3 id="Parameters" name="Parameters">参数</h3>
<dl>
<dt><code>testValue</code></dt>
<dd>要被检测的值。</dd>
</dl>
<h3 id="返回值">返回值</h3>
<p>如果给定值为 <a href="Reference/Global_Objects/NaN" title="全局属性 NaN 的值表示不是一个数字（Not-A-Number）。"><code>NaN</code></a>则返回值为<code>true</code>；否则为<code>false</code>。</p>
<h2 id="Description" name="Description">描述</h2>
<h3 id="The_necessity_of_an_isNaN_function" name="The_necessity_of_an_isNaN_function"><code>isNaN</code> 函数的必要性</h3>
<p>与 JavaScript 中其他的值不同，<a href="Reference/Global_Objects/NaN" title="全局属性 NaN 的值表示不是一个数字（Not-A-Number）。"><code>NaN</code></a>不能通过相等操作符（== 和 ===）来判断 ，因为 <code>NaN == NaN</code> 和 <code>NaN === NaN</code> 都会返回 <code>false</code>。 因此，<code>isNaN</code> 就很有必要了。</p>
<h3 id="Origin_of_NaN_values" name="Origin_of_NaN_values"><code>NaN值</code>的产生</h3>
<p><font face="Courier New, Andale Mono, monospace">当算术运算返回一个未定义的或无法表示的值时，NaN就产生了。但是，<code>NaN</code>并不一定用于表示某些值超出表示范围的情况。</font><font face="Consolas, Monaco, 'Andale Mono', monospace">将某些不能强制转换为数值的非数值转换为数值的时候，也会得到<code>NaN</code></font>。</p>
<p>例如，0 除以0会返回<code>NaN </code>—— 但是其他数除以0则不会返回<code>NaN</code>。</p>
<h3 id="令人费解的怪异行为">令人费解的怪异行为</h3>
<p>如果<code>isNaN</code>函数的参数不是<code>Number</code>类型， <code>isNaN</code>函数会首先尝试将这个参数转换为数值，然后才会对转换后的结果是否是<a href="Reference/Global_Objects/NaN" title="全局属性 NaN 的值表示不是一个数字（Not-A-Number）。"><code>NaN</code></a>进行判断。因此，对于能被强制转换为有效的非NaN数值来说（空字符串和布尔值分别会被强制转换为数值0和1），返回<code>false</code>值也许会让人感觉莫名其妙。比如说，空字符串就明显“不是数值（not a number）”。这种怪异行为起源于："不是数值（not a number）"在基于IEEE-754数值的浮点计算体制中代表了一种特定的含义。<code>isNaN</code>函数其实等同于回答了这样一个问题：被测试的值在被强制转换成数值时会不会返回IEEE-754​中所谓的“不是数值（not a number）”。</p>
<p>下一个版本的ECMAScript (ES2015)包含<a href="Reference/Global_Objects/Number/isNaN" title="Number.isNaN() 方法确定传递的值是否为 NaN和其类型是 Number。它是原始的全局isNaN()的更强大的版本。"><code>Number.isNaN()</code></a>函数。通过<code>Number.isNaN(x)</code>来检测<code>变量x</code>是否是一个<code>NaN</code><font face="'Open Sans', sans-serif">将会是一种可靠的做法</font>。然而，在缺少<code>Number.isNaN</code>函数的情况下, 通过表达式<code>(x != x)</code> 来检测<code>变量x</code>是否是<code>NaN</code>会更加可靠。</p>
<p>一个<code>isNaN</code>的 polyfill 可以理解为（这个polyfill利用了<code>NaN</code>自身永不相等于自身这一特征 ）：</p>
<pre><code  class="language-javascript"><code class="language-js"><span class="keyword token">var</span> isNaN <span class="operator token">=</span> <span class="keyword token">function</span><span class="punctuation token">(</span>value<span class="punctuation token">)</span> <span class="punctuation token">{</span>
    <span class="keyword token">var</span> n <span class="operator token">=</span> <span class="function token">Number</span><span class="punctuation token">(</span>value<span class="punctuation token">)</span><span class="punctuation token">;</span>
    <span class="keyword token">return</span> n <span class="operator token">!==</span> n<span class="punctuation token">;</span>
<span class="punctuation token">}</span><span class="punctuation token">;</span></code></code></pre>
<h2 id="Examples" name="Examples">示例</h2>
<pre><code  class="language-javascript">isNaN(NaN);       // true
isNaN(undefined); // true
isNaN({});        // true

isNaN(true);      // false
isNaN(null);      // false
isNaN(37);        // false

// strings
isNaN("37");      // false: 可以被转换成数值37
isNaN("37.37");   // false: 可以被转换成数值37.37
isNaN("37,5");    // true
isNaN('123ABC');  // true:  parseInt("123ABC")的结果是 123, 但是Number("123ABC")结果是 NaN
isNaN("");        // false: 空字符串被转换成0
isNaN(" ");       // false: 包含空格的字符串被转换成0

// dates
isNaN(new Date());                // false
isNaN(new Date().toString());     // true

isNaN("blabla")   // true: "blabla"不能转换成数值
                  // 转换成数值失败， 返回NaN
</code></pre>
<h3 id="有用的特殊行为">有用的特殊行为</h3>
<p>有许多方式来看待isNaN()：如果isNaN(x)返回false，那么x在任何算数表达式中都不会使表达式等于NaN；如果返回true，x会使所有算数表达式返回NaN。这就意味着，在JavaScript中，isNaN(x)==true等价于x-0=NaN(在JavaScript中 x-0 == NaN 总是返回false，所以你不用去测试它)。实际上， <code>isNaN(x)</code>, <code>isNaN(x - 0)</code>,<code>isNaN(Number(x))</code>, <code>Number.isNaN(x - 0)</code>,和<code>Number.isNaN(Number(x))</code> 的返回值都是一样的 并且在JavaScript中isNaN(x)是这些表达式中最短的表达。</p>
<p>举个例子，可以利用这个特殊行为来检测函数的参数是可运算的（可以像number一样进行加减乘除等运算）。如果不可运算，则可赋予这个参数一个默认的值或其他合适的内容。这样，就可以得到一个隐式转换参数值的函数，而这得益于Javascript的全功能性。</p>
<h2 id="例子">例子</h2>
<pre><code  class="language-javascript">function increment(x) {
  if (isNaN(x)) x = 0;
  return x + 1;
};

// The same effect with Number.isNaN():
function increment(x) {
  if (Number.isNaN(Number(x))) x = 0;
  return x + 1;
};

// In the following cases for the function's argument x,
// isNaN(x) is always false, although x is indeed not a
// number, but can be used as such in arithmetical
// expressions
increment("");            // 1: "" is converted to 0
increment(new String());  // 1: String object representing an empty string is converted to 0
increment([]);            // 1: [] is converted to 0
increment(new Array());   // 1: Array object representing an empty array is converted to 0
increment("0");           // 1: "0" is converted to 0
increment("1");           // 2: "1" is converted to 1
increment("0.1");         // 1.1: "0.1" is converted to 0.1
increment("Infinity");    // Infinity: "Infinity" is converted to Infinity
increment(null);          // 1: null is converted to 0
increment(false);         // 1: false is converted to 0
increment(true);          // 2: true is converted to 1
increment(new Date());    // returns current date/time in milliseconds plus 1

// In the following cases for the function's argument x,
// isNaN(x) is always false and x is indeed a number
increment(-1);            // 0
increment(-0.1);          // 0.9
increment(0);             // 1
increment(1);             // 2
increment(2);             // 3
// ... and so on ...
increment(Infinity);      // Infinity

// In the following cases for the function's argument x,
// isNaN(x) is always true and x is really not a number,
// thus the function replaces it by 0 and returns 1
increment(String);            // 1
increment(Array);             // 1
increment("blabla");          // 1
increment("-blabla");         // 1
increment(0/0);               // 1
increment("0/0");             // 1
increment(Infinity/Infinity); // 1
increment(NaN);               // 1
increment(undefined);         // 1
increment();                  // 1

// isNaN(x) is always the same as isNaN(Number(x)),
// but the presence of x is mandatory here!
isNaN(x) == isNaN(Number(x))  // true for every value of x, including x == undefined,
                              // because isNaN(undefined) == true and Number(undefined) returns NaN,
                              // but ...
isNaN() == isNaN(Number())    // false, because isNaN() == true and Number() == 0</code></pre>
<h2 id="规范">规范</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%201st%20edition,%20June%201997.pdf" hreflang="en" lang="en" rel="noopener" title="ECMAScript 1st Edition (ECMA-262)">ECMAScript 1st Edition (ECMA-262)</a></td>
<td><span class="spec-Standard">Standard</span></td>
<td>Initial definition.</td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/5.1/#sec-15.1.2.4" hreflang="en" lang="en" rel="noopener">ECMAScript 5.1 (ECMA-262)<br/><small lang="zh-CN">isNaN</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-isnan-number" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">isNaN</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
<tr>
<td><a class="external" href="https://tc39.github.io/ecma262/#sec-isnan-number" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">isNaN</small></a></td>
<td><span class="spec-Draft">Draft</span></td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容性">浏览器兼容性</h2>
<p></p><div class="blockIndicator warning"><strong><a class="external" href="https://github.com/mdn/browser-compat-data" rel="noopener">We're converting our compatibility data into a machine-readable JSON format</a></strong>.
            This compatibility table still uses the old format,
            because we haven't yet converted the data it contains.
            <strong><a class="new" href="/zh-CN/docs/MDN/Contribute/Structures/Compatibility_tables" rel="nofollow">Find out how you can help!</a></strong></div>
<div class="htab">
<a id="AutoCompatibilityTable" name="AutoCompatibilityTable"></a>
<ul>
<li class="selected"><a>Desktop</a></li>
<li><a>Mobile</a></li>
</ul>
</div><p></p>
<div id="compat-desktop">
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Chrome</th>
<th>Firefox (Gecko)</th>
<th>Internet Explorer</th>
<th>Opera</th>
<th>Safari</th>
</tr>
<tr>
<td>Basic support</td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
</tr>
</tbody>
</table>
</div>
<div id="compat-mobile">
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Android</th>
<th>Chrome for Android</th>
<th>Firefox Mobile (Gecko)</th>
<th>IE Mobile</th>
<th>Opera Mobile</th>
<th>Safari Mobile</th>
</tr>
<tr>
<td>Basic support</td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
</tr>
</tbody>
</table>
</div>
<h2 id="相关链接">相关链接</h2>
<ul>
<li><a href="Reference/Global_Objects/NaN" title="全局属性 NaN 的值表示不是一个数字（Not-A-Number）。"><code>NaN</code></a></li>
<li><a href="Reference/Global_Objects/Number/isNaN" title="Number.isNaN() 方法确定传递的值是否为 NaN和其类型是 Number。它是原始的全局isNaN()的更强大的版本。"><code>Number.isNaN()</code></a></li>
</ul>
</article>