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copied!<p>The NaN value is defined to be unequal to everything, including itself. Test if a value is NaN with the <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN" rel="nofollow noreferrer"><code>isNaN()</code></a> function, appropriately enough. (ECMAScript 6 adds a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/isNaN" rel="nofollow noreferrer"><code>Number.isNan()</code></a> function with different semantics for non-number arguments, but it's not supported in all browsers yet as of 2015).</p> <p>There are two built-in properties available with a NaN value: the global <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN" rel="nofollow noreferrer"><code>NaN</code></a> property (i.e. <code>window.NaN</code> in browsers), and <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/NaN" rel="nofollow noreferrer"><code>Number.NaN</code></a>. It is <em>not</em> a language keyword. In older browsers, the <code>NaN</code> property could be overwritten, with potentially confusing results, but with the <a href="http://www.ecma-international.org/ecma-262/5.1/#sec-15.1.1.1" rel="nofollow noreferrer">ECMAScript 5 standard it was made non-writable</a>.</p> <ul> <li>As @some pointed out in the comments, there is also the global function <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isFinite" rel="nofollow noreferrer">isFinite()</a> which may be useful.</li> </ul>

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