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copied!<p>I believe the code is trying to ask, "is <code>x</code> numeric?" with the specific case here of <code>x = null</code>. The function <code>isNaN()</code> can be used to answer this question, but semantically it's referring specifically to the value <code>NaN</code>. From Wikipedia for <a href="http://en.wikipedia.org/wiki/NaN" rel="noreferrer"><code>NaN</code></a>: </p> <blockquote> <p>NaN (<b>N</b>ot <b>a</b> <b>N</b>umber) is a value of the numeric data type representing an undefined or unrepresentable value, especially in floating-point calculations.</p> </blockquote> <p>In most cases we think the answer to "is null numeric?" should be no. However, <code>isNaN(null) == false</code> is semantically correct, because <code>null</code> is not <code>NaN</code>.</p> <p>Here's the algorithmic explanation:</p> <p>The function <code>isNaN(x)</code> attempts to convert the passed parameter to a number<sup><a href="https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/isNaN" rel="noreferrer">1</a></sup> (equivalent to <code>Number(x)</code>) and then tests if the value is <code>NaN</code>. If the parameter can't be converted to a number, <code>Number(x)</code> will return <code>NaN</code><sup><a href="https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Number" rel="noreferrer">2</a></sup>. Therefore, if the conversion of parameter <code>x</code> to a number results in <code>NaN</code>, it returns true; otherwise, it returns false. </p> <p>So in the specific case <code>x = null</code>, <code>null</code> is converted to the number 0, (try evaluating <code>Number(null)</code> and see that it returns 0,) and <code>isNaN(0)</code> returns false. A string that is only digits can be converted to a number and isNaN also returns false. A string (e.g. <code>'abcd'</code>) that cannot be converted to a number will cause <code>isNaN('abcd')</code> to return true, specifically because <code>Number('abcd')</code> returns <code>NaN</code>.</p> <p>In addition to these apparent edge cases are the standard numerical reasons for returning NaN like 0/0.</p> <p>As for the seemingly inconsistent tests for equality shown in the question, the behavior of <code>NaN</code> is specified such that any comparison <code>x == NaN</code> is false, regardless of the other operand, including <code>NaN</code> itself<sup><a href="https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/isNaN" rel="noreferrer">1</a></sup>.</p>

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