Unquoted literal constant NaN is a special value representing Not-a-Number. Since NaN always compares unequal to any number, including NaN, it is usually used to indicate an error condition for a function that should return a valid number.
Note: Use the isNaN() global function to see if a value is an NaN value.
You can access this property using the following syntax:
var val = Number.NaN;
Here dayOfMonth is assigned NaN if it is greater than 31, and a message is displayed indicating the valid range:
var dayOfMonth = 50;
if (dayOfMonth < 1 || dayOfMonth > 31)
dayOfMonth = Number.NaN
alert("Day of Month must be between 1 and 31.")
alert("Value of dayOfMonth : " + dayOfMonth );
<p>Click the following to see the result:</p>
<input type="button" value="Click Me" onclick="showValue();" />
This will produce following result:
Day of Month must be between 1 and 31.
Value of dayOfMonth : NaN