JavaScript 1.1; JScript 1.0; ECMAScript v1
x
The value to be tested.
true if x is (or can be converted to) the special not-a-number value; false if x is any other value.
isNaN( ) tests its argument to determine whether it is the value NaN, which represents an illegal number (such as the result of division by zero). This function is required, because comparing a NaN with any value, including itself, always returns false, so it is not possible to test for NaN with the == or === operators.
A common use of isNaN( ) is to test the results of parseFloat( ) and parseInt( ) to determine if they represent legal numbers. You can also use isNaN( ) to check for arithmetic errors, such as division by zero.
isNaN(0); // Returns false
isNaN(0/0); // Returns true
isNaN(parseInt("3")); // Returns false
isNaN(parseInt("hello")); // Returns true
isNaN("3"); // Returns false
isNaN("hello"); // Returns true
isNaN(true); // Returns false
isNaN(undefined); // Returns true