The isNaN() function in JavaScript is used to determine whether a given value is NaN (Not-a-Number) or not. NaN is a special value that represents the result of an operation that is not a number, such as dividing by zero or taking the square root of a negative number.
The isNaN() function takes a single argument, which can be of any data type. If the argument is not a number (i.e., it is a string, object, or any other non-numeric data type), the function will attempt to convert it to a number. If the argument cannot be converted to a number, the function will return true. If the argument is a number (including the special values NaN, Infinity, and -Infinity), the function will return false.
sNote that the isNaN() function is not the same as the Number.isNaN() method, which was introduced in ECMAScript 6. The Number.isNaN() method is a stricter version of the isNaN() function that only returns true if the argument is the special value NaN, and returns false for any other value.