The isNaN function is used to determine if the argument, testvalue, is a NaN.

A NaN, which means "Not-a-Number", is classified as a primitive value by the ECMA-262 standard and indicates that the specified value is not a legal number. The function returns true if the argument is not a number and false if the argument is a number.

Code:
document.write(isNaN("Ima String"))

output:
true

Code:
document.write(isNaN(1.414))

output:
false