What is NaN? Provide examples when NaN occurs. How to check if a value is NaN?

NaN (Not a Number) is a special value in JavaScript that indicates the result of a non-numeric operation with numbers.

Examples of NaN occurrence:

• Result of an operation with non-numeric strings: parseInt("abc") returns NaN.
• Division of 0 by 0: 0 / 0 is NaN.
• Result of mathematical operations with undefined: undefined * 5 is NaN.

Checking for NaN:

• The standard way is to use the function Number.isNaN(value), which returns true only if the value is truly NaN.
• It is not recommended to use the global function isNaN(), as it converts the argument to a number and may return true for non-numeric values.

Example:

console.log(Number.isNaN(NaN)); // true
console.log(Number.isNaN("abc")); // false
console.log(isNaN("abc")); // true (undesirable behavior)

Thus, NaN is an indicator of an error or an undefined numeric result, and it is better to use Number.isNaN() for checking.