What is NaN and in what cases does it occur?

NaN (Not a Number) is a special value in JavaScript that indicates the result of an operation that does not produce a number. It occurs when a mathematical operation or conversion cannot return a valid numeric value.

Examples of when NaN occurs:

• Division of 0 by 0: 0 / 0.
• Converting a non-numeric string to a number: Number('abc').
• Result of invalid mathematical operations, such as taking the square root of a negative number: Math.sqrt(-1).

It is important to note that NaN is not equal to itself (NaN !== NaN), so functions like isNaN() or Number.isNaN() are used for checking.

console.log(0 / 0); // NaN
console.log(Number('abc')); // NaN
console.log(Math.sqrt(-1)); // NaN
console.log(NaN === NaN); // false
console.log(Number.isNaN(NaN)); // true