What does the value NaN mean in the context of JavaScript?

In JavaScript, the value NaN stands for "Not-a-Number". It is a special numeric value indicating that the result of a computation is not a valid number.

NaN occurs, for example, when attempting to perform arithmetic operations with non-numeric values that cannot be converted to a number:

console.log(0 / 0);          // NaN
console.log(parseInt("abc")); // NaN
console.log(Math.sqrt(-1));  // NaN

Features of NaN:

• NaN is of type number.
• NaN is not equal to anything, including itself: NaN === NaN returns false.
• To check if a value is NaN, use the isNaN() function or the more reliable Number.isNaN().

Example of checking:

let value = NaN;
console.log(value === NaN);           // false
console.log(isNaN(value));            // true
console.log(Number.isNaN(value));    // true

Thus, NaN serves as an indicator of errors in numerical calculations and helps identify incorrect operations.