NaN

The NaN property represents a value that is “not a number”. This special value results from an operation that could not be performed either because one of the operands was non-numeric (e.g., "abc" / 4), or because the result of the operation is non-numeric.

While this seems straightforward enough, there are a couple of somewhat surprising characteristics of NaN that can result in hair-pulling bugs if one is not aware of them.

For one thing, although NaN means “not a number”, its type is, believe it or not, Number:

console.log(typeof NaN === "number");  // logs "true"

Additionally, NaN compared to anything – even itself! – is false:

console.log(NaN === NaN);  // logs "false"

A semi-reliable way to test whether a number is equal to NaN is with the built-in function isNaN(), but even using isNaN() is an imperfect solution.

A better solution would either be to use value !== value, which would only produce true if the value is equal to NaN. Also, ES6 offers a new Number.isNaN() function, which is a different and more reliable than the old global isNaN() function.

Reference: Toptal Interview Questions