Is NaN falsy? Why NaN === false returns false

Question

1. Why NaN === false => false, isn\'t NaN falsy?
2. Why NaN === NaN => false, but !!NaN === !!NaN => true

I\'v

Answer 1

This condition:

NaN === false

Is always false because numbers are not booleans. To test if a value is falsy you can use a ternary expression:

NaN ? "truthy" : "falsy" // falsy

Why NaN === NaN => false

This is explained in MDN; pragmatically speaking, though, two values of which you only know they're not numbers can't logically be the same thing.

... but why is !!NaN === !!NaN => true

This is because casting NaN into a boolean will make it false and booleans can be compared as per normal.