Question

I have this output in the console:

console.log((!undefined)==(!false)) // true (fine)
console.log((!!undefined)==(!!false)) // true (still fine)

As I know, !!x==x, isn’t it?

console.log((undefined)==(false)) // false

Can anyone tell me why this returns false?

Is not true that !!false==false and !!undefined==undefined?

Answer 1

Yes. !!x does not return x. !undefined coerces undefined to a boolean, false, then finds !((bool)undefined)=!false, if we use C++ cast notation. So !!undefined gives !!((bool)undefined)=!!(false)=!true=false, rather than undefined.