Answer 1

possibly it is used for indeterminant form ... for example 0/0 form or oo - oo form etc ☺

Answer 2

NaN means "Not a Number". "777" is not a number (it's text), but javascript will try to convert it to a number before it checks for isNaN-ness.

for example
isNaN(NaN) === true
isNaN(10) === false
isNaN("10") === false

isNaN({}) === true
isNaN([]) === false // what?

So funnily enough, isNaN is not a good way to check whether something is NaN or not but I don't want to go into too much detail...