checked it via if (sum!== ‘NaN’)
'NaN' is a string,
NaN is a number if you check it using
typeof :/. Also,
NaN === NaN turns out to be
false which is confusing and f*cked up but we can use
isNaN() to avoid that.
However, I don’t think I would have solved it that way. Assigning NaN to a variable seems weird I think.