parseInt returns NaN for second element whereas Number works perfectly

Question

Consider this extract from a function for squaring every digit:

const result = num.toString().split('').map(Number).map(n => n * n);

In this when I use parseInt in the map() instead of Number, the second element in the array (with index 1) always gets the value NaN but Number works perfectly fine. I read the docs but still didn't get it. Can anyone tell why this happens?