In JavaScript,
NaN stands for Not a Number.
If we perform an arithmetic operation
on a value that is not a number,
we get NaN as the result.
In the example given above,
the value stored
in the personID variable
is a string.
So when we perform
personID * 2
we are multiplying a string
with a number.
Hence the result is NaN.
Here's another example
where performing an arithmetic operation
on a value that is not a number,
results in NaN.