How to store standard error in a variable

Question

Let\'s say I have a script like the following:

useless.sh

echo \"This Is Error\" 1>&2
echo \"This Is Output\" 

And I have an

Answer 1

$ b=$( ( a=$( (echo stdout;echo stderr >&2) ) ) 2>&1 )
$ echo "a=>$a b=>$b"
a=>stdout b=>stderr