In Bash, how do I test if a variable is defined in “-u” mode

Question

I just discovered set -u in bash and it helped me find several previously unseen bugs. But I also have a scenario where I need to test if a variable is defined

Answer 1

This is what I've found works best for me, taking inspiration from the other answers:

if [ -z "${varname-}" ]; then
  ...
  varname=$(...)
fi