test process.env with Jest

I have an app that depends on environmental variables like:

const APP_PORT = process.env.APP_PORT || 8080;

and I would like to test that for example:

• APP_PORT can be set by node env variable.
• or that an express app is running on the port set with process.env.APP_PORT

How can I achieve this with Jest? Can I set these process.env variables before each test or should I mock it somehow maybe?