How to test express api on Travis

As mention in a previous post, the following lines only works on local machine for testing.

node app.js
karma start karma.conf.js --single-run

If I put it into the .travis.yml, it make Travis hanging on the "node app.js" line and fail the test after timeout. I think this is because node is supposed to start the app.js and keep listening. On the local machine, I can open 2 terminal windows and run them separately but I am not sure how to do it on Travis. Can anyone help?