The core components in YouMe can be used to request access to MIDI devices, to interact with ports, and to connect ports to each other. See the core component docs for more details.
If you would like to display onscreen controls to do things like select your own MIDI port, you can use the grades provided by YouMe as a starting point. See the UI component docs for more details.
YouMe provides components and functions designed to help test your work. Chief among them is a test component that is intended to take the place of the WebMIDI API when running tests, so that you can simulate things like connecting and disconnecting devices. See the test helper docs for more details.