Edit in JSFiddle

// request MIDI access
if (navigator.requestMIDIAccess) {
    navigator.requestMIDIAccess({
        sysex: false // this defaults to 'false' and we won't be covering sysex in this article. 
    }).then(onMIDISuccess, onMIDIFailure);
} else {
    alert("No MIDI support in your browser.");
}

// midi functions
function onMIDISuccess(midiAccess) {
    // when we get a succesful response, run this code
    console.log('MIDI Access Object', midiAccess);
}

function onMIDIFailure(e) {
    // when we get a failed response, run this code
    console.log("No access to MIDI devices or your browser doesn't support WebMIDI API. Please use WebMIDIAPIShim " + e);
}