web-midi-do-re-mi.js

const NOTE_ON = 0x90;
const NOTE_OFF = 0x80;

const delay = (ms) =>
  new Promise(resolve => setTimeout(resolve, ms));

async function playDoReMi(midi) {
  const midiOutput = Array.from(midi.outputs.values())[0];

  const notes = [60, 62, 64]; // Do-Re-Mi
  for (let note of notes) {
    midiOutput.send([NOTE_ON, note, 127]);
    await delay(500);
    midiOutput.send([NOTE_OFF, note, 0]);
  }
}

navigator.requestMIDIAccess().then(playDoReMi);