Using MIDI with JavaScript

web audio

December 30, 2020

With the Web MIDI API, we can send and receive MIDI events from javascript. This allows us to trigger notes on any instrument!

Turning MIDI into sound

Before we get started, we need to make sure we can actually hear something:

External Instrument

One way to turn MIDI into sound is to trigger an external hardware instrument. For example, we could send the MIDI from the browser though an audio interface MIDI output to the MIDI input of a synth.

Internal MIDI

As described here, MIDI can be received from the browser via IAC (at least on macOS). With it, we can control any software instrument / daw on the same computer. I am using Ableton, setting up a MIDI instrument track.

Live Coding with RunJS

A quick and simple solution to trigger midi from javascript is using RunJS and webmidi. After installation, we can use the following function to run any midi logic:

import WebMidi from 'webmidi';

function run(fn, clear = true) {
  WebMidi.enable((err) => {
    if (err) {
      throw new Error('Web Midi could not be enabled...');
    if (WebMidi.outputs.length === 0) {
      throw new Error('No output found...');
    // console.log('outputs', WebMidi.outputs)
    const output = WebMidi.outputs[0]; // use the input you like
    console.log(`using output "${}"`);
    if (clear) {
    setTimeout(() => fn(output), 5);
const stopAll = () => run(() => output.sendChannelMode('allnotesoff'));
// test chord:
run((output) => output.playNote(['C3', 'E3', 'G3', 'B3'], 1));
stopAll(); // stops all notes if no run call is active

I found it best to disable "auto-run code on change" to trigger the code only when wanted with CMD+R. If you have an instrument connected that listens for channel 1, the above code should produce a beatiful C major 7 chord.

Scale Trainer

Another way to use this is ear training. We can trigger random notes out of a given scale like this:

import { Interval, Note, Scale } from '@tonaljs/tonal';

const random = (a) => a[Math.floor(a.length * Math.random())];

const scaleTrainer = (root, scale, clear) =>
  run((output) => {
    const notes = [root, random(Scale.get(`${root} ${scale}`).notes)];
    console.log('play notes', notes);
    output.playNote(notes, 2);
  }, clear);

scaleTrainer('C4', 'dorian', false);

This is just an example how we can get creative with this tool.

Running from a Web Browser

Currently, the Web MIDI API only has limited browser support.


The following hook can be used to interface with WebMidi:

import { useEffect, useState } from 'react';
import WebMidi from 'webmidi';

export default function useWebMidi({ ready, connected, disconnected }: any) {
  const [loading, setLoading] = useState(true);
  const [outputs, setOutputs] = useState<any[]>(WebMidi?.outputs || []);
  useEffect(() => {
    WebMidi.enable((err) => {
      if (err) {
        //throw new Error("Web Midi could not be enabled...");
        console.warn('Web Midi could not be enabled..');
      // Reacting when a new device becomes available
      WebMidi.addListener('connected', (e) => {
        connected?.(WebMidi, e);

      // Reacting when a device becomes unavailable
      WebMidi.addListener('disconnected', (e) => {
        disconnected?.(WebMidi, e);
  }, [ready, connected, disconnected, outputs]);
  const outputByName = (name) => WebMidi.getOutputByName(name);
  return { loading, outputs, outputByName };


Let's use the hook to show a midi output selector:

Waiting for Web MIDI API.. Reload if nothing happens

import MidiSelect from '../components/midi/MidiSelect.tsx';
import { State } from 'react-powerplug';

<State initial={{}}>
  {({ setState, state }) => (
      <MidiSelect onChange={(output) => setState({ output })} />
      {state.output && (
          {state.output?.name} selected
          <br />
          <button onClick={() => state.output?.playNote('A4')}>play A4</button>
          <button onClick={() => state.output?.sendChannelMode('allnotesoff')}>stop</button>

Midi Instrument

We can now use the above knowledge to create a special midi instrument for the rhythmical player.

The instrument can be used like any other instrument:

  instruments={{ piano: midi('IAC-Treiber Bus 1', 1) }}
      duration: 14,
      instrument: 'piano',
      parallel: [
          ['e5', ['b4', 'c5'], 'd5', ['c5', 'b4']],
          ['a4', ['a4', 'c5'], 'e5', ['d5', 'c5']],
          ['b4', ['r', 'c5'], 'd5', 'e5'],
          ['c5', 'a4', 'a4', 'r'],
          [['r', 'd5'], ['r', 'f5'], 'a5', ['g5', 'f5']],
          ['e5', ['r', 'c5'], 'e5', ['d5', 'c5']],
          ['b4', ['b4', 'c5'], 'd5', 'e5'],
          ['c5', 'a4', 'a4', 'r'],
          ['e2', 'e3', 'e2', 'e3', 'e2', 'e3', 'e2', 'e3'],
          ['a2', 'a3', 'a2', 'a3', 'a2', 'a3', 'a2', 'a3'],
          ['g#2', 'g#3', 'g#2', 'g#3', 'e2', 'e3', 'e2', 'e3'],
          ['a2', 'a3', 'a2', 'a3', 'a2', 'a3', 'b1', 'c2'],
          ['d2', 'd3', 'd2', 'd3', 'd2', 'd3', 'd2', 'd3'],
          ['c2', 'c3', 'c2', 'c3', 'c2', 'c3', 'c2', 'c3'],
          ['b1', 'b2', 'b1', 'b2', 'e2', 'e3', 'e2', 'e3'],
          ['a1', 'a2', 'a1', 'a2', 'a1', 'a2', 'a1', 'a2'],

This is the source code for the instrument:

import WebMidi, { Output } from 'webmidi';
import * as Tone from 'tone';
import enableWebMidi from '../../midi/enableWebMidi';

export function midi(outputName, channel) {
  return () => new Promise((resolve, reject) => {
    enableWebMidi().then(() => {
      if (WebMidi.outputs.length === 0) {
        throw new Error('No output found...');
      const output = WebMidi.getOutputByName(outputName) as Output;
      const s = {
        triggerAttackRelease: (note, duration, time?, velocity?) => {
          const timingOffset = WebMidi.time - Tone.context.currentTime * 1000
          time = time * 1000 + timingOffset;
          output?.playNote(note, channel, { time, duration: duration * 1000 - 5, velocity: 0.5 });
        connect: (dest) => { return s },
        toMaster: () => { return s },
      // onload(instrument)
    }).catch(() => {
      throw new Error('Web Midi could not be enabled...');

You might wonder what enableWebMidi is doing there, and why not use WebMidi.enable? This is a workaround to be able to use WebMidi multiple times on one page. It seems that WebMidi.enable will only work once.. The enableWebMidi function looks like that:

import WebMidi from 'webmidi';

export default function enableWebMidi() {
  return new Promise((resolve, reject) => {
    if (WebMidi.enabled) {
      // if already enabled, just resolve WebMidi
    WebMidi.enable((err) => {
      if (err) {


Felix Roos 2023