Render a metronome mp3 in response to an HTTP request

metronome.js

// Google Cloud Source repos don't allow world-readable so this is the next best thing
// this is the code as of SHA b34717ce19fa9da940333402e7534c0e99f687a3
const synth = require("synth-js");
const MidiWriter = require("midi-writer-js");
const ffmpeg = require("fluent-ffmpeg");
const { Readable } = require("stream");
const concat = require("concat-stream");

const MEASURES = 100;

const THE_ONE = { pitch: ["A6"], duration: "16" };
const TWO_THREE_FOUR = {
  pitch: ["A4"],
  duration: "16",
  wait: ["16", "16", "16"],
  repeat: "3",
};

const MAX_BPM = 400;

function validateRequest(req) {
  const { bpm } = req.query;
  if (!bpm) {
    throw new Error("bpm param must be set");
  }

  if (Number.isNaN(Number.parseInt(bpm))) {
    throw new Error("bpm must be a number");
  }

  if (bpm > MAX_BPM) {
    throw new Error(`bpm must be less than ${MAX_BPM}`);
  }

  if (bpm < 40) {
    throw new Error("bpm must be less than 40");
  }
}

exports.metronome = (req, res) => {
  try {
    validateRequest(req);
    // Start with a new track
    const track = new MidiWriter.Track();
    const { bpm } = req.query;

    // Define an instrument (optional):
    track.addEvent(new MidiWriter.ProgramChangeEvent({ instrument: 1 }));
    track.setTempo(bpm);

    track.addEvent(
      [
        new MidiWriter.NoteEvent(THE_ONE),
        new MidiWriter.NoteEvent(TWO_THREE_FOUR),
      ],
      () => ({ sequential: true })
    );

    for (let x = 0; x < MEASURES; x++) {
      track.addEvent(
        [
          new MidiWriter.NoteEvent({ ...THE_ONE, wait: ["16", "16", "16"] }),
          new MidiWriter.NoteEvent(TWO_THREE_FOUR),
        ],
        () => ({ sequential: true })
      );
    }

    const midiWriter = new MidiWriter.Writer(track);

    const midiBuffer = new Buffer.from(midiWriter.buildFile());
    const wavBuffer = synth.midiToWav(midiBuffer).toBuffer();
    const metronomeWavStream = Readable.from(wavBuffer);
    const filename = `${bpm}.mp3`;

    res.setHeader("Content-disposition", "attachment; filename=" + filename);
    res.contentType("mp3");

    const concatStream = concat((buffer) => {
      res.send(buffer);
      res.end();
    });

    ffmpeg()
      .format("mp3")
      .audioCodec("libmp3lame")
      .audioBitrate(8)
      .audioFrequency(12000)
      .input(metronomeWavStream)
      .pipe(concatStream);
  } catch (err) {
    res.status(500).send(err.message);
  }
};

twilio-function.js

exports.handler = function(context, event, callback) {
	const twiml = new Twilio.twiml.VoiceResponse();
  const { bpm } = event;

  twiml.play(`💃.cloudfunctions.net/metronome?bpm=${bpm}`)

  return callback(null, twiml);
};