sound2wav.js

const parseNote = noteStr => {
  const [ note, rest = '' ] = noteStr.split('.')
  const duration = durations[rest.slice(0, -1)] / tempo
  return {
    duration,
    frequency: notes[note],
    type: types[rest[rest.length - 1]] || types[0],
  }
}

const writeString = (v,o,t) => [...t].forEach((c,i)=>v.setUint8(o+i,c.charCodeAt(0)))
const encodeWAV = async (input) => {
  const notes = input.split('+').map(parseNote)
  const duration = notes.reduce((a,n) => a + n.duration, 0)
  const offline = new OfflineAudioContext(1, 44100*duration, 44100)
  let delay = offline.currentTime
  for (const note of notes) {
    const o = offline.createOscillator()
    const g = offline.createGain()
    o.type = note.type
    o.frequency.value = note.frequency
    o.connect(g)
    g.connect(offline.destination)
    o.start(delay)
    g.gain.exponentialRampToValueAtTime(1e-9, delay += note.duration)
  }

  const samples = (await offline.startRendering()).getChannelData(0)
  const buffer = new ArrayBuffer(44 + samples.length * 2)
  const view = new DataView(buffer)
  
  writeString(view, 0, 'RIFF')                     // RIFF identifier
  view.setUint32(4, 32 + samples.length * 2, true) // file length
  writeString(view, 8, 'WAVE')                     // RIFF type
  writeString(view, 12, 'fmt ')                    // format chunk identifier
  view.setUint32(16, 16, true)                     // format chunk length
  view.setUint16(20, 1, true)                      // sample format (raw)
  view.setUint16(22, 1, true)                      // channel count
  view.setUint32(24, 44100, true)                  // sample rate
  view.setUint32(28, 44100 * 4, true)              // byte rate (sample rate * block align)
  view.setUint16(32, 4, true)                      // block align (channel count * bytes per sample)
  view.setUint16(34, 16, true)                     // bits per sample
  writeString(view, 36, 'data')                    // data chunk identifier
  view.setUint32(40, samples.length * 2, true)     // data chunk length
  var offset = 44                                  // floatTo16BitPCM
  for (var i = 0; i < samples.length; i++, offset += 2) {
    var s = Math.max(-1, Math.min(1, samples[i]))
    view.setInt16(offset, s < 0 ? s * 0x8000 : s * 0x7FFF, true)
  }

  const reader = new FileReader()
  const onload = new Promise(s => reader.onload = s)
  reader.readAsDataURL(new Blob([view], { type: 'audio/wav' }))
  const a = document.createElement('audio')
  a.src = (await onload).target.result
  a.type = 'audio/wav'
  return a
}