song.js

const scale = {
    c: 0,
    'c#': 1,
    d: 2,
    'd#': 3,
    e: 4,
    f: 5,
    'f#': 6,
    g: 7,
    'g#': 8,
    a: 9,
    'a#': 10,
    b: 11
}

const freqs = [16.35, 17.32, 18.35, 19.45, 20.60, 21.83, 23.12, 24.50, 25.96, 27.50, 29.14, 30.87]

const baseFreq = note => {
    const m = /^([a-g]#?)/.exec(note)
    return freqs[scale[m[1]]]
}

const octaveMod = note => {
    const m = /(\d+)$/.exec(note)
    if(m) {
        return +m[1]
    }
    return 0
}

const adsr = (
    attack = 0.1,
    release = 0.05
) => fn => (t, d) => {
    if (t < attack * d) {
        return t / (attack * d) * fn(t, d)
    }
    if (t > d * (1 - release)) {
       return (1 - (t - d * (1 - release)) / (d * release)) * fn(t, d)
    }
    return fn(t, d)
}

const sin = (note, octave = 4) => adsr()((t, d) =>
    Math.sin(
        2 * Math.PI *
        t * baseFreq(note) *
        (2 ** (octave + octaveMod(note)))
    ))

const msin = (note, octave = 4) => (t, d) => {
    let sample = 0;
    for (let i = 0; i <= octave; i += 1) {
        sample += sin(note, octave - i)(t, d) / (i + 1)
    }
    return 2 * sample / (octave + 1)
}

const mix = (...fns) => (t, d) =>
    fns.reduce(
        (sum, fn) => sum + fn(t, d),
        0
    ) / fns.length

const seq = duration => (...fns) => t => {
    const offset = Math.floor(t / duration * fns.length)
    return fns[offset](t - offset * duration / fns.length, duration / fns.length)
}

const rep = (duration, times) => fn =>
    t => fn(t - Math.floor(t / duration * times) * duration / times, duration / times)

const Am = mix(
    msin('a'),
    msin('c1'),
    msin('e1')
)

const F = mix(
    msin('f'),
    msin('a'),
    msin('c1')
)

const C = mix(
    msin('c'),
    msin('e'),
    msin('g')
)

const G = mix(
    msin('g'),
    msin('d1'),
    msin('b1')
)

const verseDuration = 8
const nVerses = 4

const duration = verseDuration * nVerses

const verseA = seq(verseDuration)(Am, F, C, G)
const verseB = seq(verseDuration)(
    sin('c1'), sin('e1'), sin('g1'),
    sin('a1'), sin('f1'), sin('c1'),
    sin('c1'), sin('e1'), sin('g1'),
    sin('g1'), sin('d1'), sin('b1')
)

const verse = mix(verseA, verseB)

const song = rep(duration, nVerses)(verse)

const ctx = new window.AudioContext()
const osc = ctx.createOscillator()
const buf = ctx.createBuffer(
    2,
    ctx.sampleRate * duration,
    ctx.sampleRate
)
for(let c = 0; c < 2; c += 1) {
    const data = buf.getChannelData(c)
    for (let s = 0; s < data.length; s += 1) {
        data[s] = song(s / ctx.sampleRate)
    }
}
const src = ctx.createBufferSource()
src.buffer = buf
src.connect(ctx.destination)
src.start()