I wrote a language for music

Piano.js

const tone = require("tonegenerator")
const { notes, shapes } = require("./constants.js");

module.exports = function (text) {
    const input = new InputStream(text);
    const tokenizer = new Tokenizer(input);
    const parser = new Parse(tokenizer);
    const tracks = CreateTracks(parser);
    const track = AllignTracks(tracks);
    return track;
}

function AllignTracks(tracks) {
    let track = [];
    let longest = 0;
    for (let i = 0; i < tracks.length; i++) 
        if (tracks[i].length > tracks[longest].length) longest = i;
    for (let i = 0; i < tracks[longest].length; i++) {
        let val = 0;
        for (let j = 0; j < tracks.length; j++) {
            val += tracks[j][i] || 0;
        }
        track.push(val);
    }
    return track;
}

function CreateTracks(input) {
    const tracks = [[]];
    let trk = 0;
    while (!input.eof()) {
        const token = input.next();
        if (token.type === "track") {
            trk++;
            if (!tracks[trk]) tracks[trk] = [];
            continue;
        }
        else if (Array.isArray(token)) {
            tracks[trk] = tracks[trk].concat(token);
        }
        tracks[trk] = tracks[trk].concat(tone(token))
    }
    return tracks;
}

function Parse(input) {
    function next() {
        const token = input.next();
        let shape = "triangle";
        if (token.type === "note") {
            return { freq: notes[token.note], lengthInSecs: token.length / 8, shape, sampleRate: 5512.5  };
        }
        else if (token.type === "rest") {
            return new Array(5512.5 * token.length / 8).fill(0);
        }
        else if (token.type === "tone") {
            shape = shapes[token.tone];
            return next();
        }
        return token;
    }

    return { next, eof: input.eof };
}

function Tokenizer(input) {
    function next() {
        const char = input.next();
        if (isRest(char)) {
            const chars = read(isRest);
            return { type: "rest", length: chars.length + 1};
        }
        if (isNote(char)) {
            const chars = read(isMatch(char));
            var sharp = isSharp(input.peek());
            if (sharp) input.next();
            return { type: "note", note: char, sharp, length: chars.length + 1};
        }
        if (isTone(char)) {
            return { type: "tone", tone: char};
        }
        if (isTrack(char)) {
            return { type: "track" }
        }
        return { type: "error", msg: "Invalid Character: " + char};
    }

    function read(action) {
        let res = "";
        while (!input.eof() && action(input.peek())) {
            res += input.next();
        }
        return res;
    }

    const isRest = c => c === " ";
    const isSharp = c => c === "#";
    const isNote = c => "abcdefgABCDEFG".includes(c);
    const isTrack = c => c === "|";
    const isTone = c => "1234".includes(c); 
    const isMatch = function (m) { return c => c === m };

    return { next, eof: input.eof };
}

function InputStream(text) {
    let pos = 0;
    function next() {
        return text[pos++];
    }

    function peek() {
        return text[pos];
    }

    function eof() {
        return peek() === undefined;
    }
    return {
        next,
        peek,
        eof
    };
}