token.js

// Based on https://codegists.com/snippet/javascript/indexjs_aljs_javascript
//  (found from https://forums.developer.apple.com/thread/103790)

// npm install jsonwebtoken

"use strict";

const fs = require("fs");
const jwt = require("jsonwebtoken");

const privateKey = fs.readFileSync("AuthKey.p8").toString(); //your MusicKit private key
const jwtToken = jwt.sign({}, privateKey, {
  algorithm: "ES256",
  expiresIn: "180d",
  issuer: "TEAM_ID", //your 10-character Team ID, obtained from your developer account
  header: {
    alg: "ES256",
    kid: "MUSICKIT_KEY_ID" //your MusicKit Key ID
  }
});

console.log(jwtToken);