index.js

require("dotenv").config();
const axios = require("axios");
const querystring = require("querystring");
const express = require("express");

const app = express();

const { CLIENT_ID, CLIENT_SECRET, REDIRECT_URI } = process.env;

/**
 * Generates a random string containing numbers and letters
 * @param  {number} length The length of the string
 * @return {string} The generated string
 */
const generateRandomString = (length) => {
  let text = "";
  const possible =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
  for (let i = 0; i < length; i++) {
    text += possible.charAt(Math.floor(Math.random() * possible.length));
  }
  return text;
};

const stateKey = "spotify_auth_state";

app.get("/login", (req, res) => {
  const state = generateRandomString(16);
  res.cookie(stateKey, state);

  const scope = ["user-read-private", "user-read-email", "user-top-read"].join(
    " "
  );
  const queryParams = querystring.stringify({
    client_id: CLIENT_ID,
    response_type: "code",
    redirect_uri: REDIRECT_URI,
    state: state,
    scope: scope,
  });
  res.redirect(`https://accounts.spotify.com/authorize?${queryParams}`);
});

app.get("/callback", (req, res) => {
  const code = req.query.code || null;

  axios({
    method: "post",
    url: "https://accounts.spotify.com/api/token",
    data: querystring.stringify({
      grant_type: "authorization_code",
      code: code,
      redirect_uri: REDIRECT_URI,
    }),
    headers: {
      "content-type": "application/x-www-form-urlencoded",
      Authorization: `Basic ${new Buffer.from(
        `${CLIENT_ID}:${CLIENT_SECRET}`
      ).toString("base64")}`,
    },
  })
    .then((response) => {
      if (response.status === 200) {
        const { access_token, refresh_token, expires_in } = response.data;

        const queryParams = querystring.stringify({
          access_token,
          refresh_token,
          expires_in,
        });

        res.redirect(`http://localhost:3000/?${queryParams}`);
      } else {
        res.redirect(`/?${querystring.stringify({ error: "invalid_token" })}`);
      }
    })
    .catch((error) => {
      res.send(error);
    });
});

app.get("/refresh_token", (req, res) => {
  const { refresh_token } = req.query;

  axios({
    method: "post",
    url: "https://accounts.spotify.com/api/token",
    data: querystring.stringify({
      grant_type: "refresh_token",
      refresh_token: refresh_token,
    }),
    headers: {
      "content-type": "application/x-www-form-urlencoded",
      Authorization: `Basic ${new Buffer.from(
        `${CLIENT_ID}:${CLIENT_SECRET}`
      ).toString("base64")}`,
    },
  })
    .then((response) => {
      res.send(response.data);
    })
    .catch((error) => {
      res.send(error);
    });
});

const PORT = 8888;

app.listen(PORT, () => {
  console.log(`Express is running at localhost:${PORT}`);
});