Discord bot on Heroku throws an error: agent ??= new https.Agent

This is related to my first question. I updated all of my codes. In my bot.js is:

require('dotenv').config();

let ver = process.env.DISCORD_BOT;

client.once('ready', async () => {
  if (ver === 'production') {
    client.user.setActivity(`in code land`, { type: 'PLAYING' });
  } else {
    client.user.setActivity(`over ${client.guilds.cache.size} server(s)`, {
      type: 'WATCHING',
      status: 'IDLE',
    });
  }
  console.log(`Logged in as ${client.user.tag}!`);
  console.log(`the prefix is ` + prefix);
});

Procfile:

worker: node bot.js

package.json:

 "scripts": {
    "start": "node .",
    "test": "echo \"Error: no test specified\" && exit 1",
    "production": "NODE_ENV=production&&npm start",
    "development": "set NODE_ENV=development&&npm start"
  }

.env:

DISCORD_BOT= TOKEN

When I checked the Heroku application logs here's the error thrown:

2021-12-22T03:13:05.436081+00:00 app[worker.1]: agent ??= new https.Agent({ ...this.client.options.http.agent, keepAlive: true });

2021-12-22T03:13:05.436082+00:00 app[worker.1]: ^^^

2021-12-22T03:13:05.436082+00:00 app[worker.1]:

2021-12-22T03:13:05.436082+00:00 app[worker.1]: SyntaxError: Unexpected token '??='

I've tried to search for the "SyntaxError: Unexpected token '??='" and every single result is suggesting to update node.js to v16-17 but I'm already using the latest version of both Discord.js and Node.js.

The error is coming from discord.js and it's because you're using an older version of Node.js. The logical nullish assignment operator (??=) is only available in node v15+.

You might think that you're using the latest version of Node.js, but Heroku says that "if a Node version isn't specified in the engine, the 14.x release will be used". You can add an engines prop to your package.json file to specify the version you need. As discord.js v13 requires node.js v16.6+, you can add the following:

  "engines": {
    "node": "16.6"
  }

Or to request the latest v16, add this:

  "engines": {
    "node": "16.x"
  }