discord.js/src/util/FetchRecommendedShards.js

const superagent = require('superagent');
const botGateway = require('./Constants').Endpoints.botGateway;

/**
 * Gets the recommended shard count from Discord
 * @param {string} token Discord auth token
 * @param {number} [guildsPerShard=1000] Number of guilds per shard
 * @returns {Promise<number>} the recommended number of shards
 */
function fetchRecommendedShards(token, guildsPerShard = 1000) {
  return new Promise((resolve, reject) => {
    if (!token) throw new Error('A token must be provided.');
    superagent.get(botGateway)
      .set('Authorization', `Bot ${token.replace(/^Bot\s*/i, '')}`)
      .end((err, res) => {
        if (err) reject(err);
        resolve(res.body.shards * (1000 / guildsPerShard));
      });
  });
}

module.exports = fetchRecommendedShards;