version: '3.1'
services:
  redis:
    image: 'redis'
    command: ["sh", "-c", "exec redis-server"]
    ports:
      - '6379:6379'

export const config = {
    redis: {
        host: 'localhost',
        port: 6379,
        base: 2,
    },
}

...
url: `redis://{config.redis.host}:${config.redis.port}/${config.redis.base}`,
...

const RedisStore = require('connect-redis')(session);
const sessionStore = new RedisStore({
    url: redisStore.url,
});
sessionStore.client.on('error', function(err) {
    console.log('Redis error:', err);
});

Redis error: { ReplyError: ERR invalid DB index
    at parseError (.../auth/node_modules/redis-parser/lib/parser.js:193:12)
    at parseType (.../auth/node_modules/redis-parser/lib/parser.js:303:14)
  command: 'SELECT',
  args: [ '7Bconfig.redis.host%7D:6379/2' ],
  code: 'ERR' }