callback-hell | information about async javascript programming | Reactive Programming library

I am writing a simple URL shortening web app to learn JavaScript, Node (focusing on Express), and Mongo. A user submits a URL to my web app and receives a shortened URL (if the submitted URL is a valid URL) or an error message (if the submitted URL is not a valid URL). I have implemented the following algorithm:

1. Check if the submitted URL is a valid URL (using dns.

• 1.1 If it is not a valid URL, send a JSON response {"error":"invalid URL"}
• 1.2 If it is a valid URL, check if there is already a shortened version of that URL in the database
  • 1.2.1 If there is, send a JSON response with the existing shortened URL
  • 1.2.2 If there isn't, make a new shortened URL, save it in the database, and send a JSON response with the newly created shortened URL

The JavaScript code is below. I believe it works (based on some manual testing), but I have the following questions:

• Question 1: To perform steps 1, 1.2, and 1.2.2 from the above algorithm in that order, I found myself nesting multiple function definitions as callbacks in multiple function calls and I ended up with 3 or more levels of nesting. I did this because I needed one function call to return before I start the next step (e.g., I needed to know whether the URL is a valid URL before I could do further processing with it). Is nesting so many functions within each other an appropriate/normal programming style in these kinds of web apps? I found writing this code confusing/counter-intuitive. Is there a better way to write this code and avoid all this nesting?
• Question 2: I am not sure if StackOverflow is the best place to get feedback on code style (if you are a new to a programming language) but if it is not, does anyone know of an online community where I could receive this kind of feedback (basically, a quick code review from another human being)?

Does anybody have a hands-on experience with both DB-libraries — knex vs. mysql2?

After some googling (e.g. at NPMCompare), I'm still curious to know, based on real experience, what are the pros & contra of both options?

So far, the only real advantage of using knex over mysql2, that I clearly see, is its universal support of MSSQL, MySQL, PostgreSQL, SQLite3, and Oracle, while the latter supports MySQL only, but since currently I'm focusing on MySQL only, this knex's feature seems to be less relevant.

The parameters I would consider:

• Performance & load resistance;
• Stability (production ready);
• Native ES8+ support (callback-hell-free, no extra Util.promisify wrappers, ESM/MJS support);
• Short and clear, the less verbose the better.