Callback Hell and Refactoring

Question

I have been learning JavaScript and started exploring Node. I have spent time understanding callback hell and how to fix it, but now I am more lost than ever and I'm starting to think I'm looking at it backwards (this might be a stupid question). I have read a guide on callbackhell.com and number 1 rule is to keep the code shallow.

Here is an example of connecting to Database, then reading a file, then inserting records to MongoDB and then logging:

MongoClient.connect(url, (err, db)=>{
   let dbo = db.db('test');
   fs.readFile('./data.json', (err, data)=>{
       let records = JSON.parse(data);
       dbo.collection('people').insertMany(records, (err, result) =>{
              console.log("result");
              db.close();
      })
   });
});

In the example, I have 3 anonymous callback functions and I have no idea how to refactor as the db is used throughout the callbacks. From what I understand I am should be aiming for 3 named functions (or am I missing something?) and call it like this:

MongoClient.connect(url, cb1);

and the functions something like this:

function cb1(err, db){
let dbo = db.db('test');
fs.readFile('./data.json', cb2);

}

function cb2(err, data){
    let records = JSON.parse(data);
    // ??? dbo.collection('people).insertMany(records, cb3)
    //now what?
}

function cb3(err, result){
    console.log(result);
    // ??? db.close?
}

Looking for any insight on Callback hell. Is this callback hell? How would I go about this? Am I missing something conceptually? Should it even be refactored?

PS. Promises and Async/Await can wait till I understand how to go about async programming using callbacks

Thank you!

3 callbacks isn't that bad, though more probably would be. You can consider promisifying them and `await`ing each https://stackoverflow.com/questions/22519784/how-do-i-convert-an-existing-callback-api-to-promises — CertainPerformance, Aug 27 '20 at 20:41
I was almost typing an answer, when I noticed the 'Promises can wait...' statement at end of your question. In short, you can't to that. If you use callbacks, callback hell is a possibility. Promises were explicitly created to avoid deeply nested callbacks. Since you have already realized the problem, I'd suggest move on to using Promises and write flatter code. There were some attempts to avoid cb-hell without promise (`async` module specifically) but those had their own fair share of issues. — d_shiv, Aug 27 '20 at 20:53

trincot · Accepted Answer · 2021-10-02T06:10:00.963

The nested callbacks are indeed what is commonly called callback hell.

Your attempt at separating the callbacks into named functions is fine, but you have realised one issue: that you need a reference to the db object in all callbacks, and it is not there.

You can solve this by either binding this to db, or else binding a (first) parameter for passing on that db object as argument. The principle is really the same.

Here is how it would look with binding this to the db object:

function cb1(err, db) {
    let dbo = db.db('test');
    fs.readFile('./data.json', cb2.bind(db));
}

function cb2(err, data) {
    let records = JSON.parse(data);
    this.collection('people').insertMany(records, cb3.bind(this));
}

function cb3(err, result) {
    console.log(result);
    this.close();
}

And here is how it would look with an extra parameter:

function cb1(err, db) {
    let dbo = db.db('test');
    fs.readFile('./data.json', cb2.bind(null, db));
}

function cb2(db, err, data) {
    let records = JSON.parse(data);
    db.collection('people').insertMany(records, cb3.bind(null, db));
}

function cb3(db, err, result) {
    console.log(result);
    db.close();
}

The next step would be to embrace promises and async await syntax. See for instance "How to use MongoDB with promises in Node.js?".

Callback Hell and Refactoring

1 Answers1