Refactoring RESTful API into smaller functions - node.js

Question

Background
I have a NodeJS app that is meant to be used as a RESTful API. It is connected with a MongoDB database in the backend using Mongoose. The app is built upon the idea of nested documents. It stores wikis, sections and notes with the following schema:
const noteSchema = new mongoose.Schema({ title: String, content: String });
const sectionSchema = new mongoose.Schema({ title: String, notes: [noteSchema] });
const wikiSchema = new mongoose.Schema({ title: String, sections: [sectionSchema] });
All of which are accessed via a single model of the wiki:
const wikiModel = mongoose.model("Wiki", wikiSchema);
A user can do GET, POST, PUT, DELETE requests on each of the endpoints to manipulate the data inside. If someone wants to ping the Notes endpoint (the furthest down in the hierarchy), it must first check the wiki and then the section endpoint, to ensure that each of them exists.
Here's an example:
app.get('/:wikiTitle/:sectionTitle/:noteTitle', function(req, res) {
wikiModel.findOne({ title: req.params.wikiTitle }, function(err, wiki) {
if (err) {
res.send('\nAn unkown error has occured');
console.error(err);
} else if (wiki) {
const sectionTitle = req.params.sectionTitle;
wikiModel.findOne({ 'sections.title': sectionTitle }, function(err, section) {
if (err) {
res.send('\nAn unkown error has occured');
console.error(err);
} else if (section) {
const noteTitle = req.params.noteTitle;
wikiModel.findOne({ 'sections.notes.title': noteTitle }, function(err, n) {
if (err) {
res.send('\nAn unkown error has occured');
console.error(err);
} else if (n) {
const section = n.sections.find((s) => { return s.title === sectionTitle; });
const note = section.notes.find((n) => { return n.title === noteTitle; });
if (note.content) {
res.send('\n' + note.title + '\n\n' + note.content);
} else {
res.send('\n' + note.title + '\n\n[ No content to show ]');
}
} else {
res.send('\nNo such note exists');
}
});
} else {
res.send('\nNo such section exists');
}
});
} else {
res.send('\nNo such wiki exists');
}
});
});
This is a very lengthy method and the first two queries are actually frequently throughout the app. I also understand a MongoDB query is an asynchronous operation and thus, why I put each consequent MongoDB query within it's parent (the one I wish to finish before that one begins).
Question
Is there a way to split each MongoDB query into its own method or introduce promises in a way that would shorten the code? I would rather prefer advice that ultimately causes the splitting of my code into individual methods as what you see above is one of many endpoints which all use the same queries.
So in the end result I would like to have something close to the likes of:
app.get('/:wikiTitle/:sectionTitle/:noteTitle', function(req, res) {
if (getWiki(req.params.wikiTitle)) {
// Continue with second query
if (getSection(req.params.sectionTitle)) {
// Continue with third query...
}
}
});
function getWiki(wikiTitle) {
wikiModel.findOne({ title: wikiTitle }, function(err, wiki) {
if (err) {
console.error(err);
res.send('An unknown error occured.');
} else if (wiki) {
// Send OK result to continue to next query
return wiki
} else {
res.send('No wiki found');
return null;
}
});
}
function getSection(sectionTitle) {
wikiModel.findOne({ 'sections.title': sectionTitle }, function(err, section) {
if (err) {
console.error(err);
res.send('An unknown error occured.');
} else if (section) {
// Send OK result to continue to next query
return section
} else {
res.send('No section found');
return null;
}
});
}
I am hoping this will significantly cut the length of code and also utilise re-usability of code. Any advice on how I could come close to achieving something like this is welcome.

Answer 1

You can definitely use callbacks in the same way as the ones call your model. For example:
app.get('/:wikiTitle/:sectionTitle/:noteTitle', function(req, res) {
getWiki(req.params.wikiTitle, function (err, title) {
if (err) {
return res.send(err);
}
getSection(req.params.sectionTitle, function (err, section) {
if (err) {
return res.send(err);
}
// Todo: use title and section, etc...
});
});
});
function getWiki(wikiTitle, cb) {
wikiModel.findOne({ title: wikiTitle }, function(err, wiki) {
if (err) {
console.error(err);
return cb('An unknown error occured.');
} else if (wiki) {
// Send OK result to continue to next query
return cb(null, wiki);
} else {
return cb('No wiki found');
}
});
}
function getSection(sectionTitle, cb) {
wikiModel.findOne({ 'sections.title': sectionTitle }, function(err, section) {
if (err) {
console.error(err);
return cb('An unknown error occured.');
} else if (section) {
// Send OK result to continue to next query
return cb(null, section);
} else {
return cb('No section found');
}
});
}
This is a standard way of using async functions in node. By convention, the first parameter is always an error parameter.
If you want your code to be cleaner, you can try to use guard clauses / early outs to exit error cases early. This will cut down on your need for if / else conditional statements.
You can also look into libraries like async for cleaner chaining of asynchronous calls.
When you are comfortable, you can also look into using promises and the 'async' javascript keyword (different from the async library above, confusing, I know) which will also allow you to cut down on the lines of code you have to write to get nice async code.

Answer 2

You should use async functions (Promises) like
app.get('somePath', async (req, res, next) => {
try {
const doc = await model.find({ someField: 'some value' }).exec(); // exec returns promise
res.send({ document: doc });
} catch (error) {
// here you can handle all errors or/and call next for the error middleware
next(error);
}
});