Express error handling between model and routes - node.js

Question

I have a router file which handles all the routes for country in express and call a function from Model file.
router.get('/:_id', function(req, res, next){
countriesModel.getCountry(req.params._id, function(data, err){
if(err)
{
res.json({status:0, message:"Country Not Found for id : "+req.params._id, errDetails:{err}});
res.end();
}
else
{
res.json(data);
res.end();
}
}); });
And here is the getCountry Function from model file.
exports.getCountry = function(id, callback){
return db.queryAsync('select * from tbl_countries where ID = '+id)
.then(function(countryRows){
if(countryRows.length){
return Promise.resolve(callback(countryRows));
}
else
{
return Promise.resolve(callback('No Data To Return.'));
}
});
}
It works fine when i enter correct id, however i want to push error when someone enters wrong id which is not available in database.
Can you please guide me how i can achieve this, I am new to Node & Express.
I am using mySQL with express.

Answer 1

First off, since your database is already returning a promise, you can just make your function return a rejected promise when there's an error condition. And, stop using plain callbacks at all. You already have a promise, let your caller use that:
exports.getCountry = function(id){
return db.queryAsync('select * from tbl_countries where ID = '+id)
.then(function(countryRows){
if(countryRows.length){
// make countryRows be resolved value of the promise
return countryRows;
} else {
// make promise be rejected with this error
throw new Error('Country not found'));
}
});
}
Then, in the router, use the returned promise:
router.get('/:_id', function(req, res, next){
countriesModel.getCountry(req.params._id).then(data => {
res.json(data);
}).catch(err => {
res.json({status:0, message:"Country Not Found for id : "+req.params._id, errDetails:{err}});
});
});
Notes:
There are other reasons you can get a rejected promise here (such as some sort of database error). You have to decide what you want to return in your route when that happens. Right now, it returns the country not found for all errors, but those other types of errors should probably return a 5xx status.
There is no reason to call res.end() after a res.send() or a res.json() as it is called automatically for you.
Never mix promises with plain callbacks. If you have a promise already at the lowest level, just use it and don't cover it with a plain callback.