I can't make this promise work - node.js

Question

I am trying to use Promises instead of Callback in my Lambda Function in NodeJs 4.3. What I'm trying to do is read a config file from S3 and use the configuration to connect to a DB and execute a SELECT statement on a table. When I test the function, I don't get any errors in the console.
I have defined my function as follows:
function setUpConnection(response) {
console.log("S3 Response " + JSON.stringify(response));
return new Promise(function(resolve, reject) {
config = response.Body.toString('utf-8');
config = JSON.parse( config );
// set up connection from S3 config file
var con = mysql.createConnection({
host: config.hostaddress,
user: config.user,
password: config.pass,
database: config.dbname
});
console.log("connection " + JSON.stringify(con));
console.log("config " + JSON.stringify(config));
// create connection
con.connect(function(err){
if(err){
// Output connection details for debugging
console.log('Error connecting to DB');
return Promise.reject(new Error(output));
}
});
// Run Select Query
con.query("SELECT * FROM goodsreceiptheader WHERE invoiceKey = ?", [invoicekey], function(err,res){
if(err){
console.log(err);
con.end();
return Promise.reject(new Error(output));
}
if ( res.length <= 0 ){
console.log('Response Object ' + JSON.stringify(res));
con.end();
return Promise.reject(new Error(output));
}
resolve(res);
})
})
}
The function is being called by this function:
// Setup goodsreceipt info
var goodsreceipt = data.goodsreceipt;
getObjectPromise = s3bucket.getObject(params).promise()
getObjectPromise
.then(setUpConnection)
.then(validateRecord)
.catch(callback(null, Error))
When I execute this code, I am only seeing the result of the code
console.log("S3 Response " + JSON.stringify(response));
on the second line. Is there a problem with the way I configured setUpConnection?

Answer 1

You're using rejection in a wrong way.
return Promise.reject(new Error(output));
Should be replaced with
reject(new Error(output));
return;
Also catch call looks strange: it calls callback immediately. Usually catch block looks like that:
somePromise.catch((error) => {
console.error(error); // If output required.
callback(error); // If callback provided.
});
Move con.query call inside of con.connect callback.

Answer 2

First of all, from where output is coming? I don't see it anywhere defined.
Second of all it is considered as good practice to always throw/reject with an instance of Error, in your example i guess, at least by variable name, output is not an instance of Error. Third thing in general only valid use case in my opinion of using new Promise constructor is when you want to transform callback api to promise which you are doing here (which is good) or you when deal with setTimeout/setInterval
Anyway you are using Promise.reject where you have reject in scope already which is not how you should reject your promise in this case. Next thing is that you are using con.query outside of callback that is provided to con.connect which effectively means you are calling query before connection was successfully established, try with something like:
return new Promise(function(resolve, reject) {
...
con.connect(function(err){
if(err){
return reject(new Error('...'));
}
// Run Select Query
con.query(..., ..., function (err,res) {
if(err){
...
return reject(new Error('...'));
}
if (res.length <= 0 ){
...
return reject(new Error(output));
}
return resolve(res);
})
});
})
Few sides notes:
1. Check does this lib have promise api, probably it should in that way you dont need to promisify things around on your own.
2. If it doesnt have you can always use libs like bluebird to promisfy code for you

Answer 3

You rejection of promise is wrong
Try in this way
function setUpConnection(response) {
console.log("S3 Response " + JSON.stringify(response));
return new Promise(function(resolve, reject) {
config = response.Body.toString('utf-8');
config = JSON.parse( config );
// set up connection from S3 config file
var con = mysql.createConnection({
host: config.hostaddress,
user: config.user,
password: config.pass,
database: config.dbname
});
console.log("connection " + JSON.stringify(con));
console.log("config " + JSON.stringify(config));
// create connection
con.connect(function(err){
if(err){
// Output connection details for debugging
console.log('Error connecting to DB');
return reject(err);
}
});
// Run Select Query
con.query("SELECT * FROM goodsreceiptheader WHERE invoiceKey = ?", [invoicekey], function(err,res){
if(err){
console.log(err);
con.end();
return reject(err);
}
if ( res.length <= 0 ){
console.log('Response Object ' + JSON.stringify(res));
con.end();
return reject('response less 0');
}
resolve(res);
})
})
}