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How to throw custom error in async callout

I'm making a couple async API callouts and may throw a custom error depending on the outcome.
I'm deleting Objects from S3.
try {
await s3.deleteObject(bucketParams);
//S3 API doesn't provide resp on if obj successfully deleted. Therefore, check that it doesn't exist afterwards to verify instead
const err = await s3.headObject(bucketParams);
if (err & err.code === 'NotFound') return { code:200, message:`${key} successfully deleted` }
throw `Error deleting ${key}`;
} catch (error) {
throw new Error(500, error);
}
So the main catch can handle any exceptions thrown by those couple API callouts (error with network, bad key, bad authorization, etc..)
However, if the object didn't actually get deleted (so if it still exists)..I throw a custom error to get reported back to end user.
My question is if there's a better pattern to just throwing a custom error in your try and then having it get slurped up by your catch.
Thanks!

You can do a lot to be honest, you just need to work out what you want to capture. Heres a basic example. If you were using TypeScript you would get more power and type safety
class S3ObjectNotFound extends Error {
// Add other attributes you might like
code
constructor(message) {
super(message)
this.name = 's3/object-id-not-found'
this.code = 404
}
}
const someFunction = async (bucketParmas, key) => {
try {
try {
await s3.deleteObject(bucketParams)
} catch (s3Error) {
throw new S3ObjectNotFound(`Object with id ${bucketParmas} was not found`)
}
const err = await s3.headObject(bucketParams)
if (err && err.code === 'NotFound') {
return { code: 200, message: `${key} successfully deleted` }
}
throw AnotherErrorYouCouldMake()
} catch (error) {
throw new Error(500, error)
}
}
someFunction({...YourParams}, 'YourKey')

NodeJS ASync call of MongoClient GetData routine

The code below is a mix of https://www.w3schools.com/nodejs/nodejs_mongodb_find.asp
and
https://stackoverflow.com/questions/49982058/how-to-call-an-async-function#:~:text=Putting%20the%20async%20keyword%20before,a%20promise%20to%20be%20resolved.
When you look at the console.log below the code, things seem to be running out of order. I thought by making the function async and using the .then I would avoid those issues.
I want the MongoDB data retrieval function separate from the app.get function.
No data is being returned to the get request. I guess the app.get code is falling through and ending before the function returns the value. What do I need to correct?
async function getLanguageTranslationData(fromLanguage, toLanguage) {
console.log("Started getLanguageTranslationData")
const databaseUrl = "mongodb://localhost:27017"
const databaseName = 'MyCompanyPOC'
mongoClient.connect(databaseUrl, function(err, conn) {
if (err) throw err;
const collectionName = "Phrases";
var dbo = conn.db(databaseName)
var query =
{ $and:
[ {masterLanguage: fromLanguage},
{localizedLanguage: toLanguage}
]
}
console.log("query=" + JSON.stringify(query));
console.log("about to retrieve data");
dbo.collection(collectionName).find(query).toArray(
function(err, result) {
if (err) throw err;
console.log("Back from mongoDB.find()")
console.log(JSON.stringify(result))
return result
conn.close()
})
})
}
app.get("/api/gettranslations/:fromLanguage/:toLanguage",
async function(req, res) {
console.log("Backend: /api/gettranslations method started: " +
" fromLanguage=" + req.params.fromLanguage +
" toLanguage=" + req.params.toLanguage)
getLanguageTranslationData(
req.params.fromLanguage,
req.params.toLanguage)
.then((arrayResult) => {
console.log("got arrayResult back from getLanguageTranslationData")
res.status(200).json(arrayResult)
console.log("Backend: End of /api/gettranslations process")
})
})
Node.JS Console output:
listening on port 3001
Backend: /api/gettranslations method started: fromLanguage=en-US toLanguage=es-MX
Started getLanguageTranslationData
(node:44572) DeprecationWarning: current Server Discovery and Monitoring engine is deprecated, and will be removed in a future version. To use the new Server Discover and Monitoring engine, pass option { useUnifiedTopology: true } to the MongoClient constructor.
got arrayResult back from getLanguageTranslationData
Backend: End of /api/gettranslations process
query={"$and":[{"masterLanguage":"en-US"},{"localizedLanguage":"es-MX"}]}
about to retrieve data
Back from mongoDB.find()
[{"_id":"5f403f7e5036d7bdb0adcd09","masterLanguage":"en-US","masterPhrase":"Customers","localizedLanguage":"es-MX","localizedPhrase":"Clientes"},{ etc...

The thing is getLanguageTranslationData should return a promise so that you can use it as a promise in your handler, but in your case, call to getLanguageTranslationData will return undefined as all the code inside this function will execute asynchronously due to nodejs non-blocking nature.
So what you can do is return promise from your getLanguageTranslationData function like this.
function getLanguageTranslationData(fromLanguage, toLanguage) {
const databaseUrl = "mongodb://localhost:27017"
const databaseName = 'MyCompanyPOC'
return new Promise((resolve, reject)=>{
mongoClient.connect(databaseUrl, function(err, conn) {
if (err) reject(err);
else{
const collectionName = "Phrases";
var dbo = conn.db(databaseName)
var query =
{ $and:
[ {masterLanguage: fromLanguage},
{localizedLanguage: toLanguage}
]
}
dbo.collection(collectionName).find(query).toArray(
function(err, result) {
if (err) reject(err);
else
resolve(result);
})
}
})
})
}
and then use await in your handler to use that promise returned
app.get("/api/gettranslations/:fromLanguage/:toLanguage",
async function(req, res) {
try{
let arrayResult = await getLanguageTranslationData(req.params.fromLanguage, req.params.toLanguage);
res.status(200).json(arrayResult)
}catch(err){
// return error
}
})
The above code will give you the gist of what you need to do, actual code may vary according to your needs.
You can refer async-await from here

I got it working in this way, based on this example: Node.js, Mongo find and return data
#Namar's answer is probably correct too, but I was testing this the same time he posted. As the StackOverflow question/answer above notes, the up-to-date versions of MongoClient have support for promises. That post also shows how to put in a separate module, which is something I will probably do later this week.
function getLanguageTranslationData(fromLanguage, toLanguage) {
console.log("Started getLanguageTranslationData")
const databaseUrl = "mongodb://localhost:27017"
const databaseName = 'ShedCompanyPOC'
return mongoClient.connect(databaseUrl)
.then(function(conn) {
var collectionName = "UploadedDataeFromExcel";
var dbo = conn.db(databaseName)
var query =
{ $and:
[ {masterLanguage: fromLanguage},
{localizedLanguage: toLanguage}
]
}
console.log("query=" + JSON.stringify(query));
console.log("about to retrieve data");
var collection = dbo.collection(collectionName)
return collection.find(query).toArray();
}).then(function(result) {
console.log("Back from mongoDB.find()")
console.log(JSON.stringify(result))
//conn.close()
return result
});
}
app.get("/api/gettranslations/:fromLanguage/:toLanguage",
async function(req, res) {
console.log("Backend: /api/gettranslations method started: " +
" fromLanguage=" + req.params.fromLanguage +
" toLanguage=" + req.params.toLanguage)
getLanguageTranslationData(
req.params.fromLanguage,
req.params.toLanguage)
.then(function(arrayResult) {
console.log("got arrayResult back from getLanguageTranslationData")
res.status(200).json(arrayResult)
console.log("Backend: End of /api/gettranslations process")
}, function(err) {
console.log("The promise was rejected", err, err.stack)
})
})

Why does Node style error 1st callback return undefined after callback

The following code is returning undefined when I attempt to execute the error, callback capability. I don't understand why the json isn't returning as the console log outputs the complete query.
Here is the invoked function that is producing the json into the log.
exports.getThem = (req) => {
var addy = req.body.email;
Picks.findOne({ 'email' : addy }, (err, theResults) => {
if (err) {
return ({ 'msg': err });
}
console.log("made the query " + JSON.stringify(theResults));
return theResults;
});
};
Above, theResults print "made the query " and a complete JSON string.
The invoking code will execute and return to Postman the following JSON without the theResults JSON.
The console.log "log this" never executes and the 2nd log prints "after the log" undefined.
{
"token" : someCrazyLookingToken
}
exports.loginUser = (req, res) => {
var theResults;
theResults = Picks.getPicks( req , ( err , thePicks) => {
console.log("log this" + JSON.stringify(thePicks));
if (!err){
console.log ("what happened" + err)
}
});
console.log("after the call " + JSON.stringify(theResults));
return res.status(200).json({ picks: thePicks, token: createToken(user) });
}
Why? Is there a timing issue I'm not waiting for in the callback?

It's not necessarily a timing issue, but perhaps a misunderstanding on your part in how callbacks work, particularly in their scope. In your code, theResults is never going to have a value.
Try setting it up like this, and read up on promises and callbacks to get a better understanding on both.
exports.loginUser = async (req, res) => {
try {
const theResults = await new Promise((resolve, reject) => {
Picks.getPicks(req, (err, thePicks) => {
if (err) {
return reject(err);
}
return resolve(thePicks);
})
});
return res.status(200).json({picks: theResults, token: createToken(user)});
} catch (err) {
//handle err
}
}

How come when using pg if i return the result of my query from a function, it returns undefined, but if i console log it, it prints?

This is my code:
const queryFirstNames = function (qString) {
let firstNameMatches;
client.query('SELECT * FROM famous_people WHERE first_name = $1', [qString], (err, result) => {
if (err) {
return console.error('error running query', err);
}
firstNameMatches = result.rows;
return firstNameMatches;
client.end();
});
};
console.log(queryFirstNames(qString));
});
This code return undefined and doesn't end the connection with the database
But if I console.log firstNameMatches inside the function instead of returning and then just invoke the function without console logging, I get the result I want, and the database connection closes properly.
What I want to do is return the result of this query and use it in another function, so I don't want it to console log the result, but when I try to return the result, it gives me undefined, and doesn't close the database connection either.

I believe that the issue you are having is that when you are returning firstNameMatches, you are returning it just to the callback for client.query. firstNameMatches is set inside of the context of the queryFirstNames function, however you never return that back. That being said, you are also utilizing an async call to client.query. This means that when you return from the queryFirstNames function, chances are you haven't finished querying. Per this, I am pretty sure you want to utilize promises to manage this or the events like one of the other answers.
In addition to this, you are going to want to move your return to before the client.end. To play with my answer, I created a jsfiddle that you can see here. I had to create my own promise via Promise just to mock out the client executing the query.
const queryFirstNames = function (qString) {
let firstNameMatches;
client.query('SELECT * FROM famous_people WHERE first_name = $1', [qString])
.then((result) => {
firstNameMatches = result.rows;
client.end();
return firstNameMatches;
})
.then(f => {
console.log(f);
})
.catch(e => {
if (err) {
return console.error('error running query', err);
}
});
};

You are returning the call before you even execute client.end(); This way your client connection will never end. You can do something like this:
const query = client.query(
'SELECT * FROM famous_people WHERE first_name = $1',
[qString],
(err, result) => {
if (err) {
return console.error('error running query', err);
}
});
query.on('row', (row) => {
results.push(row);
});
// After all data is returned, close connection and return results
query.on('end', () => {
return res.json(results);
});

Error handling for fetch() in Aurelia

I have an API that includes a useful description of what went wrong when an error is raised by the server (status = 500). The description comes as part of the response text. My client code, using Aurelia, calls the api via aurelia-fetch-client using a generic method to make the call:
function callRemoteService(apiName, timeout) {
return Promise.race([
this.http.fetch(apiName),
this.waitForServer(timeout || 5000) // throws after x ms
])
.then(response => response.json() )
.catch(err => {
if (err instanceof Response) {
// HERE'S THE PROBLEM.....
err.text().then(text => {
console.log('Error text from callRemoteService() error handler: ' + text);
throw new Error(text)
});
} else if (err instanceof Error) {
throw new Error(err.message);
} else {
throw new Error('Unknown error encountered from callRemoteService()');
}
});
}
Note that I want to catch the server (fetch or timeout) errors in a consistent way, and then throw back just a simple error message to the calling view. I can invoke callRemoteService successfully, catching errors when a 500 is returned with:
callRemoteService(this.apiName, this.apiTimeout)
.then(data => {
console.log('Successfully called \'' + this.apiName +
'\'! Result is:\n' + JSON.stringify(data, null, 2));
})
.catch(err => {
console.log('Error from \'' + this.apiName + '\':',err)
});
However, I'm having trouble accessing the response text, because the fetch provides the text() method that returns a promise, and that's interfering with my otherwise happy promise chaining. The code above doesn't work, leaving me with an Uncaught (in promise) error.
Hopefully there's a good way to access that response text?

This should do the trick:
function callRemoteService(apiName, timeout = 5000) {
return Promise.race([
this.http.fetch(apiName)
.then(
r => r.json(),
r => r.text().then(text => throw new Error(text))
),
this.waitForServer(timeout)
]);
}
by the way, I like what you're doing with Promise.race- nice technique!