We Keep Coding

Variable is not storing Promise result in Node [duplicate]

This question already has answers here:
Why is my variable unaltered after I modify it inside of a function? - Asynchronous code reference
(7 answers)
How do I access previous promise results in a .then() chain?
(17 answers)
Closed 2 years ago.
I'm developing a script that connects with an API, then with the JSON reponse do some operations and then reformat the JSON to send it to another API.
But now I'm stuck in the first step as I can't deal with the first part as my Promises is not working as expected. How can I store the API's response into a variable? For development puropose I stored one API response into a JSON file. This is my code:
declare var require: any;
let url = './data/big buy/big-bui-product-list.json';
const fs = require('fs');
let counter = 0;
const getProductList = () => {
return new Promise((resolve, reject) => {
fs.readFile(url, 'utf8', (err, data) => {
if(err){
return reject (err);
}
else {
return resolve(JSON.parse(data));
}
})
})
}
const getProductStock = () => {
return new Promise((resolve, reject) => {
fs.readFile('./data/big buy/big-bui-product-stock.json', 'utf8', (err, data) => {
if(err) {
return reject(err);
}
else {
return resolve(JSON.parse(data));
}
})
})
}
try {
let products;
console.log('Products:');
Promise.all([getProductList()])
.then(function(result) {
products = result[0];
});
console.log('Stocks:');
const productStock = Promise.all([getProductStock()]);
console.log(products);
}
catch(e) {
console.log((`Ha ocurrido un error: ${e.message}`));
}
finally {
}
In this code, what I do is getting a list of products and then get the stocks of all the products, later I will add a new function that will filter by stock and get me just a list of products where stock is bigger than X units. Now when I launch it from the terminal I dont' get the response stored into products variable but if I add .then((data) => console.log(data)) into the Promise I see on screen the JSON but as I dont' have it stored it in any variable I don't see how can I work with the objects I'm retrieving.

Promises are asynchronous. They are quite literally promises that a value will be yielded in the future. When you do getProductList().then(x => products = x), you're saying that Javascript should fetch the product list in the background, and once it's finished, it should set the products variable to x. The key words there are "in the background", since the code afterwards keeps running while the product list is being fetched. The products variable is only guaranteed to be set after the .then portion is run. So, you can move things into the .then:
try {
let products;
getProductList().then(list => {
products = list;
return getProductStock(); // leverage promise chaining
}).then(stocks => {
console.log("Products:", products);
console.log("Stocks:", stocks);
});
}
catch(e) {
console.log((`Ha ocurrido un error: ${e.message}`));
}
finally {
}

You seem to be missing some fundamental knowledge about promises. I suggest reading through the MDN Using Promises guide to familiarize yourself a bit with them.
A structure like below never works.
let variable;
promise.then(data => variable = data);
console.log(variable);
This is because it is executed in the following manner.
Create the variable variable.
Add a promise handler to the promise.
Log variable.
Promise gets resolved.
Execute the promise handler.
Set variable variable to data.
If you are using Node.js 10 or higher you can use the promise API of file system to simplify your code quite a bit.
const fs = require('fs/promises');
const readJSON = (path) => fs.readFile(path, "utf8").then((json) => JSON.parse(json));
const getProductList = () => readJSON("./data/big buy/big-bui-product-list.json");
const getProductStock = () => readJSON("./data/big buy/big-bui-product-stock.json");
Promise.all([
getProductList(),
getProductStock(),
]).then(([products, stocks]) => {
console.log({ products, stocks });
}).catch((error) => {
console.log("Ha ocurrido un error:", error.message);
}).finally(() => {
// ...
});

How to correctly string together Promises for synchronous code execution

I posted a similar question several days ago but I have made some changes and commenting on that question was becoming tedious, so it was recommended I ask a new question.
The idea is that I want to execute four equations synchronously. Inside those equations are HTTP requests. I have two of the equations working properly and but there is one equation that involves two POST requests and a GET requests. The second requests relies on the first and the third request relies on the second.
I have tried several different methods to get this to work. I have tried flattening my promises, returning the promises. All kinds of things, with no luck. I am not sure where I am going wrong.
Synchronous code snippet:
this.getData1(user, userID).then(() =>
{
this.getData2(user, userID)
.then(() =>
{
this.getData3(user, lan).then(() =>
{
this.userCheck(user);
})
});
});
I have getData2 and getData3 working.
getData1 looks like:
getData1(user: string, id: string){
console.log('grabbing CC information', id, user);
return new Promise((resolve, reject) =>
{
var status: string;
this._apiService.getAssertion(id).subscribe((data: any) =>
{
let assert = data.toString();
this._apiService.getToken(assert).subscribe((data: any) =>
{
let tkn = data.access_token.toString();
this._apiService.ccMeta(tkn, guid).subscribe((data: any) =>
{
parseString(data, (err, result) =>
{
if (err)
{
throw new Error(err);
}
else
{
status = result['entry']['content'][0]['m:properties'][0]['d:status'][0];
this.ccData.push(
{
key: 'userStatus',
value: status
})
}
});
});
});
});
resolve()
});
}
I also tried something like this previously. It did not work either.
apiService.getAssertion(id).then(assert =>
{
return apiService.getToken(assert.toString(), user);
}).then(data =>
{
return apiService.ccMeta(data.access_token.toString(), id);
}).then(parseStringPromise).then(information =>
{
this.ccData.push(
{
key: 'userStatus',
value: information.entry
});
});
Inside this function the getAssertion function is a POST request. The getToken function is another POST request that relies on the assertion from the first POST request. Finally, ccMeta is a get request that relies on the token from the second POST request.
I would expect getData1 to execute first, then getData2, then getData3, and finally, userCheck. Inside getData1 I need the assertion, then the token, and then get request to execute synchronously. The code snippet above is not executing correctly. The assertion is not properly being used in the getToken equation.
I would greatly appreciate some help.

Since these HTTP calls are in fact observables and not promises, I think you should look into observable composition using pipe and switchMap for instance. If you still want you method to return a promise, it could look like this:
getData1(user: string, id: string) {
console.log('grabbing CC information', id, user);
return new Promise((resolve, reject) => {
this._apiService.getAssertion(id)
.pipe(
switchMap((data: any) => {
let assert = data.toString();
return this._apiService.getToken(assert);
}),
switchMap((data: any) => {
let tkn = data.access_token.toString();
return this._apiService.ccMeta(tkn, guid);
}),
)
.subscribe(
data => {
parseString(data, (err, result) => {
if (err) {
reject(new Error(err));
return;
}
const status: string = result['entry']['content'][0]['m:properties'][0]['d:status'][0];
this.ccData.push({
key: 'userStatus',
value: status
});
resolve();
});
},
);
});
}

Defining asynchrosnous function and using it in a forEach loop

I am trying to compile tex files into PFD using data from a firestore database. After completion the script doens't terminate. I found one can use process.exit() to make it quit. However, it terminates the child processes still cimpling the tex files. So, I need an asynchronous function.
The guides I found on how to make them did not particularly help me. I am still very new to javascript and any bloat is still highly confusion to me.
The guides I found on how to make them did not particularly help me. I am still very new to javascript and any bloat is still highly confusion to me.
prepending below mentioned makePDF function with async and the call of the function with await does not work and is, to my understanding, outdated.
I tried implementing a promise, but don't understand how their syntax works. Does simply appending .then() to the function call in the for loop suffice to make the loop wait for the functions completion?
How do I make this specific asynchronous?
Does it matter that it already uses asynchronous functions in its body?
I understand that a promise is used to return what ever the producer has produced to a consumer. Now, my producer doesn't produce anything to be returned. Does this matter at all?
My function called from the for loop:
function makePDF(object){
let input = fs.readFileSync('main.tex', 'utf8');
const outNameTex = object.ID + '.tex';
const outNamePDF = object.ID + '.pdf';
makeTEX(object, input, outNameTex);
const infile = fs.createReadStream(outNameTex);
const outfile = fs.createWriteStream(outNamePDF);
const pdf = latex(infile);
pdf.pipe(outfile);
pdf.on('error', err => console.error(err));
pdf.on('finish', () => {console.log('PDF generated!')});
}
And my function with the loop:
firebase.auth().onAuthStateChanged((user) => {
if (user) {
console.log('user');
db.collection('objects').where('printed', '==', false).get().then((snapshot) => {
snapshot.forEach((doc) => {
console.table(doc.data());
makePDF(doc.data());
})
process.exit();
})
.catch((err) => {
console.log('Error getting documents', err);
});
} else {
console.log('no user');
}
});
It outputs a table for each document, but no PDF generated.

async/await can be tricky to use with for loops, that is because async functions return a promise... if you convert the async/await syntax to native promise syntax you might figure out what the issue is.
What you want to do is use Array.map to map/convert each doc to a promise that resolves once the makePDF is done, then use Promise.all to wait for all the promises to resolve..
The solution should look something like this
function makePDF(object){
return new Promise((resolve, reject) => {
let input = fs.readFileSync('main.tex', 'utf8');
const outNameTex = object.ID + '.tex';
const outNamePDF = object.ID + '.pdf';
makeTEX(object, input, outNameTex);
const infile = fs.createReadStream(outNameTex);
const outfile = fs.createWriteStream(outNamePDF);
const pdf = latex(infile);
pdf.pipe(outfile);
pdf.on('error', reject);
pdf.on('finish', () => {console.log('PDF generated!'); resolve();});
}
firebase.auth().onAuthStateChanged((user) => {
if (user) {
console.log('user');
db.collection('objects').where('printed', '==', false).get().then((snapshot) => {
const promiseArr = snapshot.docs.map((doc) => {
console.table(doc.data());
return makePDF(doc.data());
})
Promise.all(promiseArr)
.then(() => {
process.exit();
})
})
.catch((err) => {
console.log('Error getting documents', err);
});
} else {
console.log('no user');
}
});

Why this promise chain is stuck at one of then calls and catch returns empty error object?

I'm making a post request and it requires several things to happen. It's native JavaScript promise not any library. Initially used nested promise and it was working but code was not that good. So, I decided to go with Promise chain and I'm stuck. The post route always returns {success:false, err:{}}, which it should when something goes wrong. But the err object is empty object. Why is that? After some tests I found the problem is in the second then where I'm returning AvaiablexForX.findOne({isX:false});. Don't worry about variables names, for the sake idea I have changed the actual names.
router.post("/delevery_request",
passport.authenticate("jwt", {session:false}),
(req, res) => {
const requestInputFields = {};
const foundxProfile = {};
const xProfileId = "";
const newxRequestId = "";
requestInputFields.isAccepted = false;
XProfile.findOne({user:req.user.id})
.then(xProfile => {
foundxProfile= xProfile;
requestInputFields.xId = xProfile._id;
return AvaiablexForX.findOne({isX:false});
})
.then( avaiablexForX =>
{
// this does not reach here
console.log("available x for X", avaiablexForX);
requestInputFields.xToBeDonateId = avaiablexForX._id;
xProfileId = avaiablexForX.xProfileId;
return requestInputFields;
})
.then( result => new RequestxY(result).save()).
then( requestForxY => {
foundxProfile.requestedxDeleivery.unshift(requestForxY._id);
return foundxProfile.save();
}).
then( xProfile => res.json({success:true}))
.catch(err => {
// output in body of request: {success:false, err:{}}
res.status(404).json({success:false, err:err})
}
);
});

Probably the problem is you trying to set a new value for const:
foundxProfile= xProfile;
This cause error and broke the chain. Try to replace all const by let.

Short answer: as already pointed out members declared with const can't be reassigned.
Long answer: you would benefit from a better strategy for accessing previous promise results in a .then() chain
With ref to the linked topic, you are using the "inelegant and rather errorprone" mutable contextual state.
You might consider one of the other approaches :
Nesting (and) closures
Break the chain
Explicit pass-through
For example, Nesting (and) closures would give you something like this :
router.post('/delevery_request', passport.authenticate('jwt', { 'session': false }), (req, res) => {
XProfile.findOne({ 'user': req.user.id })
.then(xProfile => {
return AvaiablexForX.findOne({ 'isX': false })
.then(avaiablexForX => {
return new RequestxY({
'isAccepted': false,
'xId': xProfile._id,
'xToBeDonateId': avaiablexForX._id
}).save();
})
.then(requestForxY => {
xProfile.requestedxDeleivery.unshift(requestForxY._id);
return xProfile.save();
});
})
.then(() => res.json({ 'success': true }))
.catch(err => {
res.status(404).json({
'success': false,
'err': err
});
});
});
Due to closure, xProfile is available to the first and second nested .then().
What was requestInputFields is composed on the fly where it is used.
You lose the nice flat line up of then()s but gain by not needing a bunch of messy outer members.

How do I skip a callback when running .map in JavaScript/Node.js?

Kind of a sequel to this question, I need to accept multiple objects in a POST request and then for each object process it, save it, and then return the saved object to the frontend (so that the client can see which columns were successfully edited).
When I use .map, it does save to the database and I can confirm this. However, I have two problems:
It does not execute res.locals.retval.addData(dtoObject); correctly, and my returning payload has no data transfer objects inside of it.
My object validation cannot be done inside of the callback of map. I initially tried reduce, but that didn't work at all and just saved all the same values to each database object. How can I exclude invalid JSON objects while I'm mapping them?
var jsonObjects = req.body;
//for (var n in req.body) {
var promises = jsonObjects.map((jsonObject) => {
var transform = new Transform();
// VALIDATION OF jsonObject VARIABLE IS HERE
if (jsonObject.id == 0) {
var databaseObject = Database.getInstance().getModel(objectName).build(jsonObject);
transform.setNew(true);
transform.setJsonObject(jsonObject);
transform.setDatabaseObject(databaseObject);
transform.baseExtract()
.then(() => transform.extract())
.then(() => transform.clean())
.then(() => transform.getDatabaseObject().save())
.then(function(data) {
// PROCESSING DATA
}).catch((e) => {
// ERROR
});
} else {
var queryParameters = {
where: {id: jsonObject.id}
};
console.log("Query parameters: ");
console.log(queryParameters);
Database.getInstance().getModel(objectName).findOne(queryParameters).then((databaseObject) => {
transform.setJsonObject(jsonObject);
transform.setDatabaseObject(databaseObject);
})
.then(() => transform.baseExtract())
.then(() => transform.extract())
.then(() => transform.clean())
.then(() => transform.getDatabaseObject().save())
.then((data) => {
// PROCESSING DATA
}).catch((e) => {
// ERROR
});
}
});
Promise.all(promises)
.then((results) => {
return next();
}).catch((e) => {
throw e;
});
Here's the resulting payload:
{
"errors": [],
"warnings": [],
"data": []
}

As #KevinB said in the comments, you are missing the return calls inside of your arrow functions so the database saves are going through because they are part of the Promise chain, but pushes to the response are stalled waiting for the return, and then the Express.js call resolves before the Promises do. Add return Database.getInstance() and return transform.baseExtract() to your code to fix this.
Use Array.prototype.filter() to remove elements you want to ignore since you won't ever need to execute Promises on them, then call Array.prototype.map() on the resulting array. If you don't want to use the arrow functions, you can specify this as a parameter to filter and map:
jsonObjects.filter(function(jsonObject) {
}, this);
var promises = jsonObjects.map(function(jsonObject) {
}, this);