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ES6 Promises confusion

I have the following snippet of code inside asynchronous function -
await application.save((err) => {
console.log("hello");
if (err) {
res.status(500).send({ message: "Error encountered" });
}
});
console.log("hey");
Why does "hey" get printed out earlier than "hello"? And how to fix it so the behaviour is as expected (asynchronous save operation is waited for and only when it's done and "hello" is printed, "hey" should be printed).
Following code does actually save object to MongoDB, but when I use application.save().then(() => {}) I get an error "Cannot read property 'then' of undefined"

You are confusing between callbacks and promises.
For example, we have first task F, long task T and next task N. We want to ensure the code run in the order F -> T -> N. To do that, we need either callbacks or promises.
Callbacks
function F() {}
function T(cb) {
// do that long task here
cb(err); // then call the cb and pass is the err
}
function N() {}
function main() {
F();
cb = (err) => {
if (err) { // handle err }
else {
N() // N needs to be run in cb
}
// clean up
// N can also be put here if you want to guarantee N will run
};
T(cb);
}
Promises
function F() {}
function T(cb) {
return new Promise((resolve, reject) => {
// do that long task here
if(err) {
reject();
} else {
resolve();
}
});
}
function N() {}
// using .then .catch .final
function main() {
F();
T().then(N())
.catch(() => {
// handle error
})
.finally(() => {
// clean up
// N can also be put here if you want to guarantee N will run
})
}
// using async/await
async function main() {
F();
try {
await T();
N();
} catch {
// handle error
} finally {
// clean up
// N can also be put here if you want to guarantee N will run
}
}
In your case, save() function does not return a promise but expects a callback to be passed in. If you want a quick and simple solution, put your console.log("hey"); in the callback. A better way is to promisify that callback function so it returns a promise and you can await it. Here is a sample promisify function to turn functions accepting callbacks into functions returning promises:
From
function T(...args, cb) {
// do that long task here
cb(err, data)
}
To
function TAsync(...args) {
return new Promise((resolve, reject) => {
T(...args, function(err, data) {
if (err) reject(err)
else resolve(data)
});
});
}

if a function returns a promise you just need to use the await and assign the return to a variable which is the response. You shoud always use a try catch because promises can launch exceptions.
function divide(i, j) {
return new Promise((resolve, reject) => {
if(j == 0) {
reject("division by zero.");
} else {
resolve(i / j);
}
});
}
async function main() {
try {
let resp = await divide(5, 2);
console.log(`resp = ${resp}`);
resp = await divide(2, 0);
console.log(`resp 2 = ${resp}`);
} catch(e) {
console.log(`catch = ${e}`);
}
}
// another way to use promises
divide(5,2).then((resp) => {
console.log(resp);
}).catch((err) => {
console.log(err);
});
divide(5,0).then((resp) => {
console.log(resp);
}).catch((err) => {
console.log(err);
});
main();
probably your function isn't returning a promise.

Wait for inner functions to wait and execute then proceed execution

I am executing a cloud function which is written in nodeJS.Here the function triggers when a data from the external source comes in and in this function, I have to call and check DB at the particular table but it takes more than 5 seconds and before the execution of my getDataFromDb function my main function completed execution. Plus there is a function called updateItems(postdate); and it executes if I cannot find data in my DB when triggering getDataFromDb
I tried async await but I am not sure where I am doing wrong. my function always ends first before my DB operation ends.
exports.handler = (event, context) => {
//here i am making data ready for DB and checking for the record if that is present in db
getDataFromDb(uniqueArray);
function getDataFromDb(uniqueArray) {
var params = {
// params for get reques
}
//db get operation
db.get(params, function (err, data) {
//takes time here
if (err) {
console.log(err); // an error occurred
}
else {
//another DB operation updateItems(postdata);
//takes time here
}
else {
console.log("first run for db")
//another DB operation updateItems(postdata);
//takes time here
}
}
});
}
});
console.log("main function ended")
};
the expected result should wait for the inner function to execute then end the main function but actually, the main function ends first then DB calling function ends

Though this can be achieved through callbacks, converting it to promise chain makes it easy, as execution of inner function depends on outer function, it's better to chain promises i.e. return Promise in the call back of first function, to execute them serially.
exports.handler = (event, context) => {
getDataFromDb(uniqueArray).then(success => {
console.log('Done')
})
.catch(err => {
console.log('handle get or post err here');
});
function getDataFromDb(uniqueArray) {
var params = {};
return new Promise((resolve, reject) => {
db.get(params, (err, data) => {
if (err) {
return reject(err); // an error occurred
} else {
return resolve(data);
}
});
}).then(success => updateItems(data))
}
});

Run callback function after forEach is done

In the project, I have a loop going through a list of urls. It downloads file from every url and do some post process over the downloaded file.
After the all the process done (both download process and post process), I want to execute a callback function. Because post process includes some streaming task, it has close event. If the last item can be identified, I can pass the callback function to the close event. However, since the loop is async, I can't track which item is done at last.
For now, I use a 5 second timeout to make sure the callback is executed after the whole process. Obviously, this is not sustainable. What's a good way to handle this?
loop code:
exports.processArray = (items, process, callback) => {
var todo = items.concat();
setTimeout(function() {
process(todo.shift());
if(todo.length > 0) {
// execute download and post process each second
// however it doesn't guarantee one start after previous one done
setTimeout(arguments.callee, 1000);
} else {
setTimeout(() => {callback();}, 5000);
}
}, 1000);
};
processArray(
// First param, the array
urlList,
// Second param, download and post process
(url) => {
if(url.startsWith('http')) {
getDataReg(url, uid);
}
else if(url.startsWith('ftp')) {
getDataFtp(url, uid);
}
else {
console.log('not a valid resource');
}
},
// Third param, callback to be executed after all done
() => {
Request.get(`${config.demouri}bound=${request.query.boundary};uid=${uid}`, {
method: 'GET',
auth: auth
})
.on('response', (response) => {
console.log('response event emmits');
zipFiles(uid)
.then((path) => {
reply.file(path, { confine: false, filename: uid + '.zip', mode: 'inline'}).header('Content-Disposition');
});
});
}
);
Download and post process:
exports.getDataFtp = (url, uid) => {
console.log('get into ftp');
var usefulUrl = url.split('//')[1];
var spliter = usefulUrl.indexOf('/');
var host = usefulUrl.substring(0, spliter);
var dir = usefulUrl.substring(spliter+1, usefulUrl.length);
var client = new ftp();
var connection = {
host: host
};
var fileNameStart = dir.lastIndexOf('/') + 1;
var fileNameEnd = dir.length;
var fileName = dir.substring(fileNameStart, fileNameEnd);
console.log('filename: ', fileName);
client.on('ready', () => {
console.log('get into ftp ready');
client.get(dir, (err, stream) => {
if (err) {
console.log('get file err:', err);
return;
} else{
console.log('get into ftp get');
stream.pipe(fs.createWriteStream(datadir + `download/${uid}/${fileName}`));
stream.on('end', () => {
console.log('get into ftp close');
unzipData(datadir + `download/${uid}/`, fileName, uid);
client.end();
});
}
});
});
client.connect(connection);
};
exports.getDataReg = (url, uid) => {
console.log('get into http');
var fileNameStart = url.lastIndexOf('/') + 1;
var fileNameEnd = url.length;
var fileName = url.substring(fileNameStart, fileNameEnd);
var file = fs.createWriteStream(datadir + `download/${uid}/${fileName}`);
if (url.startsWith('https')) {
https.get(url, (response) => {
console.log('start piping file');
response.pipe(file);
file.on('finish', () => {
console.log('get into http finish');
unzipData(datadir + `download/${uid}/`, fileName, uid);
});
}).on('error', (err) => { // Handle errors
fs.unlink(datadir + `download/${uid}/${fileName}`);
console.log('download file err: ', err);
});
} else {
http.get(url, (response) => {
console.log('start piping file');
response.pipe(file);
file.on('finish', () => {
unzipData(datadir + `download/${uid}/`, fileName, uid);
});
}).on('error', (err) => {
fs.unlink(datadir + `download/${uid}/${fileName}`);
console.log('download file err: ', err);
});
}
};
function unzipData(path, fileName, uid) {
console.log('get into unzip');
console.log('creating: ', path + fileName);
fs.createReadStream(path + fileName)
.pipe(unzip.Extract({path: path}))
.on('close', () => {
console.log('get into unzip close');
var filelist = listFile(path);
filelist.forEach((filePath) => {
if (!filePath.endsWith('.zip')) {
var components = filePath.split('/');
var component = components[components.length-1];
mv(filePath, datadir + `processing/${uid}/${component}`, (err) => {
if(err) {
console.log('move file err: ');
} else {
console.log('move file done');
}
});
}
});
fs.unlink(path + fileName, (err) => {});
});
}

After the all the process done (both download process and post process), I want to execute a callback function.
The interesting thing about a series of asynchronous processes is that you can never know when exactly all processes will complete. So setting a timeout for the callback is quick&dirty way to do it, but it's not reliable for sure.
You can instead use a counter to solve this problem.
Let's say you have 10 operations to perform. At the beginning you set your counter to ten counter = 10 And after each process is completed, regardless how (it can either succeed or fail), you can decrement the counter by 1 like counter -= 1 and right after it you can check if the counter is 0, if so that means all processes are completed and we reached the end. You can now safely run your callback function, like if(counter === 0) callback();
If I were you, I would do something like this:
*Notice that the called process should return a promise, so that I can know when it finishes (again regardless how)
*If you need help about promises, this useful article might help you: https://howtonode.org/promises
*Oh and one more thing, you should avoid using arguments.callee, because it's deprecated. Here is why Why was the arguments.callee.caller property deprecated in JavaScript?
exports.processArray = (items, process, callback) => {
var todo = [].concat(items);
var counter = todo.length;
runProcess();
function runProcess() {
// Check if the counter already reached 0
if(checkCounter() === false) {
// Nope. Counter is still > 0, which means we got work to do.
var processPromise = process(todo.shift());
processPromise
.then(function() {
// success
})
.catch(function() {
// failure
})
.finally(function() {
// The previous process is done.
// Now we can go with the next one.
--counter;
runProcess();
})
}
};
function checkCounter() {
if(counter === 0) {
callback();
return true;
} else {
return false;
}
}
};

What you want to do is to make all your asynchronous processes converge into a single promise that you can use to execute the callback at the correct moment.
Lets start at the point each process is complete, which I assume is in the callback passed to the mv() function in unzipData(). You want to wrap each of these asynchronous actions in a Promise that resolves in the callback and you also want to use these promises later and for that you use the .map() method to collect the promises in an array (instead of .forEach()).
Here's the code:
var promises = filelist.map((filePath) => {
if (!filePath.endsWith('.zip')) {
var components = filePath.split('/');
var component = components[components.length-1];
return new Promise((resolve, reject) =>
mv(filePath, datadir + `processing/${uid}/${component}`, (err) => {
if(err) {
console.log('move file err: ');
reject(); // Or resolve() if you want to ignore the error and not cause it to prevent the callback from executing later
} else {
console.log('move file done');
resolve();
}
}));
}
return Promise.resolve();
});
(if the asynchronous action is not to be executed, a Promise that resolves immediately is returned instead)
Now, we can turn this list of Promises into a single Promise that resolves when all of the promises in the list has resolved:
var allPromise = Promise.all(promises);
Next, we need to look further up in the code. We can see that the code we've just been looking at is itself part of an event handler of an asynchronous action, i.e. fs.createReadStream(). You need to wrap that in a promise that gets resolved when the inner promises resolve and this is the promise that the unzipData() function shall return:
function unzipData(path, fileName, uid) {
console.log('get into unzip');
console.log('creating: ', path + fileName);
return new Promise((outerResolve) =>
fs.createReadStream(path + fileName)
.pipe(unzip.Extract({path: path}))
.on('close', () => {
console.log('get into unzip close');
var filelist = listFile(path);
// Code from previous examples
allPromise.then(outerResolve);
}));
}
Next, we look at the functions that use unzipData(): getDataReg() and getDataFtp(). They only perform one asynchronous action so all you need to do is to make them return a promise that resolves when the promise returned by unzipData() resolves.
Simplified example:
exports.getDataReg = (url, uid) => {
return new Promise((resolve, reject) => {
// ...
https.get(url, (response) => {
response.pipe(file);
file.on('finish', () => {
unzipData(datadir + `download/${uid}/`, fileName, uid)
.then(resolve);
});
}).on('error', (err) => { // Handle errors
fs.unlink(datadir + `download/${uid}/${fileName}`);
reject(); // Or resolve() if you want to ignore the error and not cause it to prevent the callback from executing later
});
// ...
});
}
Finally, we get to the processArray() function and here you need to do the same thing we did to begin with: map the processes into a list of promises. First, the process function passed needs to return the promises returned by getDataReg() and getDataFtp():
// Second param, download and post process
(url) => {
if(url.startsWith('http')) {
return getDataReg(url, uid);
}
else if(url.startsWith('ftp')) {
return getDataFtp(url, uid);
}
else {
console.log('not a valid resource');
}
return Promise.reject(); // or Promise.resolve() if you want invalid resources to be ignored and not prevent the callback from executing later
}
Now, your processArray() function can look like this:
exports.processArray = (items, process, callback) =>
Promise.all(items.map(process))
.then(callback)
.catch(() => console.log('Something went wrong somewhere'));
Your callback will get invoked when all asynchronous actions have completed, regardless of in which order they do. If any one of the promises rejects, the callback will never be executed so manage your promise rejections accordingly.
Here's a JSFiddle with the complete code: https://jsfiddle.net/upn4yqsw/

In general, since nodejs does not appear to have implemented Streams Standard to be Promise based, at least from what can gather; but rather, uses an event based or callback mechanism, you can use Promise constructor within function call, to return a fulfilled Promise object when a specific event has been dispatched
const doStuff = (...args) => new Promise((resolve, reject)) => {
/* define and do stream stuff */
doStreamStuff.on(/* "close", "end" */, => {
// do stuff
resolve(/* value */)
})
});
doStuff(/* args */)
.then(data => {})
.catch(err => {})

Issue with promises in a for loop

I'm confronted to a situation which drives me a bit mad.
So The situation is as below :
module.exports = {
generation: function (req, res) {
// Let's firstly fetch all the products from the productTmp Table
function fetchProductsTmp (){
ProductsTmp.find().then(function (products) {
return Promise.all(products.map (function (row){
Service.importProcess(row);
}));
});
}
fetchProductsTmp();
}
Here I simply call my model ProductsTmp to fetch my datas and iterate through my rows calling importProcess.
importProcess :
importProcess: function (product) {
async.series([
function (callback) {
return SousFamille.findOne({name: product.sous_famille}).then(function (sf) {
console.log('1');
if (!sf) {
return SousFamille.create({name: product.sous_famille}).then(function (_sf) {
console.log('2');
callback(null, _sf.sf_id);
});
} else {
callback(null, sf.sf_id);
}
});
},
function (callback){
console.log('3');
},
], function(err, results){
if(err) return res.send({message: "Error"});
});
}
So I got with my console log :
1
1
1
2
3
2
3
2
3
What I want to Obtain is 1 2 3 1 2 3 1 2 3 So that each function wait for the promise to finish before calling the next one.

In the generation function of the first section, replace
return Promise.all(products.map (function (row){
Service.importProcess(row);
}));
with
var results = [],
pushResult = id => results.push(id);
return products.reduce(function(prev, row){//Go through all the products
//Take the previous promise, and schedule next call to Service.importProcess to be
//made after the previous promise has been resolved
return prev.then(function(){
return Service.importProcess(row).then(pushResult);
});
}, Promise.resolve())
.then(() => results);
You also need to return a promise from importProcess for this to work. Just ditch the whole async.series thingy and do something like
return new Promise(function(resolve, reject){
...
resolve(sf.sf_id); //instead of the callback(null, sf.sf_id)
...
});
Update: This forces the calls to Service.importProcess to be sequential instead of concurrent, which does affect the overall performance of calls to generation. But I guess you have more solid reasons to do so than sequential console.logs.

Sorry, can't help the urge to do it in ES6, basically things can be reduced to single line, like Bergi said, async is redundant( using Bluebird Promise Library):
importProcess: product =>
SousFamille.findOne({name: product.sous_famille})
.then(sf => sf? sf.sf_id : SousFamille.create({name: product.sous_famille}).then(_sf => _sf.sf_id))
// the other module
module.exports = {
generation: (req, res) => ProductsTmp.find()
.then(products => Promise.mapSeries(products, Service.importProcess.bind(Service)) )
.then(ids => res.send({ids}))
.catch(error => res.send({message: 'Error'}))
}
also like noppa said, your problem is the missing return in Service.importProcess(row), same code in ES5:
module.exports = {
generation: function (req, res) {
ProductsTmp.find()
.then(function (products) {
return Promise.mapSeries(products, Service.importProcess.bind(Service)) );
}).then(function(ids){
res.send({ids: ids});
}).catch(function(error){
res.send({message: 'Error'});
})
}
importProcess: function (product) {
return SousFamille.findOne({name: product.sous_famille})
.then(function (sf) {
if (sf) return sf.sf_id;
return SousFamille.create({name: product.sous_famille})
.then(function (_sf){ return _sf.sf_id});
});
}

Node JS Sync Work flow with Async request

Currently try to learn Node JS and getting my head around Async/Sync workflow.
Try to the follow:
Step 1:
- Get data 1 with function 1
- Get data 2 with function 2
- Get data 3 with function 3
Step2:
- Work out logic with data 1,2,3
Step 3
- Do final call
I been looking at Q and Async packages but still havent really find an example.
Can some one show me how they will go about this issue in Node JS?
Thanks

Not entirely clear on your implementation, but depending on how specific your ordering needs to be you could try something like this:
var data1 = null;
var data2 = null;
var data3 = null;
async.series([
function(httpDoneCallback){
async.parallel([
function(data1Callback){
$http(...).then(function(response){
// some logic here
data1 = response;
data1Callback();
})
},
function(data2Callback){
$http(...).then(function(response){
// some logic here
data2 = response;
data2Callback();
})
},
function(data3Callback){
$http(...).then(function(response){
// some logic here
data3 = response;
data3Callback();
})
}
], function(){
//all requests dome, move onto logic
httpDoneCallback();
})
},
function(logicDoneCallback){
// do some logic, maybe more asynchronous calls with the newly acquired data
logicDoneCallback();
}
], function(){
console.log('all done');
})

Do you want function 1, 2, and 3 to trigger at the same time? If so then this should help:
var async = require('async');
async.parallel([
function(cb1) {
cb1(null, "one")
},
function(cb2){
cb2(null, "two")
},
function(cb3){
cb3(null, "three")
}
], function(err, results) {
console.log(results); // Logs ["one", "two", "three"]
finalCall();
});
To explain, every function in the array submitted as the first param to the parallel method will also receive a callback function. Activating the callback function signifies that you're done fetching your data or doing whatever you need to do in said function. All three functions will trigger at the same time, and once all three callbacks are called, the final function is called. The callback accepts two parameters: "error", and "result." If everything's successful, pass "null" as the error parameter. The results will be given to the final function as an array containing each of the results for your individual functions.

You can setup a chain of Promises to do things sequentially:
var funcA = () => {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve('some data from A')
}, 1000)
});
}
var funcB = (dataFromA) => {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(dataFromA + ' data from B')
}, 2000)
})
}
var funcC = (dataFromB) => {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(dataFromB + ' data from C')
}, 500)
})
}
// Doing the functions on after another
funcA().then(funcB).then(funcC).then((data) => {
console.log(data);
})
Or if you want to do them all at the same time you can use Promise.all():
var promises = [];
promises.push(new Promise((resolve, reject) => {
setTimeout(() => {
resolve('some data from A')
}, 1000)
}));
promises.push(new Promise((resolve, reject) => {
setTimeout(() => {
resolve('some data from B')
}, 1000)
}));
promises.push(new Promise((resolve, reject) => {
setTimeout(() => {
resolve('some data from C')
}, 1000)
}));
// Execute the array of promises at the same time, and wait for them all to complete
Promise.all(promises).then((data) => {
console.log(data);
})

Probably the best thing to do is use Promises like #Tyler here states. However, for conceptual understanding it is best to first understand the node callback pattern.
Because some tasks take time, we give the task a function and say 'When you are done, put the data you retrieved into this function'. These functions that we give to other functions are called callbacks. They must be constructed to accept the data, and also an error in case there is a problem while fetching the data. In Node the error is the first callback parameter and the data is the second.
fs.readFile('/file/to/read.txt', function callback(error, data) {
if (error) console.log(error);
else console.log(data);
});
In this example, once node reads the file, it will feed that data into the callback function. In the callback we must account for the case that there was a problem and handle the error.
In your question you want to do multiple async tasks and use their results. Therefore you must take this pattern and nest several of them. So, continuing this example, if there is no error you will begin another async task.
fs.readFile('/file/to/read.txt', function callback(error, data) {
if (error) console.log(error);
else {
someOtherAsyncThing(function callback(error, data2) {
if (error) console.log(error);
else {
console.log(data + data2)
}
});
}
});