Using multiple await's in a function, looping, and handling errors - javascript

I'm struggling to get my head around this. I've searched for similar problems.
Basically, I have 2 asynchronous functions. Let's call them getData() and processData() for the purposes of this question, which both return a promise. (these 2 functions are not defined using the async keyword)
I then have a function which calls these 2 using the async keyword. Like so:
async function init() {
// get the data
for (var i = 0; i < arr_process.length; i++) {
try {
await getData(arr_process[i]);
} catch (err) {
return Promise.reject(err);
}
}
// now process the data
for (var i = 0; i < arr_done.length; i++) {
try {
await processData(arr_done[i]);
} catch (err) {
return Promise.reject(err);
}
}
}
My question is; is this the correct way to handle promise rejections. My understanding is as soon as you define a function using the async keyword it will return a promise. I want to reject that promise if either of the functions using the await keyword (getData or processData) reject a promise - and I don't want the remainder of the function to execute. So if the getData promise rejects above, I don't want the loop to continue or the second loop to start - I want to exit the function returning a rejected promise. Is this the correct implementation of what I'm after? Hopefully that all makes sense!

All the try/catches are unnecessary. If a Promise that is awaited rejects, the whole containing async function will reject, and the rejection value will be the thrown error. You may simply do:
async function init() {
// get the data
for (var i = 0; i < arr_process.length; i++) {
await getData(arr_process[i]);
}
// now process the data
for (var i = 0; i < arr_done.length; i++) {
await processData(arr_done[i]);
}
}
and then, just like with your original code, the caller of init will catch any errors with a catch:
init()
.catch((err) => {
console.log('error:', err);
});

You can throw an error from within an async function by simply using the throw keyword.
For example:
async function init() {
throw new Error('error message')
}
or in your case, just remove the try catch blocks and any errors will be thrown automatically.
async function init() {
// get the data
for (var i = 0; i < arr_process.length; i++) {
await getData(arr_process[i]);
}
// now process the data
for (var i = 0; i < arr_done.length; i++) {
await processData(arr_done[i]);
}
}

Related

Cannot print object through res.json in express JS

I am trying to build an API through which I can get whois detail in the JSON output like below
For this, I installed the whois package from npm (https://www.npmjs.com/package/whois[whois Link]2). I tried to convert the string to object and print it in JSON format but I am not getting any output on the web but it console i can get the data easily. Can you guys please fix my error.
function whoisFunction() {
var whois = require('whois')
whois.lookup(url,async function(err, data) {
try {
a = await data.split('\n')
}
catch (e) {
console.log(e)
return e
}
c=[]
for(i = 0; i < a.length; i++){
c.push(a[i])
}
await console.log(typeof (c))
console.log(c)
return a
})
}
// res.json({'Headers':+whoisFunction()})
res.status(200).json(whoisFunction())
There are async and awaits sprinkled seemingly randomly all over your function.
You should realize that the only thing that is asynchronous here is whois.lookup().
console.log is not asynchronous. Array.prototype.split is not asynchronous. The callback (err, data) => {...} is not asynchronous.
If you want to use the callback pattern, then you need to use res.send() inside of the callback
(err, data) => {
res.send(data)
}
But we got fed up with callback-patterns because of how messy it is to nest them. So instead we went over to using promises. If you have callbacks but want use promises, then you wrap the callback in a promise. You do it once, and you do it as tight to the offending callback as you can:
function promisifiedLookup(url){
return new Promise( (resolve, reject) => {
whois.lookup(url, function(err, data) {
if(err) reject(err)
else resolve(data)
})
})
}
So, to use async/await we need that:
the calling function is declared async
the called function is returning a promise (or else there is nothing to wait for)
async function whoisFunction() {
let data = await promisifiedLookup(url) // _NOW_ we can use await
data = data.split('\n')
// ...
return data; // Because this funtion is declared async, it will automatically return promise.
}
If your express-handler is defined as async, then you now can use await here as well:
res.status(200).json(await whoisFunction())

Javascript promise chaining with promise.all not working?

I have the following code. When i run it i get this error " Cannot read property 'then' of undefined" at the first line in my code when calling getQueryToShift. It seems like getQueryToShift is not working like I intended. What is the correct way to use Promise.all so that the original promise I am declaring on the first line waits for all the promises in the promises array declared inside the getQueryToShift function to resolve before executing what is inside the then block ?
promise = getQueryToShift(hourDiff, options, map, baseBuildStart, oldFrom).then(values => { // error is here
// wait for promise before handling data in map passed as parameter
});
function getQueryToShift(hourDiff, options, map, baseBuildStart, oldFrom) {
let promises = [];
datasourceSrv.get(options.targets[0].datasource).then(ds => {
for (let i = 0; i < daysDiff - 1; i++) {
options.range.from._d = dateToMoment(oldFrom, false).add(i, 'h').toDate();
options.range.to._d = dateToMoment(options.range.from._d, false).add(1, 'h').toDate();
ds.query(options).then(result => {
promises.push(createQueryPromise(map, baseBuildStart, result.data));
});
}
return Promise.all(promises);
});
}
function createQueryPromise(map, baseBuildStart, data) {
return new Promise((resolve) => {
data.forEach(datum => {
//parsing data and adding it to map passed in as parameter
})
resolve();
});
}
The question is incomplete because createQueryPromise() function makes no sense by itself. It does not show an asynchronous operation at all, thus without one there is no need for a promise. And, if there is an asynchronous operation inside your .forEach() look, then you will need to coordinate that async operation, but you don't show the code for that so we can't help you fix that and we need to understand what that function is actually doing before we can help with a full and correct implementation of getQueryToShift().
On top of that, here are a couple other things that need fixing:
You aren't returning your promise from getQueryToShift(). Change this:
datasourceSrv.get(options.targets[0].datasource).then(ds => {
to this:
return datasourceSrv.get(options.targets[0].datasource).then(ds => {
And, you are ignoring the promise returned by:
ds.query(options).then(...)
Thus, your code wouldn't wait for that operation to finish. This is probably the promise that you need to collect in the array and use Promise.all() with, not the one you are doing it with.
Your code does the Promise.all on an empty array - you need to return the promise:
function* getDays(daysDiff, oldFrom) {
for (let i = 0; i < daysDiff - 1; i++) {
var options = {};
options.range.from._d = dateToMoment(oldFrom, false).add(i, 'h').toDate();
options.range.to._d = dateToMoment(options.range.from._d, false).add(1, 'h').toDate();
yield options;
}
}
function getQueryToShift(hourDiff, options, map, baseBuildStart, oldFrom) {
// return here
return datasourceSrv.get(options.targets[0].datasource).then(ds => {
return Promise.all(
Array.from(getDays(daysDiff, oldFrom), option => db.query(option))
);
});
}
Although I warmly recommend using an async function instead:
async function getQueryToShift(hourDiff, options, map, baseBuildStart, oldFrom) {
const ds = await datasourceSrv.get(options.targets[0].datasource);
await Promise.all(Array.from(getDays(daysDiff, oldFrom), db.query));
}

Promises and cycles

There's a chain of promises, the first takes user's id from data base after i need to get his groups and names of constructor from data base
And these can be multiple copies
As a result cycle into cycle. eventually I receive object only of one user
async(takeIds()
.then(ids => {
// console.log("ids", ids);
for (var i = 0; i < ids.length; i++) {
data.id = ids[i];
await (takeIdvk(ids[i])
.then(idm => {
data.idvk = idm;
takeIdg(data.id)
.then(res => {
takeNamesCycle(res, data)
.then(data => {
console.log("data", data);
})
.catch(err => {
console.log(err);
});
})
.catch(err => {
console.log(err);
});
})
.catch(err => {
console.log(err);
}));
}
})
.catch(function(err) {
console.log(err);
}));
And function takeNamesCycle cos it differs from other
( in the rest simple requests to data base)
var takeNamesCycle = async(function(arr, obj) {
return new Promise((resolve, reject) => {
for (var i = 0; i < arr.length; i++) {
var idg = arr[i];
await (takeNames(arr[i])
.then(names => {
obj.idg[idg] = names;
})
.catch(err => {
console.log(err);
}));
}
resolve(obj);
});
});
Is it possible to simplify this?
I'd be grateful
Consider using real async/await syntax instead of a library with async() and await() functions.
const ids = await takeIds();
// console.log("ids", ids);
try {
for (var i = 0; i < ids.length; i++) {
data.id = ids[i];
const idem = await takeIdvk(ids[i]);
data.idvk = idm;
const res = await takeIdg(data.id);
console.log("data", await takeNamesCycle(res, data));
}
} catch(err) {
console.log(err);
}
and make sure to avoid the Promise constructor antipattern
async function takeNamesCycle(arr, obj) {
for (const idg of arr) {
const names = await takeNames(idg);
obj.idg[idg] = names;
}
return obj;
}
The posted code is mixing promise chain style of code within await and async functions within bad code.
First await does not permit .then or .catch methods to be called on the value it returns. The reason is that the value returned by await is never a promise. await only resumes execution after a successful promise operation and returns the result of the operation. So for example
await (takeIdvk(ids[i])
.then(idm => { // incorrect
data.idvk = idm;
// ... more code
becomes
let idm = await takeIdvk(ids[i]) // await returns fulfilled value
data.idvk = idm;
// ... more code
In other words, code to handle data for a successful operation is written inline after await instead of in a separate onfulfilled handler passed as a parameter to .then.
Promise Rejections
If the promise being waited on becomes rejected, the await operator throws the rejection reason without resuming function execution.
The rejection can be caught in a try/catch block in order to ignore the error, repeat the operation, or replace it with a different operation as determined by application requirements. E.G.
let idm;
try {
idm = await takeIdvk(ids[i]) // await returns fulfilled value
data.idvk = idm;
// ... more code
} catch( err) {
console.log( "ignoring error + err);
// the value of idm is undefined
}
On the other hand, if rejection is to be considered fatal don't put await inside a try/catch block - the thrown value will be caught and used to reject the promise returned by the async function, much like throwing an error inside a then or catch handler.
If calls to async functions are nested, each called using await without any try/catch blocks, rejection errors will bubble up from deeply nested functions and reject the promise returned by the outermost async function call. A single .catch clause on the outermost promise can be used to prevent uncaught promise rejections and notify that a failure occurred.
Conclusion
Try writing the code using plain promise chains if you are more familiar with that syntax. By all means continue with converting it to async/await syntax but be aware that async/await is an alternative syntax and they don't mix well in the same function. As already stated by #Bergi , returning a new promise from an async function is an anti pattern - async functions return a promise already.

Anti-Pattern to aggregate errors and pass to promise resolve js?

I'm processing a list in a loop that's run async returning a promise and I do not want to exit processing on exception, so I aggregate them and pass them to the resolve callback in an outer finally block.
I'll like to know if this is an anti-pattern and if so, please provide a pointer on how to do correctly.
Thanks.
Example
async doSomething(list) {
let errorCount = 0
let errors = []
return new Promise(async (resolve, reject) => {
try {
list.forEach(async (item) => {
try {
actionThatThrows(item)
} catch (e) {
errorCount++
errors[errorCount] = e
}
})
} catch (e) {
errorCount++
errors[errorCount] = e
} finally {
if (errorCount > 0) {
resolve(errors)
} else {
resolve()
}
}
})
}
Yes, this code employs several antipatterns:
Never pass an async function to a Promise constructor
Never use the Promise constructor when you already have promises around
forEach does not work with async functions
I do not want to exit processing on exception but aggregate them
You might want to have a look at Wait until all ES6 promises complete, even rejected promises for that.
But you can do it without those as well, assuming you want sequential iteration:
async function doSomething(list) {
const errors = [];
for (let item of list) {
try {
await actionThatThrows(item);
} catch (e) {
errors.push(e);
}
}
if (errors.length)
return errors;
else
return …;
}
The errors are the result of your asynchronous computation so it globally looks legit.
Assuming that actionThatThrows returns a promise (it's unclear in your question and code), it looks like it could be written like this:
function doSomething(list) {
let errors = []
return Promise.all(list.map(
item => actionThatThrows(item).catch(e => {
errors.push(e);
})
)).then(()=>{
return errors.length ? errors : undefined;
});
}
1) async doSomething is not invoking an await, so remove async
2) async in list.forEach is not invoking an await, so remove async
3) First catch will catch all. Second catch will never be hit, so remove second catch and finally
Code can be simplified to:
doSomething(list) {
let errorCount = 0,
errors = [];
for (let item of list) {
try {
actionThatThrows(item); //I suppose this is not returning a promise
} catch (e) {
errorCount += 1;
errors[errorCount] = e;
}
}
if (errorCount > 0) {
return errors; //return Promise.reject(errors);
} else {
//return Promise.resolve();
}
}

Loop with native promises;

I'm trying to make an asynchronous loop with native ES6 promises It kind of works, but incorrectly. I suppose I made a huge mistake somewhere and I need someone to tell me where it is and how it's done correctly
var i = 0;
//creates sample resolver
function payloadGenerator(){
return function(resolve) {
setTimeout(function(){
i++;
resolve();
}, 300)
}
}
// creates resolver that fulfills the promise if condition is false, otherwise rejects the promise.
// Used only for routing purpose
function controller(condition){
return function(resolve, reject) {
console.log('i =', i);
condition ? reject('fin') : resolve();
}
}
// creates resolver that ties payload and controller together
// When controller rejects its promise, main fulfills its thus exiting the loop
function main(){
return function(resolve, reject) {
return new Promise(payloadGenerator())
.then(function(){
return new Promise(controller(i>6))
})
.then(main(),function (err) {
console.log(err);
resolve(err)
})
.catch(function (err) {
console.log(err , 'caught');
resolve(err)
})
}
}
new Promise(main())
.catch(function(err){
console.log('caught', err);
})
.then(function(){
console.log('exit');
process.exit()
});
Now the output:
/usr/local/bin/iojs test.js
i = 1
i = 2
i = 3
i = 4
i = 5
i = 6
i = 7
fin
error: [TypeError: undefined is not a function]
error: [TypeError: undefined is not a function]
error: [TypeError: undefined is not a function]
error: [TypeError: undefined is not a function]
error: [TypeError: undefined is not a function]
error: [TypeError: undefined is not a function]
error: [TypeError: undefined is not a function]
caught [TypeError: undefined is not a function]
exit
Process finished with exit code 0
The good part: it reaches the end.
The bad part: it catches some errors and I don't know why.
Any helper function with promise looping I have seen actually made it much worse than what you can do out of the box with recursion.
It is a little nicer with .thenReturn but yeah:
function readFile(index) {
return new Promise(function(resolve) {
setTimeout(function() {
console.log("Read file number " + (index +1));
resolve();
}, 500);
});
}
// The loop initialization
Promise.resolve(0).then(function loop(i) {
// The loop check
if (i < len) { // The post iteration increment
return readFile(i).thenReturn(i + 1).then(loop);
}
}).then(function() {
console.log("done");
}).catch(function(e) {
console.log("error", e);
});
See it in jsfiddle http://jsfiddle.net/fd1wc1ra/
This is pretty much exactly equivalent to:
try {
for (var i = 0; i < len; ++i) {
readFile(i);
}
console.log("done");
} catch (e) {
console.log("error", e);
}
If you wanted to do nested loops it is exactly the same:
http://jsfiddle.net/fd1wc1ra/1/
function printItem(item) {
return new Promise(function(resolve) {
setTimeout(function() {
console.log("Item " + item);
resolve();
}, 500);
});
}
var mdArray = [[1,2], [3,4], [5,6]];
Promise.resolve(0).then(function loop(i) {
if (i < mdArray.length) {
var array = mdArray[i];
return Promise.resolve(0).then(function innerLoop(j) {
if (j < array.length) {
var item = array[j];
return printItem(item).thenReturn(j + 1).then(innerLoop);
}
}).thenReturn(i + 1).then(loop);
}
}).then(function() {
console.log("done");
}).catch(function(e) {
console.log("error", e);
});
If all you're trying to do is count to 7 with promises, then this will do it:
function f(p, i) {
return p.then(function() {
return new Promise(function(r) { return setTimeout(r, 300); });
})
.then(function() { console.log(i); });
}
var p = Promise.resolve();
for (var i = 0; i < 8; i++) {
p = f(p, i);
}
p.then(function() { console.log('fin'); })
.catch(function(e) { console.log(e.message); });
Looping with promises is hard, because it's almost impossible not to fall into JavaScript's closures in a loop trap, but it is doable. The above works because it pushes all use of .then() into a sub-function f of the loop (i.e. away from the loop).
A safer solution, that I use, is to forgo loops altogether and seek out patterns like forEach and reduce whenever I can, because they effectively force the sub-function on you:
[0,1,2,3,4,5,6,7].reduce(f, Promise.resolve())
.then(function() { console.log('fin'); })
.catch(function(e) { console.log(e.message); });
here f is the same function as above. Try it.
Update: In ES6 you can also use for (let i = 0; i < 8; i++) to avoid the "closures in a loop" trap without pushing code into a sub-function f.
PS: The mistake in your example is .then(main(), - it needs to be .then(function() { return new Promise(main()); }, but really, I think you're using the pattern wrong. main() should return a promise, not be wrapped by one.
Try logging err.stack instead of just err when catching promise errors.
In this case, it looks like resolve and reject are not defined within the anonymous function that gets return from main after the initial iteration is complete. I can't totally follow your control flow, but that seems to make sense - after the 7 iterations are complete, there should no longer be any new promises. However, it seems like the code is still trying to run like there are more promises to resolve.
Edit: This is the problem .then(main(),function (err) {. Invoking main on its own will cause resolve and reject inside the anonymous function to be undefined. From the way I read it, main can only be invoked as an argument to the Promise constructor.
I had a similar need and tried the accepted answer, but I was having a problem with the order of operations. Promise.all is the solution.
function work(context) {
return new Promise((resolve, reject) => {
operation(context)
.then(result => resolve(result)
.catch(err => reject(err));
});
}
Promise
.all(arrayOfContext.map(context => work(context)))
.then(results => console.log(results))
.catch(err => console.error(err));
I've looked around for various solutions too and couldn't find one that satisfied me, so I ended up creating my own. Here it is in case it's useful to someone else:
The idea is to create an array of promise generators and to give this array to a helper function that's going to execute the promises one after another.
In my case the helper function is simply this:
function promiseChain(chain) {
let output = new Promise((resolve, reject) => { resolve(); });
for (let i = 0; i < chain.length; i++) {
let f = chain[i];
output = output.then(f);
}
return output;
}
Then, for example, to load multiple URLs one after another, the code would be like this:
// First build the array of promise generators:
let urls = [......];
let chain = [];
for (let i = 0; i < urls.length; i++) {
chain.push(() => {
return fetch(urls[i]);
});
}
// Then execute the promises one after another:
promiseChain(chain).then(() => {
console.info('All done');
});
The advantage of this approach is that it creates code that's relatively close to a regular for loop, and with minimal indentation.

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