Having this async function that returns a Promise:
async function doSomething(userId) {
return new Promise(async (resolve, reject) => {
const query = 'my-query';
const roomAggregate = Room.aggregate(query).allowDiskUse(true);
Room.aggregatePaginate(roomAggregate, async function (err, data) {
if (err || !data) {
throw err;
return {
success: false,
rooms: [],
};
} else {
const rooms = [];
for (let room of data.docs) {
const roomId = room._id;
room = await computeAThing(room, {
loadWriteUps: false,
loadCreators: false,
});
room.userCompleted = await computeBThing(userId, roomId);
rooms.push(room);
}
return resolve({
rooms,
success: true,
paginator: {
// some data related to pagination
},
});
}
});
});
}
I'm not sure if it really needs to contain new Promise inside as it is already declared as async function. Is it mandatory in this case?
Because when that part was removed and at the end instead of return resolve({...}) it is only return {...} it seems to not settle.
Here is the modified code with new Promise and different return:
async function doSomething(userId) {
const query = 'my-query';
const roomAggregate = Room.aggregate(query).allowDiskUse(true);
Room.aggregatePaginate(roomAggregate, async function (err, data) {
if (err || !data) {
throw err;
return {
success: false,
rooms: [],
};
} else {
const rooms = [];
for (let room of data.docs) {
const roomId = room._id;
room = await computeAThing(room, {
loadWriteUps: false,
loadCreators: false,
});
room.userCompleted = await computeBThing(userId, roomId);
rooms.push(room);
}
return {
rooms,
success: true,
paginator: {
// some data related to pagination
},
};
}
});
}
This method is used somewhere else in this way:
const myInfo = await someObj.doSomething('myUserId');
and then checked:
if (myInfo.success) { ... }
For the first way of writing the function it works fine, for the second one myInfo is undefined and it throws and error that cannot read success of undefined.
Is something missing from the implementation?
For the second version I can see that you actually not returning anything from doSomething so I think you should do the below:
async function doSomething(userId) {
const query = 'my-query';
const roomAggregate = Room.aggregate(query).allowDiskUse(true);
const obj = Room.aggregatePaginate(roomAggregate, async function (err, data) {
if (err || !data) {
throw err;
return {
success: false,
rooms: [],
};
} else {
const rooms = [];
for (let room of data.docs) {
const roomId = room._id;
room = await computeAThing(room, {
loadWriteUps: false,
loadCreators: false,
});
room.userCompleted = await computeBThing(userId, roomId);
rooms.push(room);
}
return {
rooms,
success: true,
paginator: {
// some data related to pagination
},
};
}
});
return obj;
}
In general, you don't need to explicitly return a Promise in an async function as by default the returned value will be wrapped in a Promise , whatever it is. You just need to return something
Room.aggregatePaginat is written in coninuation-passing style, which does not interface well with promises. A generic promisify function can be used to convert any continuation-passing style function into a promise-based one. If you don't wish to write this function yourself, it is provided by Node as util.promisify -
const promisify = f => (...args) =>
new Promise((resolve, reject) =>
f(...args, (err, data) => err ? reject(err) : resolve(data))
)
Now it's easy to refactor your doSomething. Note use of Promise.all means all rooms data is processed in parallel rather than in series -
async function doSomething(userId) {
const query = 'my-query'
const roomAggregate = Room.aggregate(query).allowDiskUse(true)
const {docs:rooms} = await promisify(Room.aggregatePaginate)(roomAggregate)
return {
rooms:
await Promise.all(rooms.map(async room => ({
...await computeAThing(room, {loadWriteUps: false, loadCreators: false}),
userCompleted: await computeBThing(userId, room._id)
}))),
success: true,
paginator: ...
}
}
Also you should avoid things like { success: true } because a resolved promise is inherently "successful". A rejected one is not.
And watch out for return occurring after a throw. In this case the return is unreachable so it doesn't do what you think it's doing.
if (err || !data) {
throw err;
return { // unreachable
success: false, // these four lines
rooms: [], // are completely ignored
}; // by javascript runtime
}
Again, { success: false } goes against the Promise pattern anyway. If you want to recover from a rejection and recover with an empty list of { rooms: [] }, do it like this instead -
doSomething(user.id)
.catch(err => ({ rooms: [] }))
.then(res => console.log(res.rooms))
Better yet, you can try..catch inside doSomething and return the appropriate empty response in the event of an error. This prevents the error from bubbling up and forcing the user to handle it -
async function doSomething(userId) {
try { // try
const query = 'my-query'
const roomAggregate = Room.aggregate(query).allowDiskUse(true)
const {docs:rooms} = await promisify(Room.aggregatePaginate)(roomAggregate)
return {
rooms:
await Promise.all(rooms.map(async room => ({
...await computeAThing(room, {loadWriteUps: false, loadCreators: false}),
userCompleted: await computeBThing(userId, room._id)
}))),
paginator: ...
}
}
catch (err) { // catch
return { rooms: [] }
}
}
doSomething(user.id)
.then(res => console.log(res.rooms))
Related
I am working on something that needs nested foreach loops in order to process some data.
Intention is there is an array of ID's I need to look up, each ID related to a user and I just need to extract their names from the response of an API call. Service A has the list of ID's and then sends a HTTP GET request to service B for each ID (can't change this), which then responds with the correct info in the format of
{
success: true,
user: {
name: 'John Doe'
}
}
Code that doesn't work but is my current code
incidents.forEach((name) => {
foo = {
nameID: names
}
const asyncForEach = async (array, callback) => {
for(let index = 0; index < array.length; index++) {
await callback(array[index], index, array)
}
}
const startMulti = async () => {
await asyncForEach(foo.nameID, async (obj, index) => {
await userController.userInfo(obj)
.then((userInfo) => {
foo.nameID[index] = userInfo.user.name
})
.then(() => {
foobarArray.push(foo)
})
})
return res.json({success: true, foobar: foobarArray})
}
startMulti()
})
Took the origional idea of nested foreach loops from this blog post which allowed me to do another before though this one won't work
https://codeburst.io/javascript-async-await-with-foreach-b6ba62bbf404
edit show variables
let foo
let foobarArray = []
Added await to usercontroller now getting proper output but error message saying Cannot set headers after they are sent to the client
Names is coming from outside the code and is just needed where it is. Not sure how to explain it without explaining the project in whole/detail.
edit show code for usercontroller.userinfo
exports.userInfo = function(id) {
return new Promise((resolve, reject) => {
let user = {
_id: id
}
options.json = {user}
request.get('/userInfo', options, (err, response, body) => {
resolve(body)
})
})
}
This code work perfectly as expected - ie it sends request with proper payload and returns proper response.
Edit current code attempt
let foobarArray =[]
let names = []
let foo
for (const incident of incidents) {
foo = {
nameID: names
}
for (const id of incident.names) {
const userInfo = await userController.userInfo(id)
names.push(userInfo.user.name)
}
}
return res.json({success: true, fooreturned: foobarArray})
Error message caused by await in front of userController
SyntaxError: await is only valid in async function
edit attempt at making async function (I normally don't use async/await instead use promises)
Even after attempt code below it still gives the error message above - I had tried same code before I edited to show error message
exports.userInfo = async function(id) {
return new Promise((resolve, reject) => {
let user = {
_id: id
}
options.json = {user}
request.get('/userInfo', options, (err, response, body) => {
resolve(body)
})
})
}
Full code below except the userInfo function above which already is shown above.
exports.monitor = function(req, res, next) {
const fooID = req.params.id
let foobarArray =[]
let names = []
let foo
Incident.find({fooID})
.exec((err, incidents) => {
if(err) {
console.log(err)
return res.json({success: false, foobar: []})
}
if(incidents != []) {
for (const incident of incidents) {
foo = {
nameID: incident.foo[0].names
}
for (const id of foo.responded) {
const userInfo = await userController.userInfo(id)
names.push(userInfo.user.name)
}
}
return res.json({success: true, foobar: foobarArray})
}
})
}
That's pretty much the whole code except some logging lines I still have to add. I pretty much need the foobar: foobarArray to be an array of objects - foo - where nameID is the array of proper names not ID's. The proper names are fetched via the userController.userInfo where the ID is passed.
edit - New code after async and promisify - not sure I did the promisify correctly
exports.monitor = async function(req, res, next) {
const fooID = req.params.id
const incidents = await userController.incidentFind(fooID)
}
exports.incidentFind = async function(id) {
return new Promise((resolve, reject) => {
const sevenAgo = moment().subtract(7, 'days').toISOString()
let alertArray =[]
let names = []
let alert
Incident.find({monitorID, createdAt: {$gte: sevenAgo}})
.exec((err, incidents) => {
if(err) {
console.log(err)
return res.json({success: false, foobar: []})
}
if(incidents != []) {
for (const incident of incidents) {
foo = {
nameID: incident.foo[0].names
}
for (const id of foo.responded) {
const userInfo = await userController.userInfo(id)
names.push(userInfo.user.name)
}
}
return res.json({success: true, foobar: foobarArray})
}
})
})
}
Not sure what the actual controller monitor should contain. Bit lost
Error message
/home/me/Projects/app/incidents/controllers/users.js:100
const userInfo = await userController.userInfo(id)
^^^^^
SyntaxError: await is only valid in async function
Looks like the function should be async already (put async before function name).
I think you're looking for a simple
exports.monitor = async function(req, res, next) {
const fooID = req.params.id
const foobarArray = […]
const names = […]
try {
const incidents = await Incident.find({fooID}).exec()
for (const name of incidents) {
const foo = {
nameID: []
}
for (const userId of names) {
const userInfo = await userController.userInfo(userId)
foo.nameID.push(userInfo.user.name)
}
foobarArray.push(foo)
}
res.json({success: true, foobar: foobarArray})
} catch(err) {
console.log(err)
res.json({success: false, foobar: []})
}
}
I have an async function where there is some operations like this:
async function myAsyncFunction(argument) {
let arrayTasks = [
{
const run = async () => {
let response = await getDataAsync();
}
},
{
const run = async () => {
let response = await getDataAsync();
}
}
]
for (const index in arrayTasks) {
await arrayTasks[index].run();
}
}
The function above is a simplified version of my code, it works. But I am not sure where do I need to put try-catch block:
Wrapping all content function:
async function myAsyncFunction(argument) {
try{
// All code
}catch (e) {
// catch code
}
}
Or inside my asyncs functions and for operator:
async function myAsyncFunction(argument) {
let arrayTasks = [
{
const run = async () => {
try{
let response = await getDataAsync();
}catch (e) {
// catch code
}
}
},
{
const run = async () => {
try{
let response = await getDataAsync();
}catch (e) {
// catch code
}
}
}
]
for (const index in arrayTasks) {
try{
await arrayTasks[index].run();
}catch (e) {
// catch code
}
}
}
What is a correct way?
The arrayTasks variable is dynamic length on my original code.
Depends how and where you want to handle failure.
One approach to "an array of asynchronous tasks whose execution may fail" is a pattern like this:
async function myAsyncFunction(argument) {
const success = async res => ({ success: await res }) // Promise<{success: res}>
const error = async err => ({ error: await err }) // Promise<{error: e}>
const arrayTasks = [
{
run: async () => getDataAsync()
}
},
{
run: async () => getDataAsync()
}
]
const runWithResult = task => task.run().then(success).catch(error)
const outcomes = await Promise.all(arrayTasks.map(runWithResult))
console.log(outcomes) // array of {error: e} | {success: res}
}
You can chain a .catch() handler on an async function, and it has the same effect as wrapping it in try/catch.
More here, if you are interested. The section "Refactor without fp-ts" shows how to reduce that array from [{error: e} | {success: res}] to {error: [], success: []}, which is way easier to work with:
const { error, success } = outcomes.reduce((acc, o) => o.error ?
{ error: [...acc.error, o.error], success: acc.success } :
{ error: acc.error, success: [...acc.success, o.success] },
{ error: [], success: [] })
This is an FP type called Either - an operation may return "either" (in this case) a value or an error.
Your code doesn't throw with this approach.
If you know that something may fail, it's not exceptional. Exceptions are when some unexpected failure occurs, IMHO.
"Tasks that may fail" that are known ahead of time just need the error path code written.
If you take this approach, I recommend building it as a first-class state reducing machine, like this:
// Takes an array of async tasks that may throw of shape {run: () => Promise<result>}
// Returns Promise<{error: error[], success: result[]}>
async function executeAsyncTasks(arrayOfAsyncTasks) {
const success = async res => ({ success: await res }) // Promise<{success: res}>
const error = async err => ({ error: await err }) // Promise<{error: e}>
const runWithResult = task => task.run().then(success).catch(error)
const outcomes = await Promise.all(arrayOfAsyncTasks.map(runWithResult))
const outcomesFlat = outcomes.reduce((acc, o) => o.error ?
{ error: [...acc.error, o.error], success: acc.success } :
{ error: acc.error, success: [...acc.success, o.success] },
{ error: [], success: [] })
return outcomesFlat
}
It all depends... My rule of thumb is only catch something if you intend on doing something with it, i.e some error handling. If the handling of the error is going to be the same across all cases, wrap the it all with one try/catch, else wrap individual cases, with their own error handling code.
let getProjects = function() {
try {
return axios.get('https://app.asana.com/api/1.0/projects/')
} catch (error) {
console.error(error)
}
}
let getTasks = function(project) {
try {
return axios.get('https://app.asana.com/api/1.0/projects/'+project+'/tasks')
} catch (error) {
console.error(error)
}
}
async function getAsanaData() {
let projects = await getProjects()
projects = projects.data.data
projects.map(async (project) => {
//project.attachments = []
let tasks = await getTasks(project.gid)
if(tasks != undefined){
tasks = tasks.data.data
project.tasks = tasks
//console.log(projects)
}
})
console.log(projects)
return projects
}
Promise.try(() => {
return getAsanaData();
}).then((result) => {
//console.log(util.inspect(result, {showHidden: false, depth: null}))
//var asanaData = safeJsonStringify(result);
//fs.writeFile("thing.json", asanaData);
})
.catch(err=>console.log(err))
In getAsanaData(), projects has a new value after project.tasks = tasks.
However, it's original value is printed by console.log(projects) before return projects.
This of course also means that the original value rather than the necessary new value will be returned.
What is the cause and how do I resolve this?
Try this:
async function getAsanaData() {
let projects = await getProjects()
return Promise.all(projects.data.data.map(async (project) => {
let tasks = await getTasks(project.gid)
project.tasks = !!tasks ? tasks.data.data : project.tasks
return project
}))
}
This will go through the async calls to getTasks and return for each the project. Then you should get them all resolved via Promise.all
How do I return the value from callback function and pass it to resolve function in Graphql?
Here's the dummy code to show the concept:
This function runs the sql query:
function runQuery(query, cb){
....
var value = "Something";
cb(null, value);
}
This takes the value from callback function pass it to resolve function in graphql:
function getTitle() {
return runQuery("...", function(err, value){
return value;
});
}
Graphql schema:
var SampleType = new GraphQLObjectType({
name: 'Sample',
fields: () => ({
title: { type: GraphQLString },
}),
});
query: new GraphQLObjectType({
name: 'Query',
fields: () => ({
sample: {
type: SampleType,
resolve: () => getTitle(),
},
}),
}),
You can make use of promises and async to accomplish this.
async function getTitle() {
const queryResult = await runQuery("...");
// ...
// Do post-query stuff here that you currently have in your callback
// ...
return queryResult
}
async function runQuery() {
const value = 'something';
// ...
return value;
}
Node fully supports async/await as of 7.10.0. Use TypeScript or Babel if you're in the browser or are locked into a lower version of node.
Basically you can create a promise around that runQuery so that you can use async await when using the query data
const getTitle = () => {
return new Promise((resolve, reject) => {
runQuery("...", (error, response) => !error
? resolve(response)
: reject(error))
})
}
const asyncFunction = async () => {
const data = await getTitle()
.then((response) => {
// handle response and return what you want
return response.data
})
.catch((error) => {
// handle error, log it, etc, in whatever way you want
console.log(error.message)
return null
})
if(data) { // data is valid
// do what you want with the valid data (no error)
} else { // there was an error
// handle if there is an error
}
}
asyncFunction()
Please forgive the fairly case-specific question, though I think the general end goal could be of use to other people.
Goal: Populate a MongoDB with data requested from multiple JSON API URLs.
Short question: So far I've had some success with request-promise, which uses Bluebird:
var rp = require('request-promise');
var options = {
uri: 'http://www.bbc.co.uk/programmes/b006qsq5.json',
headers: {
'User-Agent': 'Request-Promise'
},
json: true
};
rp(options)
.then(function (body) {
// Mongoose allows us query db for existing PID and upsert
var query = {pid: body.programme.pid},
update = {
name: body.programme.title,
pid: body.programme.pid,
desc: body.programme.short_synopsis
},
options = { upsert: true, new: true };
// Find the document
Programme.findOneAndUpdate(query, update, options, function(err, result) {
if (err) return res.send(500, { error: err });
return res.send("succesfully saved");
});
})
.catch(function (err) {
return res.send(err);
})
But how do I loop over an array of URLs, without the program failing if any of the promises are rejected?
Something like this for example, using Bluebird, fails if any of the URLs errors.
const urls = ['http://google.be', 'http://google.uk']
Promise.map(urls, rp)
.map((htmlOnePage, index) => {
return htmlOnePage;
})
.then(console.log)
.catch((e) => console.log('We encountered an error' + e));
As I want to write to the DB with successful requests, and ignore those that might not be responding right then, I need something that skips over rejected promises, which .all does not do.
Long question:
I've been reading up about promises all day and it's making my head hurt! But I've found some good resources, such as https://pouchdb.com/2015/05/18/we-have-a-problem-with-promises.html, which mentions the use of a Promise factory. Would this work for my case? I initially thought I should make each request, process the result and add it to the DB, then move on to the next request; but having seen .all I thought I should do all the requests, save the results in an array and loop over that with my DB saving function.
Should I even be using Promises for this? Maybe I should just make use of something like async.js and run my requests in series.
Thanks very much for any help or ideas.
But how do I loop over an array of URLs, without the program failing if any of the promises are rejected?
if you return a value from .catch other than a rejected promise, you will return a resolved promise
So, your .then for each individual request could return an object like
{
success: true,
result: whateverTheResultIs
}
and your catch returns
{
success: false,
error: whateverTheErrorIs
}
Really you don't NEED the success property, it's a convenience though
So the code would be - assuming process(url) returns a Promise
Promise.map(urls, url =>
process(url)
.then(result => ({result, success:true}))
.catch(error => ({error, success:false}))
)
.then(results => {
let succeeded = results.filter(result => result.success).map(result => result.result);
let failed = results.filter(result => !result.success).map(result => result.error);
});
Or, in ES5
Promise.map(urls, function (url) {
return process(url).then(function (result) {
return { result: result, success: true };
}).catch(function (error) {
return { error: error, success: false };
});
}).then(function (results) {
var succeeded = results.filter(function (result) {
return result.success;
}).map(function (result) {
return result.result;
});
var failed = results.filter(function (result) {
return !result.success;
}).map(function (result) {
return result.error;
});
});
I don't know if this fit your case, but I think You can use a counter to check when all promises has returned, regardless of the fact that each one has been resolved or rejected
var heroes = [
'Superman',
'Batman',
'Spiderman',
'Capitan America',
'Ironman',
];
function getHero(hero) {
return new Promise((resolve, reject) => {
setTimeout(() => {
return Math.round(Math.random()) ? resolve(hero + ' lives') : reject(hero + ' dead');
}, Math.random() * 3000)
})
}
function checkHeroes() {
var checked = heroes.length;
heroes.forEach((hero) => {
getHero(hero)
.then((res) => {
checked --;
console.log(res);
if (!checked) done();
})
.catch((err) => {
checked --;
console.log(err);
if (!checked) done();
});
})
}
function done() {
console.log('All heroes checked');
}
checkHeroes();
I think your issue is less about the bluebird api than structuring your promise chain.
const reducePropsToRequests = (props) => Promise.resolve(Object
.keys(props)
.reduce((acc, key) => {
acc[key] = request(sources[key]);
return acc;
}, {}));
const hashToCollection = (hash) => Promise.resolve(Object
.keys(hash)
.reduce((acc, k) => {
return [...acc, {source: k, data: hash[k]}];
}, []));
const fetchFromSources = (sources) => Promise.props(sources);
const findSeveralAndUpdate = (results) => Promise
.each(results.map(obj => {
// you have access to original {a: 'site.com'}
// here, so use that 'a' prop to your advantage by abstracting out
// your db config somewhere outside your service
return Programme.findOneAndUpdate(someConfig[obj.source], obj.data);
}))
const requestFromSeveralAndUpdate = (sources) => reducePropsToRequests(sources)
.then(fetchFromSources)
.then(hashToCollection)
.then(findSeveralAndUpdate)
.catch(/* some err handler */);
requestFromSeveralAndUpdate({ a: 'site.com', b: 'site.net' });
I'd just use request and write my own promise with try catch inside that only resolves. Pseudo example below
var request = require('request')
var urls = ['http://sample1.com/json', 'http://sample2.com/json']
var processUrl = (url) => {
return new Promise((resolve,reject)=> {
var result;
try {
var myRequest = {
uri: url,
method: 'GET',
header: {...}
};
request(option, (res,body,err)=> {
if(err) {
result = err;
return;
}
result = body;
})
}
catch(e) {
result = e;
}
finally {
resolve(result)
}
})
}