Related
When the code below runs I expect score to have a value (lets say: {"A": 1, "B": 2}), but when I print it I get an empty dict ({}).
I have tried to use promises but the result is the same.
driversBySeason(season) {
var query = `SELECT raceId FROM races WHERE year = ${season}`;
var score = {};
this.con.query(query, (err, drop) => {
if (err) {
console.error(err);
}
drop.forEach((element) => {
var raceId = element["raceId"];
query = `SELECT driverId, points FROM results WHERE raceId = ${raceId}`;
this.con.query(query, (err, drop) => {
if (err) {
console.error(err);
}
drop.forEach((element) => {
if (score[element["driverId"]] == undefined) {
score[element["driverId"]] = 0;
} else if (score[element["points"]] != undefined) {
score[element["driverId"]] += element["points"];
}
});
});
});
console.log(score);
});
}
First of all you need to change your .forEach loop to for const of loop or just a regular for loop because it just fires the code inside the callback and never waits for it. And then you need to change your this.con.query(...) function which has callback to promises too. Your code should be like this:
const asyncQuery = (query) => new Promise((resolve, reject) => {
this.con.query(query, (err, drop) => {
if (!!err) {
console.log(error);
return reject(err)
}
return resolve(drop);
})
});
async function driversBySeason(season) {
var query = `SELECT raceId FROM races WHERE year = ${season}`;
var score = {};
const drop = await asyncQuery(query).catch(err => err /* some error handling */);
for (const element of drop) {
var raceId = element["raceId"];
query = `SELECT driverId, points FROM results WHERE raceId = ${raceId}`;
const drop2 = await asyncQuery(query).catch(err => err /* some error handling */);
drop2.forEach((element) => {
if (score[element["driverId"]] == undefined) {
score[element["driverId"]] = 0;
} else if (score[element["points"]] != undefined) {
score[element["driverId"]] += element["points"];
}
});
}
console.log(score);
}
You are outside of your callback. When this executes the code continues to run past "this.con.query". You are seeing your empty object that was assigned at the top do to this. Go inside the callback after the drop.forEach where you assign the values, or convert to an async/await approach.
when you call driversBySeason your score variable has value of {}
and that is value for console.log() when you call it because of how closers are works and you updating score with callback function that happen later in time...
you get better understanding if you use promise...not callback
EDIT: This is for an Electron project, with a local server spun up on the user's system. So, any concerns about what happens if multiple users attempt simultaneous access can be ignored!
My client code is generating an array of JavaScript objects, which I'm calling packets. There can be potentially infinite packets, although between 1 and 10 is the most common use-case.
I need to make API calls to a backend route, one per packet, passing a packet as an argument to each call.
However, the backend does some heavy computational work on each packet, to the point where attempting to crunch more than 2-3 packets at once crashes the server.
Is it possible to resolve Promises synchronously, such that the second Promise only fires when the first resolves, the third firing after the second, and so on?
It's my understanding that Promise.all() makes all calls simultaneously, which doesn't help me here.
(I also know that this is an anti-pattern, but for my specific set of requirements, I'm not sure what else to do)
I know this is purely abstract, but any thoughts would be appreciated!!!
Get weird with Promises
An async queue, a spin-off one of my previous answers; I've added random completion time to simulate a real environment:
class Queue {
constructor() {
this.queue = [];
}
enqueue(obj) {
return this.queue.push(obj);
}
dequeue() {
return this.queue.shift();
}
hasWork() {
return (this.queue.length > 0);
}
}
class AsyncQueue extends Queue {
constructor(job) {
super();
this.job = job;
}
process(cb) {
return this.job(this.dequeue()).then(data => {
cb(data);
if (this.hasWork())
return this.process(cb);
});
}
}
//MUST RETURN Promise
function work() {
var duration = chooseDelay();
console.log('START JOB, I.E., MAKE REQUEST (will take %s)', duration);
return t_o(duration);
}
function report() {
console.log('JOB DONE');
}
function done() {
console.log('ALL WORK DONE');
}
function t_o(delay) {
return new Promise(function (resolve, reject) {
setTimeout(function () {
resolve();
}, delay);
});
}
function chooseDelay() {
var delays = [200, 1000, 4000, 100, 50, 7000];
return delays[parseInt(Math.random() * 100) % delays.length];
}
var q = new AsyncQueue(work);
for (var packet = 0; packet < 10; ++packet)
q.enqueue(packet);
q.process(report).then(done);
As an alternative to an otherwise good answer, here's a really simple queue that works (work function shamelessly copied and expanded from other answer)
// Here is the queue "creator"
let promiseQueue = fn => {
let q = Promise.resolve();
return (...args) => q = q.then(() => fn(...args));
};
// that's it, that's the whole code for a promise queue
// here we create a queue
var q = promiseQueue(work);
// simple mock asynchronous function
function work({index, data}) {
var duration = parseInt(Math.random() * 100) + 100;
console.log('START JOB %s, I.E., MAKE REQUEST (will take %s) and should result with %s', index, duration, (index +1) * data);
return new Promise(resolve => setTimeout(resolve, duration)) // mock a delay
.then(() => ({index, result:(index + 1) * data})); // and some "processing"
}
// simulating two "chunks" of packets, generated a millisecond apart, but still, the sequence will be maintained
setTimeout(() => {
var packets = Array.from({length:10}, (_, index) => ({index, data:parseInt(Math.random() * 10000)}));
var promises = packets.map(packet => q(packet));
// the results in promise all are all the results of this batch of "packets"
Promise.all(promises).then(results => console.log(results));
}, 100);
setTimeout(() => {
var packets = Array.from({length:10}, (_, index) => ({index: index + 10, data:parseInt(Math.random() * 10000)}));
var promises = packets.map(packet => q(packet));
Promise.all(promises).then(results => console.log(results));
}, 101);
the simple function to execute promises sequentially
const sequentiallyExecuting = (promises) => {
let promise = Promise.resolve();
promises.forEach((task) => {
promise = promise.then((data) => {
return task;
})
});
return promise;
}
// pass array of promises to this function
sequentiallyExecuting(promises).then((data) =>{
console.log("all requests completed sequentially");
})
'use strict';
// job to be done
function job(params) {
return function () {
console.log('job started', params);
return new Promise(function (resolve) {
setTimeout(function () {
console.log('job finished');
resolve();
}, 1000);
})
}
}
// data to be processed sequentially
var params = [
1,
2,
3,
4,
5
];
// reduce data to Promise sequence
params.reduce(function (cum, cur) {
return cum.then(job(cur));
}, Promise.resolve());
With async/await it becomes trivial:
while (promiseArray.length > 0)
await promiseArray.shift();
I need to iterate through an array and save every object to the Database.
At the end I need a callback with an array of all of the saved and failed object.
Below is the code I have:
exports.addList = (app, body, callback) => {
var savedObjects = []
var failedObjects = []
body.forEach((element, index) => {
body[index] = _.pick(element, 'userAId','userBId')
db.List.create(element).then((list) => {
savedObjects.push(element)
if (index == body.length - 1) {
callback(savedObjects, failedObjects)
}
}).catch((error) => {
if (error.name === "SequelizeUniqueConstraintError") {
failedObjects.push(element)
if (index == body.length - 1) {
callback(savedObjects, failedObjects)
}
})
})
}
The code above works. Is there a way better to accomplish this?
I would recommend the following approach using Promise.all() to run the db.List.create() in parallel as it will return a Promise. By mapping the body array elements to Promises you can achieve better performance as they will run in parallel (and not have to track the complete count).
exports.addList = (app, body, callback) => {
var savedObjects = [];
var failedObjects = [];
Promise.all(
// map the array to return Promises
body.map(element => {
const list = _.pick(element, 'userAId','userBId');
return db.List.create(list)
.then(() => savedObjects.push(list))
.catch((error) => {
if (error.name === 'SequelizeUniqueConstraintError') {
failedObjects.push(list)
}
})
})
)
// when all Promises have resolved return the callback
.then(() => callback(savedObjects, failedObjects));
}
In your example your complete callback will always fire after the first promise is complete. This is because the create function is asynchronous while the surrounding loop is not, therefore the loop will have already completed by the time your first callback is triggered.
In your scenario this means element and index will always be the last in the loop. One way around this would be to move your promise chain into it's own function.
In this example I've used an extra flag to track the number of completed promises to trigger your complete method.
exports.addList = (app, body, callback) => {
var savedObjects = []
var failedObjects = []
var complete = 0;
function createElement(element){
db.List.create(element).then((list) => {
savedObjects.push(element)
}).catch((error) => {
if (error.name === "SequelizeUniqueConstraintError") {
failedObjects.push(element)
}
}).finally(() => {
complete++;
if(complete == body.length) {
callback(savedObjects, failedObjects)
}
});
}
body.forEach((element, index) => {
body[index] = _.pick(element, 'userAId','userBId');
createElement(element);
})
}
I am new to the world of node.js and Javascript and I have a loop that goes over an array of objects
at a certain condition I need to call a function that does asnyc work
and the loop to stop while the function isn't done
fucntion foo1(arr){
for(var i=0 ; arr.length>i ; i++){
if(i==8){//or any other condition
doAsyncStuff(hits[i])
}
}
}
function doAsyncStuff(item){
parser.parseURL(someurl,function(error,result){
item.someprop=result.someprop;
})
}
the problem is no matter what I do, I can't seem to make the function wait it end's before I have the result and doesn't update the item I need it to update.
I understand it's a common issue but none of the solution I found worked.
any help would be welcome.
Thanks!
Looping and doing async stuff is a little tricky in JS. You could use one of the libraries that #smnbbrv mentioned in his comment. But you could also do it yourself, which can help you understand how some of these libraries work.
function foo1(arr) {
next(arr, 0)
}
function doAsyncStuff(item, cb) {
parser.parseURL(someurl, function(error, result) {
item.someprop = result.someprop;
cb(result);
})
}
function next(arr, i) {
// Stop when we reach the end of the array.
if (i >= arr.length) {
return;
}
if (i == 8) { // or any condition
// Move to the next item only when the async work is done.
doAsyncStuff(arr[i], function() {
next(arr, i + 1)
})
} else {
next(arr, i + 1)
}
}
I would use Bluebird Promises and the reducer pattern.
var Promise = require('bluebird');
// iterates over the array 'arr' and calls fn
// Myfn repeatedly, optionally passing an initial.
// for the 1st iteration. For subsequent iterations,
// the value returned by prev invocation of Myfn is passed.
Promise.reduce(arr, Myfn, someInitialValue);
function Myfn(prev, item) {
//return a value or a promise.
}
see documentation of Reduce here: http://bluebirdjs.com/docs/api/promise.reduce.html
If I understand you correctly then you could use a Promise chain, serialised using reduce (a for loop would also work), something like this
function doAsyncStuff(item) {
return new Promise(resolve => {
const time = Math.ceil(Math.random() * 2000 + 1000);
window.setTimeout(() => {
item.someprop = time;
resolve();
}, time);
});
}
function foo1(arr) {
return arr.reduce(
(promise, item, index) => index % 2 === 0 ? promise.then(() => doAsyncStuff(item)) : promise,
Promise.resolve()
);
}
const hits = new Array(9).fill().map(() => ({}));
foo1(hits).then(() => {
console.log(hits);
});
There is also Promise.all, which you could probably use (though not sure how wonderful that would be, I'm not a frequent Promise user).
Update: Using Promise.all
function doAsyncStuff(item) {
return new Promise(resolve => {
const time = Math.ceil(Math.random() * 2000 + 1000);
window.setTimeout(() => {
item.someprop = time;
resolve();
}, time);
});
}
function foo1(arr) {
return Promise.all(
arr.map((item, index) => index % 2 === 0 && doAsyncStuff(item))
);
}
const hits = new Array(9).fill().map(() => ({}));
foo1(hits).then(() => {
console.log(hits);
});
I still haven't figured out the best way to format ES6, it always seems to end up with longish lines. (personal styling issue) :)
As the title suggests. How do I do this?
I want to call whenAllDone() after the forEach-loop has gone through each element and done some asynchronous processing.
[1, 2, 3].forEach(
function(item, index, array, done) {
asyncFunction(item, function itemDone() {
console.log(item + " done");
done();
});
}, function allDone() {
console.log("All done");
whenAllDone();
}
);
Possible to get it to work like this? When the second argument to forEach is a callback function which runs once it went through all iterations?
Expected output:
3 done
1 done
2 done
All done!
Array.forEach does not provide this nicety (oh if it would) but there are several ways to accomplish what you want:
Using a simple counter
function callback () { console.log('all done'); }
var itemsProcessed = 0;
[1, 2, 3].forEach((item, index, array) => {
asyncFunction(item, () => {
itemsProcessed++;
if(itemsProcessed === array.length) {
callback();
}
});
});
(thanks to #vanuan and others) This approach guarantees that all items are processed before invoking the "done" callback. You need to use a counter that gets updated in the callback. Depending on the value of the index parameter does not provide the same guarantee, because the order of return of the asynchronous operations is not guaranteed.
Using ES6 Promises
(a promise library can be used for older browsers):
Process all requests guaranteeing synchronous execution (e.g. 1 then 2 then 3)
function asyncFunction (item, cb) {
setTimeout(() => {
console.log('done with', item);
cb();
}, 100);
}
let requests = [1, 2, 3].reduce((promiseChain, item) => {
return promiseChain.then(() => new Promise((resolve) => {
asyncFunction(item, resolve);
}));
}, Promise.resolve());
requests.then(() => console.log('done'))
Process all async requests without "synchronous" execution (2 may finish faster than 1)
let requests = [1,2,3].map((item) => {
return new Promise((resolve) => {
asyncFunction(item, resolve);
});
})
Promise.all(requests).then(() => console.log('done'));
Using an async library
There are other asynchronous libraries, async being the most popular, that provide mechanisms to express what you want.
Edit
The body of the question has been edited to remove the previously synchronous example code, so i've updated my answer to clarify.
The original example used synchronous like code to model asynchronous behaviour, so the following applied:
array.forEach is synchronous and so is res.write, so you can simply put your callback after your call to foreach:
posts.foreach(function(v, i) {
res.write(v + ". index " + i);
});
res.end();
If you encounter asynchronous functions, and you want to make sure that before executing the code it finishes its task, we can always use the callback capability.
For example:
var ctr = 0;
posts.forEach(function(element, index, array){
asynchronous(function(data){
ctr++;
if (ctr === array.length) {
functionAfterForEach();
}
})
});
Note: functionAfterForEach is the function to be executed after foreach tasks are finished. asynchronous is the asynchronous function executed inside foreach.
Hope this will fix your problem, i usually work with this when i need to execute forEach with asynchronous tasks inside.
foo = [a,b,c,d];
waiting = foo.length;
foo.forEach(function(entry){
doAsynchronousFunction(entry,finish) //call finish after each entry
}
function finish(){
waiting--;
if (waiting==0) {
//do your Job intended to be done after forEach is completed
}
}
with
function doAsynchronousFunction(entry,callback){
//asynchronousjob with entry
callback();
}
It's odd how many incorrect answers has been given to asynchronous case!
It can be simply shown that checking index does not provide expected behavior:
// INCORRECT
var list = [4000, 2000];
list.forEach(function(l, index) {
console.log(l + ' started ...');
setTimeout(function() {
console.log(index + ': ' + l);
}, l);
});
output:
4000 started
2000 started
1: 2000
0: 4000
If we check for index === array.length - 1, callback will be called upon completion of first iteration, whilst first element is still pending!
To solve this problem without using external libraries such as async, I think your best bet is to save length of list and decrement if after each iteration. Since there's just one thread we're sure there no chance of race condition.
var list = [4000, 2000];
var counter = list.length;
list.forEach(function(l, index) {
console.log(l + ' started ...');
setTimeout(function() {
console.log(index + ': ' + l);
counter -= 1;
if ( counter === 0)
// call your callback here
}, l);
});
With ES2018 you can use async iterators:
const asyncFunction = a => fetch(a);
const itemDone = a => console.log(a);
async function example() {
const arrayOfFetchPromises = [1, 2, 3].map(asyncFunction);
for await (const item of arrayOfFetchPromises) {
itemDone(item);
}
console.log('All done');
}
My solution without Promise (this ensures that every action is ended before the next one begins):
Array.prototype.forEachAsync = function (callback, end) {
var self = this;
function task(index) {
var x = self[index];
if (index >= self.length) {
end()
}
else {
callback(self[index], index, self, function () {
task(index + 1);
});
}
}
task(0);
};
var i = 0;
var myArray = Array.apply(null, Array(10)).map(function(item) { return i++; });
console.log(JSON.stringify(myArray));
myArray.forEachAsync(function(item, index, arr, next){
setTimeout(function(){
$(".toto").append("<div>item index " + item + " done</div>");
console.log("action " + item + " done");
next();
}, 300);
}, function(){
$(".toto").append("<div>ALL ACTIONS ARE DONE</div>");
console.log("ALL ACTIONS ARE DONE");
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="toto">
</div>
There are many solutions and ways to achieve this on this thread!.
But, if you need do this with map and async/await then here it is
// Execution Starts
console.log("start")
// The Map will return promises
// the Execution will not go forward until all the promises are resolved.
await Promise.all(
[1, 2, 3].map( async (item) => {
await asyncFunction(item)
})
)
// Will only run after all the items have resolved the asynchronous function.
console.log("End")
The output will be something like this! May vary based on the asynchronous function.
start
2
3
1
end
Note: If you use await in a map, it will always return promises array.
This is the solution for Node.js which is asynchronous.
using the async npm package.
(JavaScript) Synchronizing forEach Loop with callbacks inside
My solution:
//Object forEachDone
Object.defineProperty(Array.prototype, "forEachDone", {
enumerable: false,
value: function(task, cb){
var counter = 0;
this.forEach(function(item, index, array){
task(item, index, array);
if(array.length === ++counter){
if(cb) cb();
}
});
}
});
//Array forEachDone
Object.defineProperty(Object.prototype, "forEachDone", {
enumerable: false,
value: function(task, cb){
var obj = this;
var counter = 0;
Object.keys(obj).forEach(function(key, index, array){
task(obj[key], key, obj);
if(array.length === ++counter){
if(cb) cb();
}
});
}
});
Example:
var arr = ['a', 'b', 'c'];
arr.forEachDone(function(item){
console.log(item);
}, function(){
console.log('done');
});
// out: a b c done
I try Easy Way to resolve it, share it with you :
let counter = 0;
arr.forEach(async (item, index) => {
await request.query(item, (err, recordset) => {
if (err) console.log(err);
//do Somthings
counter++;
if(counter == tableCmd.length){
sql.close();
callback();
}
});
request is Function of mssql Library in Node js. This can replace each function or Code u want.
GoodLuck
var i=0;
const waitFor = (ms) =>
{
new Promise((r) =>
{
setTimeout(function () {
console.log('timeout completed: ',ms,' : ',i);
i++;
if(i==data.length){
console.log('Done')
}
}, ms);
})
}
var data=[1000, 200, 500];
data.forEach((num) => {
waitFor(num)
})
var counter = 0;
var listArray = [0, 1, 2, 3, 4];
function callBack() {
if (listArray.length === counter) {
console.log('All Done')
}
};
listArray.forEach(function(element){
console.log(element);
counter = counter + 1;
callBack();
});
//First Example
save() {
this.holdImageData.forEach((val,i) => {
this.UploadToMinio(val.file, val.index, res => {
if (res && i+1 == this.holdImageData.length) {
this.FinallySave();
}
})
})
}
UploadToMinio(files, index, callback) {
this._fileUploadService.uploadFile(files[0], files[0].name, 'rms').subscribe(data => {
if (data) {
console.log('data >>> ', data);
callback(true);
}
})
}
FinallySave() {}
//Second Example
var sum = 0; // It can be global variable
startFunction() {
this.sumFunction(2, 4, res => {
if (res == true && sum == 6) {
this.saveFunction();
} else {
//call to another function
}
})
}
sumFunction(num1, num2, callback) {
if ((num1 + num2) == 6) {
callback(true)
}
callback(false);
}
You shouldn't need a callback for iterating through a list. Just add the end() call after the loop.
posts.forEach(function(v, i){
res.write(v + ". Index " + i);
});
res.end();
How about setInterval, to check for complete iteration count, brings guarantee. not sure if it won't overload the scope though but I use it and seems to be the one
_.forEach(actual_JSON, function (key, value) {
// run any action and push with each iteration
array.push(response.id)
});
setInterval(function(){
if(array.length > 300) {
callback()
}
}, 100);