I'm trying to write a simple promise queue function, the function will process 50 tasks with 10 concurrent promises:
const sendTasks = () => {
let tasks = 50;
const concurrentCount = 10;
let promisePool = 0;
while (tasks > 0) {
console.log(`current tasks: ${tasks}`);
while (promisePool < concurrentCount && task > 0) {
console.log("create promise");
tasks--;
promisePool++;
const promise = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("haha");
}, Math.floor(Math.random() * 3) * 1000);
});
promise.then((value) => {
console.log(value);
promisePool--;
});
console.log(`current promisePool: ${promisePool}`);
}
}
return "done";
};
But when I execute it, the promises seems never resolve, then keep stuck in the task>0 while loop. Can someone explain to me why the promises never resolve?
A while loop inside a synchronous function will never yield control flow to Promise .thens, or to anything else. You will need to restructure the code to wait for the promises to resolve without fully terminating the sendTasks function and also without blocking the engine.
One approach is to push each Promise to an array, then await a Promise.any on that array. Have the Promises remove themselves from the array when they're finished, and recursively push more Promises to the array. Then return when no more Promises exist in the array.
const sendTasks = async () => {
let tasks = 50;
const concurrentCount = 10;
let promisePool = 0;
let promises = [];
const enqueueNext = () => {
if (!tasks) return;
// creating this variable just for the sake of logging
const thisTask = tasks--;
console.log("create promise", thisTask);
const prom = new Promise((resolve) => {
setTimeout(() => {
promises = promises.filter(p => p !== prom);
console.log('resolving', thisTask);
resolve("haha");
// recursive asynchronous call; init the next promise, if there is one
enqueueNext();
}, Math.floor(Math.random() * 3) * 1000);
});
promises.push(prom);
};
for (let i = 0; i < concurrentCount; i++) {
enqueueNext();
}
while (promises.length) {
await Promise.any(promises);
}
console.log("done");
};
sendTasks();
JavaScript is single-threaded except when it explicitly isn’t (web workers, Node multiprocessing, … – not promises), so your while (tasks > 0) { is a busy loop that never returns control to whatever event loop and gives timers the chance to fire.
You need to give a function (.then) or continuation (async/await) to your promises so that you can yield back to the event loop and be told when to continue processing.
const sendTasks = async () => {
let tasks = 50;
const concurrentCount = 10;
const promisePool = new Set();
while (tasks > 0) {
console.log(`current tasks: ${tasks}`);
while (promisePool.size < concurrentCount) {
console.log("create promise");
tasks--;
const promise = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("haha");
}, Math.floor(Math.random() * 3) * 1000);
});
promise.then((value) => {
console.log(value);
promisePool.delete(promise);
});
promisePool.add(promise);
console.log(`current promisePool: ${[...promisePool]}`);
}
await Promise.race(promisePool);
}
// all tasks have been created here, but not necessarily completed
await Promise.all(promisePool);
return "done";
};
sendTasks().then(console.log);
If you were given an array of async functions and the task is to create a class that takes this array and runs the functions as fast as possible with a constraint that only 15 functions can run at the same time, what would be a way to do that?
If there wasn't a constraint for 15 functions, I believe Promise.all would be the way to go.
Using just async/await and waiting for one function to resolve to add the next one is very slow as we must have to wait for 1 function to resolve until we can add another one and we can thus have a bottleneck function.
Adding 15 functions to array and running them with Promise.all and after that resolves, adding another 15 or the rest of them, is again, not very efficient as what we want to do is to call another function as soon as one of the functions resolves.
Any ideas?
Let's create a stack that has an async popAsync method:
const createAsyncStack = () => {
const stack = [];
const waitingConsumers = [];
const push = (v) => {
if (waitingConsumers.length > 0) {
const resolver = waitingConsumers.shift();
resolver && resolver(v);
}
else {
stack.push(v);
}
};
const popAsync = () => {
if (stack.length > 0) {
const queueItem = stack.pop();
return typeof queueItem !== 'undefined'
? Promise.resolve(queueItem)
: Promise.reject(Error('unexpected'));
}
else {
return new Promise((resolve) => waitingConsumers.push(resolve));
}
};
return [push, popAsync];
};
This means that any consumer calling popAsync will be returned a Promise that only completes if / when an item is available in the stack.
We can now use this stack as a "gatekeeper" for a simple higher-order function (i.e. a function that returns a function).
Say we only want to allow maxDOP (maximum degrees-of-parallelism) concurrent invocations of an async function, we push maxDOP tokens into the stack (here, I've used empty objects as the tokens), then require that in order to proceed, it is necessary to acquire a token from this stack. When our function call is finished, we return our token to the stack (using push), where that token can then be consumed by any waiting consumers.
const withMaxDOP = (f, maxDop) => {
const [push, popAsync] = createAsyncStack();
for (let x = 0; x < maxDop; ++x) {
push({});
}
return async (...args) => {
const token = await popAsync();
try {
return await f(...args);
}
finally {
push(token);
}
};
};
The function returns a new function that can be called in exactly the same way as the function that is supplied to it (i.e. is has the same signature).
Now, let's create a function that simply calls a supplied function with the supplied arguments:
const runAsync = (asyncFn, ...args) => asyncFn(...args);
and wrap it using the higher-order withMaxDOP function, which will return a new function with an identical signature to the wrapped function:
const limitedRunAsync = withMaxDOP(runAsync, 15);
Now we can use this function to call the functions in our array:
Promise.all(asyncFns.map(f => limitedRunAsync(f)))
.then((returnValues) => console.log("all finished", returnValues));
which will ensure that there are only ever 15 "in-flight" invocations ever permitted at one time.
See this runnable snippet for a full example:
const createAsyncStack = () => {
const stack = [];
const waitingConsumers = [];
const push = (v) => {
if (waitingConsumers.length > 0) {
const resolver = waitingConsumers.shift();
resolver && resolver(v);
} else {
stack.push(v);
}
};
const popAsync = () => {
if (stack.length > 0) {
const queueItem = stack.pop();
return typeof queueItem !== 'undefined' ? Promise.resolve(queueItem) : Promise.reject(Error('unexpected'));
} else {
return new Promise((resolve) => waitingConsumers.push(resolve));
}
};
return [push, popAsync];
};
const withMaxDOP = (f, maxDop) => {
const [push, popAsync] = createAsyncStack();
for (let x = 0; x < maxDop; ++x) {
push({});
}
return async(...args) => {
const token = await popAsync();
try {
return await f(...args);
} finally {
push(token);
}
};
};
const runAsync = (asyncFn, ...args) => asyncFn(...args);
const limitedRunAsync = withMaxDOP(runAsync, 15);
// set up an array of async functions
const delay = (durationMS) => new Promise((resolve) => setTimeout(() => resolve(), durationMS));
const asyncFns = [...Array(50)].map((_, i) => () => {
console.log("starting " + i);
return delay(Math.random() * 5000).then(v => {
console.log("finished " + i);
return i;
});
});
// ...then wrap and call them all at once
Promise.all(asyncFns.map(f => limitedRunAsync(f))).then((returnValues) => console.log("all finished", returnValues));
...and see this TypeScript Playground Link for a fully type-annotated version of the same code.
Here's something I whipped up in the last 20 minutes that should do the job
I'm sure if I thought about it I could probably do it without the Promise constructor, but ... 20 minutes is 20 minutes :p
Please, if someone can rewrite this without the Promise constructor, I'd love to see it - because in the back of my mind, I'm sure there is a way
Note, this will run regardless of rejections
Results will be either
result: actualResult
or
error: rejectionReason
So you can process results/rejections
function runPromises(arrayOfFunctions, maxLength) {
return new Promise(resolve => {
const queue = arrayOfFunctions.map((fn, index) => ({fn, index}));
const results = new Array(arrayOfFunctions.length);
let finished = 0;
const doQ = () => {
++finished;
if (queue.length) {
const {fn, index} = queue.shift();
fn()
.then(result => results[index] = {result})
.catch(error => results[index] = {error})
.finally(doQ);
} else {
if (finished === arrayOfFunctions.length) {
resolve(results);
}
}
};
queue.splice(0, maxLength).forEach(({fn, index}) => fn()
.then(result => results[index] = {result})
.catch(error => results[index] = {error})
.finally(doQ)
);
});
}
//
// demo and show that maximum 15 inflight requests
//
let inFlight = 0;
let maxInFlight = 0;
const fns = Array.from({length:50}, (_, i) => {
return () => new Promise(resolve => {
++inFlight;
maxInFlight = Math.max(inFlight, maxInFlight);
setTimeout(() => {
--inFlight;
resolve(i);
}, Math.random() * 200 + 100,)
});
});
runPromises(fns, 15).then(results => console.log(maxInFlight, JSON.stringify(results)));
I tried to return the value from a Promise and assign it to a local variable inside the function, but the Promise gets fired at last.
The function (which will get fired in Vue.js mounted()):
getPriceForYesterday(){
let yesterdayUSD = [];
for (let i = 0; i < this.cryptos.length; i++) {
let cryptoName = this.cryptos[i].title;
let timestamp = new Date(this.cryptos[i].purchaseDate);
timestamp.setDate(timestamp.getDate() - 1);
timestamp = timestamp.toISOString();
let priceYesterday = getPriceForTimestamp(cryptoName, timestamp);
Promise.all([priceYesterday]).then((values) => {
console.log("values", values)
yesterdayUSD.push(values[0]);
}).catch(e => console.error(e));
}
console.log("yesterdayUSD", yesterdayUSD);
console.log("yesterdayUSD[0]", yesterdayUSD[0]);
}
Output:
yesterdayUSD []
yesterdayUSD[0] undefined
values [13308.06]
values [1278.69]
I want to use yesterdayUSD to compare it with a local storage value and then return the "result" of the comparison to the vue data.
Maybe you can rewrite this method/function to return a promise, too
getPriceForYesterday(){
return new Promise( resolve => {
let yesterdayUSD = [];
for (let i = 0; i < this.cryptos.length; i++) {
let cryptoName = this.cryptos[i].title;
let timestamp = new Date(this.cryptos[i].purchaseDate);
timestamp.setDate(timestamp.getDate() - 1);
timestamp = timestamp.toISOString();
let priceYesterday = getPriceForTimestamp(cryptoName, timestamp);
Promise.all([priceYesterday])
.then((values) => {
yesterdayUSD.push(values[0]);
})
.then( () => {
resolve(yesterdayUSD)
})
}
})
}
and then use it in the mount() method
mount() {
...
getPriceForYesterday().then( yesterdayUSDArray => {
//do something with this yesterdayUSDArray
})
}
The reason that you're not seeing anything when trying to write out yesterdayUSD is because the promises are executing asynchronously and they're not complete before reaching the lines where you use console.log();
I'm going to assume that you meant to use Promise.All() to wait for all of the promises to complete because that's typically what it is used for. I rewrote your example to populate an array of promises and then wait after they all resolve before writing out the values in yesterdayUSD. That should at least allow you to print out the values after all of the promises complete.
getPriceForYesterday(){
let yesterdayUSD = [];
let promises = [];
for (let i = 0; i < this.cryptos.length; i++) {
let cryptoName = this.cryptos[i].title;
let timestamp = new Date(this.cryptos[i].purchaseDate);
timestamp.setDate(timestamp.getDate() - 1);
timestamp = timestamp.toISOString();
let priceYesterday = getPriceForTimestamp(cryptoName, timestamp)
.then((values) => {
console.log("values", values)
yesterdayUSD.push(values[0]);
})
.catch(e => console.error(e));
promises.push(priceYesterday);
}
Promise.all(promises).finally(() => {
console.log("yesterdayUSD", yesterdayUSD);
console.log("yesterdayUSD[0]", yesterdayUSD[0]);
});
}
I'm trying to create a form that allows you to create multiple resources in sequential order.
Example below
Floor 1
Floor 2
Floor 3
...
Floor 9
The problem with the code is that the order is not guarantee.
My code below
let startAt = this.addAreasForm.controls['startAt'].value
const name = this.addAreasForm.controls['name'].value
const newArea = {name: name}
for (let i = 1; i < (amount + 1); i++) {
newArea.name = name + ' ' + startAt
startAt++
this.areasService.createArea(newArea, parentId)
.subscribe(
area => this.added.emit(area)
)
}
Can come back like
Floor 2
Floor 3
Floor 1
Floor 5
Floor 4
How do you handle async api calls to guarantee sequential order?
You can use async / await for that purpose with the Promise resolve:
for (let i = 1; i < (amount + 1); i++) {
await new Promise(resolve => {
newArea.name = name + ' ' + startAt
startAt++
this.areasService.createArea(newArea, parentId)
.subscribe(
area => {
this.added.emit(area);
resolve();
});
});
}
Remember to put async before your function. See this demo on StackBlitz.
You can try something like this, I don't exactly all your code from your services, but the main idea is this: In order to execute async code in order, you can build an array of promises and then to use Promise.all to take each result in the same order from the creation:
Promise.all
let startAt = this.addAreasForm.controls['startAt'].value;
const name = this.addAreasForm.controls['name'].value;
const newArea = {name: name};
Keep your services into variables I don't know from where your context comes.
const areasService = this.areasService,
added = this.added;
Make a function that create a promise for your subscribe:
function createAreaPromise(newArea, parentId) {
return new Promise((resolve, reject) => {
areasService.createArea(newArea, parentId)
.subscribe(area => resolve(area));
});
}
Than another function to build multiple an array of promises:
function buildPromises() {
let promises = [];
for (let i = 1; i < (amount + 1); i++) {
newArea.name = name + ' ' + startAt
startAt++
promises.push(createAreaPromise(newArea, parentId));
}
return promises;
}
Then solve them with Promise.all, to obtain the same order from creation
let promises = buildPromises();
Promise.all(promises)
.then(results => {
results.forEach(result => added.emit(result));
});
Here a live example:
function random() {
return Math.floor(Math.random() * 5);
}
function makePromise(index) {
return new Promise((resolve) => {
setTimeout(() => {
resolve(index);
}, random() * 1000);
});
}
function buildPromises() {
let promises = [];
for(let i = 0; i < 5; i++) {
promises.push(makePromise(i));
}
return promises;
}
let promises = buildPromises();
Promise.all(promises)
.then(results => {
results.forEach(result => {
console.log(result);
});
});
Sometimes I need to wait for a .forEach() method to finish, mostly on 'loader' functions. This is the way I do that:
$q.when(array.forEach(function(item){
//iterate on something
})).then(function(){
//continue with processing
});
I can't help but feel that this isn't the best way to wait for a .forEach() to finish. What is the best way to do this?
If there is no asynchronous code inside the forEach, forEach is not asynchronous, for example in this code:
array.forEach(function(item){
//iterate on something
});
alert("Foreach DONE !");
you will see the alert after forEach finished.
Otherwise (You have something asynchronous inside), you can wrap the forEach loop in a Promise:
var bar = new Promise((resolve, reject) => {
foo.forEach((value, index, array) => {
console.log(value);
if (index === array.length -1) resolve();
});
});
bar.then(() => {
console.log('All done!');
});
Credit: #rolando-benjamin-vaz-ferreira
The quickest way to make this work using ES6 would be just to use a for..of loop.
const myAsyncLoopFunction = async (array) => {
const allAsyncResults = []
for (const item of array) {
const asyncResult = await asyncFunction(item)
allAsyncResults.push(asyncResult)
}
return allAsyncResults
}
Or you could loop over all these async requests in parallel using Promise.all() like this:
const myAsyncLoopFunction = async (array) => {
const promises = array.map(asyncFunction)
await Promise.all(promises)
console.log(`All async tasks complete!`)
}
var foo = [1,2,3,4,5,6,7,8,9,10];
If you're actually doing async stuff inside the loop, you can wrap it in a promise ...
var bar = new Promise((resolve, reject) => {
foo.forEach((value, index, array) => {
console.log(value);
if (index === array.length -1) resolve();
});
});
bar.then(() => {
console.log('All done!');
});
If you have an async task inside a loop and you want to wait. you can use for await
for await (const i of images) {
let img = await uploadDoc(i);
};
let x = 10; //this executes after
Use for of instead of forEach. Like this:
for (const item of array) {
//do something
}
console.log("finished");
"finished" will be logged after finishing the loop.
forEach() doesn't return anything, so a better practice would be map() + Promise.all()
var arr = [1, 2, 3, 4, 5, 6]
var doublify = (ele) => {
return new Promise((res, rej) => {
setTimeout(() => {
res(ele * 2)
}, Math.random() ); // Math.random returns a random number from 0~1
})
}
var promises = arr.map(async (ele) => {
// do some operation on ele
// ex: var result = await some_async_function_that_return_a_promise(ele)
// In the below I use doublify() to be such an async function
var result = await doublify(ele)
return new Promise((res, rej) => {res(result)})
})
Promise.all(promises)
.then((results) => {
// do what you want on the results
console.log(results)
})
A universal solution for making sure that all forEach() elements finished execution.
const testArray = [1,2,3,4]
let count = 0
await new Promise( (resolve) => {
testArray.forEach( (num) => {
try {
//some real logic
num = num * 2
} catch (e) {
// error handling
console.log(e)
} fanally {
// most important is here
count += 1
if (count == testArray.length) {
resolve()
}
}
})
})
The idea is same with the answer using index to count. But in real case, if error happened, the index way cannot count correctly. So the solution is more robust.
Thx
const array = [1, 2, 3];
const results = [];
let done = 0;
const asyncFunction = (item, callback) =>
setTimeout(() => callback(item * 10), 100 - item * 10);
new Promise((resolve, reject) => {
array.forEach((item) => {
asyncFunction(item, (result) => {
results.push(result);
done++;
if (done === array.length) resolve();
});
});
}).then(() => {
console.log(results); // [30, 20, 10]
});
// or
// promise = new Promise(...);
// ...
// promise.then(...);
The order of results in the "results" array can be different than the order of items in the original array, depending on the time when the asyncFunction() finishes for each of the items.
Alter and check a counter at the end of every possible unique branch of code, including callbacks. Example:
const fs = require('fs');
/**
* #description Delete files older than 1 day
* #param {String} directory - The directory to purge
* #return {Promise}
*/
async function purgeFiles(directory) {
const maxAge = 24*3600000;
const now = Date.now();
const cutoff = now-maxAge;
let filesPurged = 0;
let filesProcessed = 0;
let purgedSize = 0;
await new Promise( (resolve, reject) => {
fs.readdir(directory, (err, files) => {
if (err) {
return reject(err);
}
if (!files.length) {
return resolve();
}
files.forEach( file => {
const path = `${directory}/${file}`;
fs.stat(path, (err, stats)=> {
if (err) {
console.log(err);
if (++filesProcessed === files.length) resolve();
}
else if (stats.isFile() && stats.birthtimeMs < cutoff) {
const ageSeconds = parseInt((now-stats.birthtimeMs)/1000);
fs.unlink(path, error => {
if (error) {
console.log(`Deleting file failed: ${path} ${error}`);
}
else {
++filesPurged;
purgedSize += stats.size;
console.log(`Deleted file with age ${ageSeconds} seconds: ${path}`);
}
if (++filesProcessed === files.length) resolve();
});
}
else if (++filesProcessed === files.length) resolve();
});
});
});
});
console.log(JSON.stringify({
directory,
filesProcessed,
filesPurged,
purgedSize,
}));
}
// !!DANGER!! Change this line! (intentional syntax error in ,')
const directory = ,'/tmp'; // !!DANGER!! Changeme
purgeFiles(directory).catch(error=>console.log(error));
I'm not sure of the efficiency of this version compared to others, but I used this recently when I had an asynchronous function inside of my forEach(). It does not use promises, mapping, or for-of loops:
// n'th triangular number recursion (aka factorial addition)
function triangularNumber(n) {
if (n <= 1) {
return n
} else {
return n + triangularNumber(n-1)
}
}
// Example function that waits for each forEach() iteraction to complete
function testFunction() {
// Example array with values 0 to USER_INPUT
var USER_INPUT = 100;
var EXAMPLE_ARRAY = Array.apply(null, {length: USER_INPUT}).map(Number.call, Number) // [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, n_final... ] where n_final = USER_INPUT-1
// Actual function used with whatever actual array you have
var arrayLength = EXAMPLE_ARRAY.length
var countMax = triangularNumber(arrayLength);
var counter = 0;
EXAMPLE_ARRAY.forEach(function(entry, index) {
console.log(index+1); // show index for example (which can sometimes return asynchrounous results)
counter += 1;
if (triangularNumber(counter) == countMax) {
// function called after forEach() is complete here
completionFunction();
} else {
// example just to print counting values when max not reached
// else would typically be excluded
console.log("Counter index: "+counter);
console.log("Count value: "+triangularNumber(counter));
console.log("Count max: "+countMax);
}
});
}
testFunction();
function completionFunction() {
console.log("COUNT MAX REACHED");
}
I had to deal with the same problem (forEach using multiple promises inside) and none of the solutions presented at the current date were helpful for me. So I implemented a check array, were each promise updates its complete status. We have a general promise that wraps the process. We only resolve the general promise when each promise completed. Snippet code:
function WaitForEachToResolve(fields){
var checked_fields = new Array(fields.length).fill(0);
const reducer = (accumulator, currentValue) => accumulator + currentValue;
return new Promise((resolve, reject) => {
Object.keys(fields).forEach((key, index, array) => {
SomeAsyncFunc(key)
.then((result) => {
// some result post process
checked_fields[index] = 1;
if (checked_fields.reduce(reducer) === checked_fields.length)
resolve();
})
.catch((err) => {
reject(err);
});
}
)}
}
I like to use async-await instead of .then() syntax so for asynchronous processing of data, modified the answer of #Ronaldo this way -
let finalData = [];
var bar = new Promise(resolve => {
foo.forEach((value, index) => {
const dataToGet = await abcService.getXyzData(value);
finalData[index].someKey = dataToGet.thatOtherKey;
// any other processing here
if (finalData[dataToGet.length - 1].someKey) resolve();
});
});
await Promise.all([bar]);
console.log(`finalData: ${finalData}`);
NOTE: I've modified the if condition where it resolves the promise to meet my conditions. You can do the same in your case.
You can use this, because we are using async/await inside the forEach loop. You can use your own logic inside the loop.
let bar = new Promise((resolve, reject) => {
snapshot.forEach(async (doc) => {
"""Write your own custom logic and can use async/await
"""
const result = await something()
resolve(result);
});
});
let test = []
test.push(bar)
let concepts = await Promise.all(test);
console.log(concepts);
For simple compare code i like use for statement.
doit();
function doit() {
for (var i = 0; i < $('span').length; i++) {
console.log(i,$('span').eq(i).text() );
if ( $('span').eq(i).text() == "Share a link to this question" ) { // span number 59
return;
}
}
alert('never execute');
}
If there are async (observable) method calls inside the for loop, you could use the following method:
await players.reduce(async (a, player) => {
// Wait for the previous item to finish processing
await a;
// Process this item
await givePrizeToPlayer(player);
}, Promise.resolve());
Check here: https://gist.github.com/joeytwiddle/37d2085425c049629b80956d3c618971
I've been using this and it works best .forEach()
//count
var expecting = myArray.length;
myArray.forEach(function(item){
//do logic here
var item = item
//when iteration done
if (--expecting === 0) {
console.log('all done!');
}
})