We Keep Coding

How to run multiple async functions as fast as possible (JS)?

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)));

Executing list of async functions

I have an exercise to make a function executeFunctions which takes as arguments a list of async functions and an argument, e.g. number.
The functions have to happen one after another, so if fun1 ends, fun2 needs to start with the value which was returned from fun1.
The problem is that I can't use async and await. I wanted to do it using reduce, but I guess that it wants to execute const res1 and go further before it returns a value (because of setTimeout).
Is there any way to do it without async and await?
const fun1 = function(value) {
return setTimeout(() => value*2, 3000)
}
const fun2 = function(value) {
return setTimeout(() => value*4, 3000)
}
const cb2 = (value) => {
return value*10
}
const executeFunctions = (funTab, cb) => (n) => {
const res1= funTab[0](n)
console.log(res1)
const resReduce = funTab.reduce((prev,curr) => {
const res2 = curr(prev)
return prev+res2
}, res1)
return cb(resReduce)
};
executeFunctions([fun1,fun2], cb2)(2)

We can use Promise-chaining:
const fun1 = function(value) {
return Promise.resolve(value * 2);
}
const fun2 = function(value) {
return Promise.resolve(value * 2);
}
const fun3 = function(value) {
return Promise.resolve(value * 2);
}
const executeFunctions = (funcList) => (n) => {
let chain = Promise.resolve(n); // initial value
for (let i = 0; i < funcList.length; i++) {
chain = chain.then(funcList[i]); // keep chaining
}
return chain; // last promise
};
const main = () => {
// we need to wait for the last promise in order to print the result
executeFunctions([fun1, fun2, fun3])(2).then(x => console.log('solution is:', x));
}
main() // prints: "solution is: 16"
or, we can also use a modified version of the suggested reduce solution, by changing the implementation of executeFunctions as follows (the rest of the code should remain as in the previous snippet):
const executeFunctions = (funcList) => (n) => {
const init = Promise.resolve(n);
const res = funcList.reduce((p, c) => {
return p.then(c)
}, init);
return res;
};

Generating dataset with promises

I'm currently trying to generate an array of pairs with asynchronous calls in javascript however I just cant seem to get it in the right order.
generateDataPoints: function (iterable, source, arg) {
let pairs = []
let prevTime = 0
for (let index in iterable) {
let event = iterable[index]
getTime(event.valueOf()).then(function (time) {
query(source[arg], event.valueOf()).then(function(val) {
if (time !== prevTime) {
prevTime = time
pairs.push([time, val])
console.log(pairs) // This works as expected but only happens after the program returns
} else {
Promise.resolve()
}
})
})
}
return Promise.resolve(pairs)
}
The problem is that the "pairs.push.." command happens after I return. I am unable to figure out what exactly is going wrong and how I can synchronise this process. Any help is appreciated.

You are mixing synchronous code (like the for loop) with async code. Try something like this:
function generateDataPoints(iterable, source, arg) {
let prevTime = 0
[...iterable].reduce((promiseChain, event) => {
let pairs;
return promiseChain
.then(p => {
pairs = p;
return Promise.all([
getTime(event.valueOf()),
query(source[arg], event.valueOf())
])
})
.then(([time, val]) => {
if (time !== prevTime) {
prevTime = time
return pairs.concat([[time, val]]);
} else {
return pairs;
}
});
})
}, Promise.resolve([]));
}
This reduces iterable into a sequential Promise chain, which will eventually contain all the pairs.
If your environment allows for it, you could also convert your function with few changes to an async function:
generateDataPoints: async function (iterable, source, arg) {
let pairs = []
let prevTime = 0
for (let index in iterable) {
let event = iterable[index]
const [time, val] = await Promise.all([
getTime(event.valueOf()),
query(source[arg], event.valueOf())
]);
if (time !== prevTime) {
prevTime = time
pairs.push([time, val])
}
}
return pairs;
}
Edit:
On closer look, it doesn't look like you need to sequentialize these Promises. You may be able to run them in parallel instead:
function generateDataPoints(iterable, source, arg) {
Promise.all(
[...iterable].map(event => Promise.all([
getTime(event.valueOf()),
query(source[arg], event.valueOf())
])
).then(pairs => {
let prevTime = 0
return pairs.filter(([time, val]) => {
if (time !== prevTime) {
prevTime = time;
return true;
} else {
return false;
}
});
});
}

ES6 Promise replacement of async.eachLimit / async.mapLimit

In async, if I need to apply a asynchronousfunction to 1000 items, I can do that with:
async.mapLimit(items, 10, (item, callback) => {
foo(item, callback);
});
so that only 10 item are processed at the same time, limiting overhead and allowing control.
With ES6 promise, while I can easily do:
Promise.all(items.map((item) => {
return bar(item);
}));
that would process all 1000 items at the same time which may cause a lot of problems.
I know Bluebird have ways to handle that, but I am searching a ES6 solution.

If you don't care about the results, then it's quick to whip one up:
Promise.eachLimit = async (funcs, limit) => {
let rest = funcs.slice(limit);
await Promise.all(funcs.slice(0, limit).map(async func => {
await func();
while (rest.length) {
await rest.shift()();
}
}));
};
// Demo:
var wait = ms => new Promise(resolve => setTimeout(resolve, ms));
async function foo(s) {
await wait(Math.random() * 2000);
console.log(s);
}
(async () => {
let funcs = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".split("").map(s => () => foo(s));
await Promise.eachLimit(funcs, 5);
})();
A key performance property is running the next available function as soon as any function finishes.
Preserving results
Preserving the results in order makes it a little less elegant perhaps, but not too bad:
Promise.mapLimit = async (funcs, limit) => {
let results = [];
await Promise.all(funcs.slice(0, limit).map(async (func, i) => {
results[i] = await func();
while ((i = limit++) < funcs.length) {
results[i] = await funcs[i]();
}
}));
return results;
};
// Demo:
var wait = ms => new Promise(resolve => setTimeout(resolve, ms));
async function foo(s) {
await wait(Math.random() * 2000);
console.log(s);
return s.toLowerCase();
}
(async () => {
let funcs = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".split("").map(s => () => foo(s));
console.log((await Promise.mapLimit(funcs, 5)).join(""));
})();

There's nothing built in, but you can of course group them yourself into promise chains, and use a Promise.all on the resulting array of chains:
const items = /* ...1000 items... */;
const concurrencyLimit = 10;
const promise = Promise.all(items.reduce((promises, item, index) => {
// What chain do we add it to?
const chainNum = index % concurrencyLimit;
let chain = promises[chainNum];
if (!chain) {
// New chain
chain = promises[chainNum] = Promise.resolve();
}
// Add it
promises[chainNum] = chain.then(_ => foo(item));
return promises;
}, []));
Here's an example, showing how many concurrent promises there are any given time (and also showing when each "chain" is complete, and only doing 200 instead of 1,000):
const items = buildItems();
const concurrencyLimit = 10;
const promise = Promise.all(items.reduce((promises, item, index) => {
const chainNum = index % concurrencyLimit;
let chain = promises[chainNum];
if (!chain) {
chain = promises[chainNum] = Promise.resolve();
}
promises[chainNum] = chain.then(_ => foo(item));
return promises;
}, []).map(chain => chain.then(_ => console.log("Chain done"))));
promise.then(_ => console.log("All done"));
function buildItems() {
const items = [];
for (let n = 0; n < 200; ++n) {
items[n] = n;
}
return items;
}
var outstanding = 0;
function foo(item) {
++outstanding;
console.log("Starting " + item + " (" + outstanding + ")");
return new Promise(resolve => {
setTimeout(_ => {
--outstanding;
console.log("Resolving " + item + " (" + outstanding + ")");
resolve(item);
}, Math.random() * 500);
});
}
.as-console-wrapper {
max-height: 100% !important;
}
I should note that if you want to track the result of each of those, you'd have to modify the above; it doesn't try to track the results (!). :-)

Using Array.prototype.splice
while (funcs.length) {
await Promise.all( funcs.splice(0, 100).map(f => f()) )
}

This is the closest one to async.eachLimit
Promise.eachLimit = async (coll, limit, asyncFunc) => {
let ret = [];
const splitArr = coll.reduce((acc,item,i)=> (i%limit) ? acc :[...acc,coll.slice(i,i+limit)],[])
for(let i =0; i< splitArr.length;i++){
ret[i]=await Promise.all(splitArr[i].map(ele=>asyncFunc(ele)));
}
return ret;
}
const wait = ms => new Promise(resolve => setTimeout(resolve, ms));
async function foo(s) {
await wait(Math.random() * 2000);
console.log(s);
return s.toLowerCase();
}
(async () => {
let arr = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".split("");
console.log((await Promise.eachLimit(arr, 5, foo)));
})();

Best way to wait for .forEach() to complete

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!');
}
})