Develop Reference

Unordered resolution of a list of promises

How to convert a dynamic Set<Promise<T>> into AsyncIterable<T> (unordered)?
The resulting iterable must produce values as they get resolved, and it must end just as the source runs empty.
I have a dynamic cache of promises to be resolved, and values reported, disregarding the order.
NOTE: The source is dynamic, which means it can receive new Promise<T> elements while we progress through the resulting iterator.
UPDATE
After going through all the suggestions, I was able to implement my operator. And here're the official docs.
I'm adding a bounty to reward anyone who can improve it further, though at this point a PR is preferable (it is for a public library), or at least something that fits the same protocol.

Judging from your library implementation, you actually want to transform an AsyncIterable<Promise<T>> into an AsyncIterator<T> by racing up to N of the produced promises concurrently. I would implement that as follows:
async function* limitConcurrent<T>(iterable: AsyncIterable<Promise<T>>, n: number): AsyncIterator<T> {
const pool = new Set();
for await (const p of iterable) {
const promise = Promise.resolve(p).finally(() => {
pool.delete(promise); // FIXME see below
});
promise.catch(() => { /* ignore */ }); // mark rejections as handled
pool.add(promise);
if (pool.size >= n) {
yield /* await */ Promise.race(pool);
}
}
while (pool.size) {
yield /* await */ Promise.race(pool);
}
}
Notice that if one of the promises in the pool rejects, the returned iterator will end with the error and the results of the other promises that are currently in the pool will be ignored.
However, above implementation presumes that the iterable is relatively fast, as it will need to produce n promises before the pool is raced for the first time. If it yields the promises slower than the promises take to resolve, the results are held up unnecessarily.
And worse, the above implementation may loose values. If the returned iterator is not consumed fast enough, or the iterable is not yielding fast enough, multiple promise handlers may delete their respective promise from the pool during one iteration of the loop, and the Promise.race will consider only one of them.
So this would work for a synchronous iterable, but if you actually have an asynchronous iterable, you would need a different solution. Essentially you got a consumer and a producer that are more or less independent, and what you need is some queue between them.
Yet with a single queue it still wouldn't handle backpressure, the producer just runs as fast as it can (given the iteration of promises and the concurrency limit) while filling the queue. What you really need then is a channel that allows synchronisation in both directions, e.g. using two queues:
class AsyncQueue<T> {
resolvers: null | ((res: IteratorResult<T> | Promise<never>) => void)[];
promises: Promise<IteratorResult<T>>[];
constructor() {
// invariant: at least one of the arrays is empty.
// when `resolvers` is `null`, the queue has ended.
this.resolvers = [];
this.promises = [];
}
putNext(result: IteratorResult<T> | Promise<never>): void {
if (!this.resolvers) throw new Error('Queue already ended');
if (this.resolvers.length) this.resolvers.shift()(result);
else this.promises.push(Promise.resolve(result));
}
put(value: T): void {
this.putNext({done: false, value});
}
end(): void {
for (const res of this.resolvers) res({done: true, value: undefined});
this.resolvers = null;
}
next(): Promise<IteratorResult<T>> {
if (this.promises.length) return this.promises.shift();
else if (this.resolvers) return new Promise(resolve => { this.resolvers.push(resolve); });
else return Promise.resolve({done: true, value: undefined});
}
[Symbol.asyncIterator](): AsyncIterator<T> {
// Todo: Use AsyncIterator.from()
return this;
}
}
function limitConcurrent<T>(iterable: AsyncIterable<Promise<T>>, n: number): AsyncIterator<T> {
const produced = new AsyncQueue<T>();
const consumed = new AsyncQueue<void>();
(async () => {
try {
let count = 0;
for await (const p of iterable) {
const promise = Promise.resolve(p);
promise.then(value => {
produced.put(value);
}, _err => {
produced.putNext(promise); // with rejection already marked as handled
});
if (++count >= n) {
await consumed.next(); // happens after any produced.put[Next]()
count--;
}
}
while (count) {
await consumed.next(); // happens after any produced.put[Next]()
count--;
}
} catch(e) {
// ignore `iterable` errors?
} finally {
produced.end();
}
})();
return (async function*() {
for await (const value of produced) {
yield value;
consumed.put();
}
}());
}

function createCache() {
const resolve = [];
const sortedPromises = [];
const noop = () => void 0;
return {
get length() {
return sortedPromises.length
},
add(promiseOrValue) {
const q = new Promise(r => {
resolve.push(r);
const _ = () => {
resolve.shift()(promiseOrValue);
}
Promise.resolve(promiseOrValue).then(_, _);
});
q.catch(noop); // prevent q from throwing when rejected.
sortedPromises.push(q);
},
next() {
return sortedPromises.length ?
{ value: sortedPromises.shift() } :
{ done: true };
},
[Symbol.iterator]() {
return this;
}
}
}
(async() => {
const sleep = (ms, value) => new Promise(resolve => setTimeout(resolve, ms, value));
const cache = createCache();
const start = Date.now();
function addItem() {
const t = Math.floor(Math.random() ** 2 * 8000), // when to resolve
val = t + Date.now() - start; // ensure that the resolved value is in ASC order.
console.log("add", val);
cache.add(sleep(t, val));
}
// add a few initial items
Array(5).fill().forEach(addItem);
// check error handling with a rejecting promise.
cache.add(sleep(1500).then(() => Promise.reject("a rejected Promise")));
while (cache.length) {
try {
for await (let v of cache) {
console.log("yield", v);
if (v < 15000 && Math.random() < .5) {
addItem();
}
// slow down iteration, like if you'd await some API-call.
// promises now resolve faster than we pull them.
await sleep(1000);
}
} catch (err) {
console.log("error:", err);
}
}
console.log("done");
})()
.as-console-wrapper{top:0;max-height:100%!important}
works with both for(const promise of cache){ ... } and for await(const value of cache){ ... }
Error-handling:
for(const promise of cache){
try {
const value = await promise;
}catch(error){ ... }
}
// or
while(cache.length){
try {
for await(const value of cache){
...
}
}catch(error){ ... }
}
rejected Promises (in the cache) don't throw until you .then() or await them.
Also handles backpressure (when your loop is iterating slower than the promises resolve)
for await(const value of cache){
await somethingSlow(value);
}

Detect the end of multiple async loops [duplicate]

As far as I understand, in ES7/ES2016 putting multiple await's in code will work similar to chaining .then() with promises, meaning that they will execute one after the other rather than in parallel. So, for example, we have this code:
await someCall();
await anotherCall();
Do I understand it correctly that anotherCall() will be called only when someCall() is completed? What is the most elegant way of calling them in parallel?
I want to use it in Node, so maybe there's a solution with async library?
EDIT: I'm not satisfied with the solution provided in this question: Slowdown due to non-parallel awaiting of promises in async generators, because it uses generators and I'm asking about a more general use case.

You can await on Promise.all():
await Promise.all([someCall(), anotherCall()]);
To store the results:
let [someResult, anotherResult] = await Promise.all([someCall(), anotherCall()]);
Note that Promise.all fails fast, which means that as soon as one of the promises supplied to it rejects, then the entire thing rejects.
const happy = (v, ms) => new Promise((resolve) => setTimeout(() => resolve(v), ms))
const sad = (v, ms) => new Promise((_, reject) => setTimeout(() => reject(v), ms))
Promise.all([happy('happy', 100), sad('sad', 50)])
.then(console.log).catch(console.log) // 'sad'
If, instead, you want to wait for all the promises to either fulfill or reject, then you can use Promise.allSettled. Note that Internet Explorer does not natively support this method.
const happy = (v, ms) => new Promise((resolve) => setTimeout(() => resolve(v), ms))
const sad = (v, ms) => new Promise((_, reject) => setTimeout(() => reject(v), ms))
Promise.allSettled([happy('happy', 100), sad('sad', 50)])
.then(console.log) // [{ "status":"fulfilled", "value":"happy" }, { "status":"rejected", "reason":"sad" }]
Note: If you use Promise.all actions that managed to finish before rejection happen are not rolled back, so you may need to take care of such situation. For example
if you have 5 actions, 4 quick, 1 slow and slow rejects. Those 4
actions may be already executed so you may need to roll back. In such situation consider using Promise.allSettled while it will provide exact detail which action failed and which not.

TL;DR
Use Promise.all for the parallel function calls, the answer behaviors not correctly when the error occurs.
First, execute all the asynchronous calls at once and obtain all the Promise objects. Second, use await on the Promise objects. This way, while you wait for the first Promise to resolve the other asynchronous calls are still progressing. Overall, you will only wait for as long as the slowest asynchronous call. For example:
// Begin first call and store promise without waiting
const someResult = someCall();
// Begin second call and store promise without waiting
const anotherResult = anotherCall();
// Now we await for both results, whose async processes have already been started
const finalResult = [await someResult, await anotherResult];
// At this point all calls have been resolved
// Now when accessing someResult| anotherResult,
// you will have a value instead of a promise
JSbin example: http://jsbin.com/xerifanima/edit?js,console
Caveat: It doesn't matter if the await calls are on the same line or on different lines, so long as the first await call happens after all of the asynchronous calls. See JohnnyHK's comment.
Update: this answer has a different timing in error handling according to the #bergi's answer, it does NOT throw out the error as the error occurs but after all the promises are executed.
I compare the result with #jonny's tip: [result1, result2] = Promise.all([async1(), async2()]), check the following code snippet
const correctAsync500ms = () => {
return new Promise(resolve => {
setTimeout(resolve, 500, 'correct500msResult');
});
};
const correctAsync100ms = () => {
return new Promise(resolve => {
setTimeout(resolve, 100, 'correct100msResult');
});
};
const rejectAsync100ms = () => {
return new Promise((resolve, reject) => {
setTimeout(reject, 100, 'reject100msError');
});
};
const asyncInArray = async (fun1, fun2) => {
const label = 'test async functions in array';
try {
console.time(label);
const p1 = fun1();
const p2 = fun2();
const result = [await p1, await p2];
console.timeEnd(label);
} catch (e) {
console.error('error is', e);
console.timeEnd(label);
}
};
const asyncInPromiseAll = async (fun1, fun2) => {
const label = 'test async functions with Promise.all';
try {
console.time(label);
let [value1, value2] = await Promise.all([fun1(), fun2()]);
console.timeEnd(label);
} catch (e) {
console.error('error is', e);
console.timeEnd(label);
}
};
(async () => {
console.group('async functions without error');
console.log('async functions without error: start')
await asyncInArray(correctAsync500ms, correctAsync100ms);
await asyncInPromiseAll(correctAsync500ms, correctAsync100ms);
console.groupEnd();
console.group('async functions with error');
console.log('async functions with error: start')
await asyncInArray(correctAsync500ms, rejectAsync100ms);
await asyncInPromiseAll(correctAsync500ms, rejectAsync100ms);
console.groupEnd();
})();

Update:
The original answer makes it difficult (and in some cases impossible) to correctly handle promise rejections. The correct solution is to use Promise.all:
const [someResult, anotherResult] = await Promise.all([someCall(), anotherCall()]);
Original answer:
Just make sure you call both functions before you await either one:
// Call both functions
const somePromise = someCall();
const anotherPromise = anotherCall();
// Await both promises
const someResult = await somePromise;
const anotherResult = await anotherPromise;

There is another way without Promise.all() to do it in parallel:
First, we have 2 functions to print numbers:
function printNumber1() {
return new Promise((resolve,reject) => {
setTimeout(() => {
console.log("Number1 is done");
resolve(10);
},1000);
});
}
function printNumber2() {
return new Promise((resolve,reject) => {
setTimeout(() => {
console.log("Number2 is done");
resolve(20);
},500);
});
}
This is sequential:
async function oneByOne() {
const number1 = await printNumber1();
const number2 = await printNumber2();
}
//Output: Number1 is done, Number2 is done
This is parallel:
async function inParallel() {
const promise1 = printNumber1();
const promise2 = printNumber2();
const number1 = await promise1;
const number2 = await promise2;
}
//Output: Number2 is done, Number1 is done

I've created a gist testing some different ways of resolving promises, with results. It may be helpful to see the options that work.
Edit: Gist content as per Jin Lee's comment
// Simple gist to test parallel promise resolution when using async / await
function promiseWait(time) {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(true);
}, time);
});
}
async function test() {
return [
await promiseWait(1000),
await promiseWait(5000),
await promiseWait(9000),
await promiseWait(3000),
]
}
async function test2() {
return {
'aa': await promiseWait(1000),
'bb': await promiseWait(5000),
'cc': await promiseWait(9000),
'dd': await promiseWait(3000),
}
}
async function test3() {
return await {
'aa': promiseWait(1000),
'bb': promiseWait(5000),
'cc': promiseWait(9000),
'dd': promiseWait(3000),
}
}
async function test4() {
const p1 = promiseWait(1000);
const p2 = promiseWait(5000);
const p3 = promiseWait(9000);
const p4 = promiseWait(3000);
return {
'aa': await p1,
'bb': await p2,
'cc': await p3,
'dd': await p4,
};
}
async function test5() {
return await Promise.all([
await promiseWait(1000),
await promiseWait(5000),
await promiseWait(9000),
await promiseWait(3000),
]);
}
async function test6() {
return await Promise.all([
promiseWait(1000),
promiseWait(5000),
promiseWait(9000),
promiseWait(3000),
]);
}
async function test7() {
const p1 = promiseWait(1000);
const p2 = promiseWait(5000);
const p3 = promiseWait(9000);
return {
'aa': await p1,
'bb': await p2,
'cc': await p3,
'dd': await promiseWait(3000),
};
}
let start = Date.now();
test().then((res) => {
console.log('Test Done, elapsed', (Date.now() - start) / 1000, res);
start = Date.now();
test2().then((res) => {
console.log('Test2 Done, elapsed', (Date.now() - start) / 1000, res);
start = Date.now();
test3().then((res) => {
console.log('Test3 Done, elapsed', (Date.now() - start) / 1000, res);
start = Date.now();
test4().then((res) => {
console.log('Test4 Done, elapsed', (Date.now() - start) / 1000, res);
start = Date.now();
test5().then((res) => {
console.log('Test5 Done, elapsed', (Date.now() - start) / 1000, res);
start = Date.now();
test6().then((res) => {
console.log('Test6 Done, elapsed', (Date.now() - start) / 1000, res);
});
start = Date.now();
test7().then((res) => {
console.log('Test7 Done, elapsed', (Date.now() - start) / 1000, res);
});
});
});
});
});
});
/*
Test Done, elapsed 18.006 [ true, true, true, true ]
Test2 Done, elapsed 18.009 { aa: true, bb: true, cc: true, dd: true }
Test3 Done, elapsed 0 { aa: Promise { <pending> },
bb: Promise { <pending> },
cc: Promise { <pending> },
dd: Promise { <pending> } }
Test4 Done, elapsed 9 { aa: true, bb: true, cc: true, dd: true }
Test5 Done, elapsed 18.008 [ true, true, true, true ]
Test6 Done, elapsed 9.003 [ true, true, true, true ]
Test7 Done, elapsed 12.007 { aa: true, bb: true, cc: true, dd: true }
*/

In my case, I have several tasks I want to execute in parallel, but I need to do something different with the result of those tasks.
function wait(ms, data) {
console.log('Starting task:', data, ms);
return new Promise(resolve => setTimeout(resolve, ms, data));
}
var tasks = [
async () => {
var result = await wait(1000, 'moose');
// do something with result
console.log(result);
},
async () => {
var result = await wait(500, 'taco');
// do something with result
console.log(result);
},
async () => {
var result = await wait(5000, 'burp');
// do something with result
console.log(result);
}
]
await Promise.all(tasks.map(p => p()));
console.log('done');
And the output:
Starting task: moose 1000
Starting task: taco 500
Starting task: burp 5000
taco
moose
burp
done
(async function(){
function wait(ms, data) {
console.log('Starting task:', data, ms);
return new Promise(resolve => setTimeout(resolve, ms, data));
}
var tasks = [
async () => {
var result = await wait(1000, 'moose');
// do something with result
console.log(result);
},
async () => {
var result = await wait(500, 'taco');
// do something with result
console.log(result);
},
async () => {
var result = await wait(5000, 'burp');
// do something with result
console.log(result);
}
]
await Promise.all(tasks.map(p => p()));
console.log('done');
})();

await Promise.all([someCall(), anotherCall()]); as already mention will act as a thread fence (very common in parallel code as CUDA), hence it will allow all the promises in it to run without blocking each other, but will prevent the execution to continue until ALL are resolved.
another approach that is worth to share is the Node.js async that will also allow you to easily control the amount of concurrency that is usually desirable if the task is directly linked to the use of limited resources as API call, I/O operations, etc.
// create a queue object with concurrency 2
var q = async.queue(function(task, callback) {
console.log('Hello ' + task.name);
callback();
}, 2);
// assign a callback
q.drain = function() {
console.log('All items have been processed');
};
// add some items to the queue
q.push({name: 'foo'}, function(err) {
console.log('Finished processing foo');
});
q.push({name: 'bar'}, function (err) {
console.log('Finished processing bar');
});
// add some items to the queue (batch-wise)
q.push([{name: 'baz'},{name: 'bay'},{name: 'bax'}], function(err) {
console.log('Finished processing item');
});
// add some items to the front of the queue
q.unshift({name: 'bar'}, function (err) {
console.log('Finished processing bar');
});
Credits to the Medium article autor (read more)

You can call multiple asynchronous functions without awaiting them. This will execute them in parallel. While doing so, save the returned promises in variables, and await them at some point either individually or using Promise.all() and process the results.
You can also wrap the function calls with try...catch to handle failures of individual asynchronous actions and provide fallback logic.
Here's an example:
Observe the logs, the logs printed at the beginning of execution of the individual asynchronous functions get printed immediately even though the first function takes 5 seconds to resolve.
function someLongFunc () {
return new Promise((resolve, reject)=> {
console.log('Executing function 1')
setTimeout(resolve, 5000)
})
}
function anotherLongFunc () {
return new Promise((resolve, reject)=> {
console.log('Executing function 2')
setTimeout(resolve, 5000)
})
}
async function main () {
let someLongFuncPromise, anotherLongFuncPromise
const start = Date.now()
try {
someLongFuncPromise = someLongFunc()
}
catch (ex) {
console.error('something went wrong during func 1')
}
try {
anotherLongFuncPromise = anotherLongFunc()
}
catch (ex) {
console.error('something went wrong during func 2')
}
await someLongFuncPromise
await anotherLongFuncPromise
const totalTime = Date.now() - start
console.log('Execution completed in ', totalTime)
}
main()

// A generic test function that can be configured
// with an arbitrary delay and to either resolve or reject
const test = (delay, resolveSuccessfully) => new Promise((resolve, reject) => setTimeout(() => {
console.log(`Done ${ delay }`);
resolveSuccessfully ? resolve(`Resolved ${ delay }`) : reject(`Reject ${ delay }`)
}, delay));
// Our async handler function
const handler = async () => {
// Promise 1 runs first, but resolves last
const p1 = test(10000, true);
// Promise 2 run second, and also resolves
const p2 = test(5000, true);
// Promise 3 runs last, but completes first (with a rejection)
// Note the catch to trap the error immediately
const p3 = test(1000, false).catch(e => console.log(e));
// Await all in parallel
const r = await Promise.all([p1, p2, p3]);
// Display the results
console.log(r);
};
// Run the handler
handler();
/*
Done 1000
Reject 1000
Done 5000
Done 10000
*/
Whilst setting p1, p2 and p3 is not strictly running them in parallel, they do not hold up any execution and you can trap contextual errors with a catch.

This can be accomplished with Promise.allSettled(), which is similar to Promise.all() but without the fail-fast behavior.
async function Promise1() {
throw "Failure!";
}
async function Promise2() {
return "Success!";
}
const [Promise1Result, Promise2Result] = await Promise.allSettled([Promise1(), Promise2()]);
console.log(Promise1Result); // {status: "rejected", reason: "Failure!"}
console.log(Promise2Result); // {status: "fulfilled", value: "Success!"}
Note: This is a bleeding edge feature with limited browser support, so I strongly recommend including a polyfill for this function.

I create a helper function waitAll, may be it can make it sweeter.
It only works in nodejs for now, not in browser chrome.
//const parallel = async (...items) => {
const waitAll = async (...items) => {
//this function does start execution the functions
//the execution has been started before running this code here
//instead it collects of the result of execution of the functions
const temp = [];
for (const item of items) {
//this is not
//temp.push(await item())
//it does wait for the result in series (not in parallel), but
//it doesn't affect the parallel execution of those functions
//because they haven started earlier
temp.push(await item);
}
return temp;
};
//the async functions are executed in parallel before passed
//in the waitAll function
//const finalResult = await waitAll(someResult(), anotherResult());
//const finalResult = await parallel(someResult(), anotherResult());
//or
const [result1, result2] = await waitAll(someResult(), anotherResult());
//const [result1, result2] = await parallel(someResult(), anotherResult());

I vote for:
await Promise.all([someCall(), anotherCall()]);
Be aware of the moment you call functions, it may cause unexpected result:
// Supposing anotherCall() will trigger a request to create a new User
if (callFirst) {
await someCall();
} else {
await Promise.all([someCall(), anotherCall()]); // --> create new User here
}
But following always triggers request to create new User
// Supposing anotherCall() will trigger a request to create a new User
const someResult = someCall();
const anotherResult = anotherCall(); // ->> This always creates new User
if (callFirst) {
await someCall();
} else {
const finalResult = [await someResult, await anotherResult]
}

Async Delay in sending data [duplicate]

As far as I understand, in ES7/ES2016 putting multiple await's in code will work similar to chaining .then() with promises, meaning that they will execute one after the other rather than in parallel. So, for example, we have this code:
await someCall();
await anotherCall();
Do I understand it correctly that anotherCall() will be called only when someCall() is completed? What is the most elegant way of calling them in parallel?
I want to use it in Node, so maybe there's a solution with async library?
EDIT: I'm not satisfied with the solution provided in this question: Slowdown due to non-parallel awaiting of promises in async generators, because it uses generators and I'm asking about a more general use case.

You can await on Promise.all():
await Promise.all([someCall(), anotherCall()]);
To store the results:
let [someResult, anotherResult] = await Promise.all([someCall(), anotherCall()]);
Note that Promise.all fails fast, which means that as soon as one of the promises supplied to it rejects, then the entire thing rejects.
const happy = (v, ms) => new Promise((resolve) => setTimeout(() => resolve(v), ms))
const sad = (v, ms) => new Promise((_, reject) => setTimeout(() => reject(v), ms))
Promise.all([happy('happy', 100), sad('sad', 50)])
.then(console.log).catch(console.log) // 'sad'
If, instead, you want to wait for all the promises to either fulfill or reject, then you can use Promise.allSettled. Note that Internet Explorer does not natively support this method.
const happy = (v, ms) => new Promise((resolve) => setTimeout(() => resolve(v), ms))
const sad = (v, ms) => new Promise((_, reject) => setTimeout(() => reject(v), ms))
Promise.allSettled([happy('happy', 100), sad('sad', 50)])
.then(console.log) // [{ "status":"fulfilled", "value":"happy" }, { "status":"rejected", "reason":"sad" }]
Note: If you use Promise.all actions that managed to finish before rejection happen are not rolled back, so you may need to take care of such situation. For example
if you have 5 actions, 4 quick, 1 slow and slow rejects. Those 4
actions may be already executed so you may need to roll back. In such situation consider using Promise.allSettled while it will provide exact detail which action failed and which not.

TL;DR
Use Promise.all for the parallel function calls, the answer behaviors not correctly when the error occurs.
First, execute all the asynchronous calls at once and obtain all the Promise objects. Second, use await on the Promise objects. This way, while you wait for the first Promise to resolve the other asynchronous calls are still progressing. Overall, you will only wait for as long as the slowest asynchronous call. For example:
// Begin first call and store promise without waiting
const someResult = someCall();
// Begin second call and store promise without waiting
const anotherResult = anotherCall();
// Now we await for both results, whose async processes have already been started
const finalResult = [await someResult, await anotherResult];
// At this point all calls have been resolved
// Now when accessing someResult| anotherResult,
// you will have a value instead of a promise
JSbin example: http://jsbin.com/xerifanima/edit?js,console
Caveat: It doesn't matter if the await calls are on the same line or on different lines, so long as the first await call happens after all of the asynchronous calls. See JohnnyHK's comment.
Update: this answer has a different timing in error handling according to the #bergi's answer, it does NOT throw out the error as the error occurs but after all the promises are executed.
I compare the result with #jonny's tip: [result1, result2] = Promise.all([async1(), async2()]), check the following code snippet
const correctAsync500ms = () => {
return new Promise(resolve => {
setTimeout(resolve, 500, 'correct500msResult');
});
};
const correctAsync100ms = () => {
return new Promise(resolve => {
setTimeout(resolve, 100, 'correct100msResult');
});
};
const rejectAsync100ms = () => {
return new Promise((resolve, reject) => {
setTimeout(reject, 100, 'reject100msError');
});
};
const asyncInArray = async (fun1, fun2) => {
const label = 'test async functions in array';
try {
console.time(label);
const p1 = fun1();
const p2 = fun2();
const result = [await p1, await p2];
console.timeEnd(label);
} catch (e) {
console.error('error is', e);
console.timeEnd(label);
}
};
const asyncInPromiseAll = async (fun1, fun2) => {
const label = 'test async functions with Promise.all';
try {
console.time(label);
let [value1, value2] = await Promise.all([fun1(), fun2()]);
console.timeEnd(label);
} catch (e) {
console.error('error is', e);
console.timeEnd(label);
}
};
(async () => {
console.group('async functions without error');
console.log('async functions without error: start')
await asyncInArray(correctAsync500ms, correctAsync100ms);
await asyncInPromiseAll(correctAsync500ms, correctAsync100ms);
console.groupEnd();
console.group('async functions with error');
console.log('async functions with error: start')
await asyncInArray(correctAsync500ms, rejectAsync100ms);
await asyncInPromiseAll(correctAsync500ms, rejectAsync100ms);
console.groupEnd();
})();

Update:
The original answer makes it difficult (and in some cases impossible) to correctly handle promise rejections. The correct solution is to use Promise.all:
const [someResult, anotherResult] = await Promise.all([someCall(), anotherCall()]);
Original answer:
Just make sure you call both functions before you await either one:
// Call both functions
const somePromise = someCall();
const anotherPromise = anotherCall();
// Await both promises
const someResult = await somePromise;
const anotherResult = await anotherPromise;

There is another way without Promise.all() to do it in parallel:
First, we have 2 functions to print numbers:
function printNumber1() {
return new Promise((resolve,reject) => {
setTimeout(() => {
console.log("Number1 is done");
resolve(10);
},1000);
});
}
function printNumber2() {
return new Promise((resolve,reject) => {
setTimeout(() => {
console.log("Number2 is done");
resolve(20);
},500);
});
}
This is sequential:
async function oneByOne() {
const number1 = await printNumber1();
const number2 = await printNumber2();
}
//Output: Number1 is done, Number2 is done
This is parallel:
async function inParallel() {
const promise1 = printNumber1();
const promise2 = printNumber2();
const number1 = await promise1;
const number2 = await promise2;
}
//Output: Number2 is done, Number1 is done

I've created a gist testing some different ways of resolving promises, with results. It may be helpful to see the options that work.
Edit: Gist content as per Jin Lee's comment
// Simple gist to test parallel promise resolution when using async / await
function promiseWait(time) {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(true);
}, time);
});
}
async function test() {
return [
await promiseWait(1000),
await promiseWait(5000),
await promiseWait(9000),
await promiseWait(3000),
]
}
async function test2() {
return {
'aa': await promiseWait(1000),
'bb': await promiseWait(5000),
'cc': await promiseWait(9000),
'dd': await promiseWait(3000),
}
}
async function test3() {
return await {
'aa': promiseWait(1000),
'bb': promiseWait(5000),
'cc': promiseWait(9000),
'dd': promiseWait(3000),
}
}
async function test4() {
const p1 = promiseWait(1000);
const p2 = promiseWait(5000);
const p3 = promiseWait(9000);
const p4 = promiseWait(3000);
return {
'aa': await p1,
'bb': await p2,
'cc': await p3,
'dd': await p4,
};
}
async function test5() {
return await Promise.all([
await promiseWait(1000),
await promiseWait(5000),
await promiseWait(9000),
await promiseWait(3000),
]);
}
async function test6() {
return await Promise.all([
promiseWait(1000),
promiseWait(5000),
promiseWait(9000),
promiseWait(3000),
]);
}
async function test7() {
const p1 = promiseWait(1000);
const p2 = promiseWait(5000);
const p3 = promiseWait(9000);
return {
'aa': await p1,
'bb': await p2,
'cc': await p3,
'dd': await promiseWait(3000),
};
}
let start = Date.now();
test().then((res) => {
console.log('Test Done, elapsed', (Date.now() - start) / 1000, res);
start = Date.now();
test2().then((res) => {
console.log('Test2 Done, elapsed', (Date.now() - start) / 1000, res);
start = Date.now();
test3().then((res) => {
console.log('Test3 Done, elapsed', (Date.now() - start) / 1000, res);
start = Date.now();
test4().then((res) => {
console.log('Test4 Done, elapsed', (Date.now() - start) / 1000, res);
start = Date.now();
test5().then((res) => {
console.log('Test5 Done, elapsed', (Date.now() - start) / 1000, res);
start = Date.now();
test6().then((res) => {
console.log('Test6 Done, elapsed', (Date.now() - start) / 1000, res);
});
start = Date.now();
test7().then((res) => {
console.log('Test7 Done, elapsed', (Date.now() - start) / 1000, res);
});
});
});
});
});
});
/*
Test Done, elapsed 18.006 [ true, true, true, true ]
Test2 Done, elapsed 18.009 { aa: true, bb: true, cc: true, dd: true }
Test3 Done, elapsed 0 { aa: Promise { <pending> },
bb: Promise { <pending> },
cc: Promise { <pending> },
dd: Promise { <pending> } }
Test4 Done, elapsed 9 { aa: true, bb: true, cc: true, dd: true }
Test5 Done, elapsed 18.008 [ true, true, true, true ]
Test6 Done, elapsed 9.003 [ true, true, true, true ]
Test7 Done, elapsed 12.007 { aa: true, bb: true, cc: true, dd: true }
*/

In my case, I have several tasks I want to execute in parallel, but I need to do something different with the result of those tasks.
function wait(ms, data) {
console.log('Starting task:', data, ms);
return new Promise(resolve => setTimeout(resolve, ms, data));
}
var tasks = [
async () => {
var result = await wait(1000, 'moose');
// do something with result
console.log(result);
},
async () => {
var result = await wait(500, 'taco');
// do something with result
console.log(result);
},
async () => {
var result = await wait(5000, 'burp');
// do something with result
console.log(result);
}
]
await Promise.all(tasks.map(p => p()));
console.log('done');
And the output:
Starting task: moose 1000
Starting task: taco 500
Starting task: burp 5000
taco
moose
burp
done
(async function(){
function wait(ms, data) {
console.log('Starting task:', data, ms);
return new Promise(resolve => setTimeout(resolve, ms, data));
}
var tasks = [
async () => {
var result = await wait(1000, 'moose');
// do something with result
console.log(result);
},
async () => {
var result = await wait(500, 'taco');
// do something with result
console.log(result);
},
async () => {
var result = await wait(5000, 'burp');
// do something with result
console.log(result);
}
]
await Promise.all(tasks.map(p => p()));
console.log('done');
})();

await Promise.all([someCall(), anotherCall()]); as already mention will act as a thread fence (very common in parallel code as CUDA), hence it will allow all the promises in it to run without blocking each other, but will prevent the execution to continue until ALL are resolved.
another approach that is worth to share is the Node.js async that will also allow you to easily control the amount of concurrency that is usually desirable if the task is directly linked to the use of limited resources as API call, I/O operations, etc.
// create a queue object with concurrency 2
var q = async.queue(function(task, callback) {
console.log('Hello ' + task.name);
callback();
}, 2);
// assign a callback
q.drain = function() {
console.log('All items have been processed');
};
// add some items to the queue
q.push({name: 'foo'}, function(err) {
console.log('Finished processing foo');
});
q.push({name: 'bar'}, function (err) {
console.log('Finished processing bar');
});
// add some items to the queue (batch-wise)
q.push([{name: 'baz'},{name: 'bay'},{name: 'bax'}], function(err) {
console.log('Finished processing item');
});
// add some items to the front of the queue
q.unshift({name: 'bar'}, function (err) {
console.log('Finished processing bar');
});
Credits to the Medium article autor (read more)

You can call multiple asynchronous functions without awaiting them. This will execute them in parallel. While doing so, save the returned promises in variables, and await them at some point either individually or using Promise.all() and process the results.
You can also wrap the function calls with try...catch to handle failures of individual asynchronous actions and provide fallback logic.
Here's an example:
Observe the logs, the logs printed at the beginning of execution of the individual asynchronous functions get printed immediately even though the first function takes 5 seconds to resolve.
function someLongFunc () {
return new Promise((resolve, reject)=> {
console.log('Executing function 1')
setTimeout(resolve, 5000)
})
}
function anotherLongFunc () {
return new Promise((resolve, reject)=> {
console.log('Executing function 2')
setTimeout(resolve, 5000)
})
}
async function main () {
let someLongFuncPromise, anotherLongFuncPromise
const start = Date.now()
try {
someLongFuncPromise = someLongFunc()
}
catch (ex) {
console.error('something went wrong during func 1')
}
try {
anotherLongFuncPromise = anotherLongFunc()
}
catch (ex) {
console.error('something went wrong during func 2')
}
await someLongFuncPromise
await anotherLongFuncPromise
const totalTime = Date.now() - start
console.log('Execution completed in ', totalTime)
}
main()

// A generic test function that can be configured
// with an arbitrary delay and to either resolve or reject
const test = (delay, resolveSuccessfully) => new Promise((resolve, reject) => setTimeout(() => {
console.log(`Done ${ delay }`);
resolveSuccessfully ? resolve(`Resolved ${ delay }`) : reject(`Reject ${ delay }`)
}, delay));
// Our async handler function
const handler = async () => {
// Promise 1 runs first, but resolves last
const p1 = test(10000, true);
// Promise 2 run second, and also resolves
const p2 = test(5000, true);
// Promise 3 runs last, but completes first (with a rejection)
// Note the catch to trap the error immediately
const p3 = test(1000, false).catch(e => console.log(e));
// Await all in parallel
const r = await Promise.all([p1, p2, p3]);
// Display the results
console.log(r);
};
// Run the handler
handler();
/*
Done 1000
Reject 1000
Done 5000
Done 10000
*/
Whilst setting p1, p2 and p3 is not strictly running them in parallel, they do not hold up any execution and you can trap contextual errors with a catch.

This can be accomplished with Promise.allSettled(), which is similar to Promise.all() but without the fail-fast behavior.
async function Promise1() {
throw "Failure!";
}
async function Promise2() {
return "Success!";
}
const [Promise1Result, Promise2Result] = await Promise.allSettled([Promise1(), Promise2()]);
console.log(Promise1Result); // {status: "rejected", reason: "Failure!"}
console.log(Promise2Result); // {status: "fulfilled", value: "Success!"}
Note: This is a bleeding edge feature with limited browser support, so I strongly recommend including a polyfill for this function.

I create a helper function waitAll, may be it can make it sweeter.
It only works in nodejs for now, not in browser chrome.
//const parallel = async (...items) => {
const waitAll = async (...items) => {
//this function does start execution the functions
//the execution has been started before running this code here
//instead it collects of the result of execution of the functions
const temp = [];
for (const item of items) {
//this is not
//temp.push(await item())
//it does wait for the result in series (not in parallel), but
//it doesn't affect the parallel execution of those functions
//because they haven started earlier
temp.push(await item);
}
return temp;
};
//the async functions are executed in parallel before passed
//in the waitAll function
//const finalResult = await waitAll(someResult(), anotherResult());
//const finalResult = await parallel(someResult(), anotherResult());
//or
const [result1, result2] = await waitAll(someResult(), anotherResult());
//const [result1, result2] = await parallel(someResult(), anotherResult());

I vote for:
await Promise.all([someCall(), anotherCall()]);
Be aware of the moment you call functions, it may cause unexpected result:
// Supposing anotherCall() will trigger a request to create a new User
if (callFirst) {
await someCall();
} else {
await Promise.all([someCall(), anotherCall()]); // --> create new User here
}
But following always triggers request to create new User
// Supposing anotherCall() will trigger a request to create a new User
const someResult = someCall();
const anotherResult = anotherCall(); // ->> This always creates new User
if (callFirst) {
await someCall();
} else {
const finalResult = [await someResult, await anotherResult]
}

properly using async and await

The function below calls several asynchronous functions in a for loop. It's parsing different CSV files to build a single JavaScript object. I'd like to return the object after the for loop is done. Its returning the empty object right away while it does the asynchronous tasks. Makes sense, however I have tried various Promise / async /await combinations hopes of running something once the for loop has completed. I am clearly not understanding what is going on. Is there a better pattern to follow for something like this or am I thinking about it incorrectly?
async function createFormConfig(files: string[]): Promise<object>
return new Promise(resolve => {
const retConfig: any = {};
for (const file of files) {
file.match(matchFilesForFormConfigMap.get('FIELD')) ?
parseCsv(file).then(parsedData => {
retConfig.fields = parsedData.data;
})
: file.match(matchFilesForFormConfigMap.get('FORM'))
? parseCsv(file).then(parsedData => retConfig.formProperties = parsedData.data[0])
: file.match(matchFilesForFormConfigMap.get('PDF'))
? parseCsv(file).then(parsedData => retConfig.jsPdfProperties = parsedData.data[0])
: file.match(matchFilesForFormConfigMap.get('META'))
? parseCsv(file).then(parsedData => {
retConfig.name = parsedData.data[0].name;
retConfig.imgType = parsedData.data[0].imgType;
// console.log(retConfig); <- THIS CONSOLE WILL OUTPUT RETCONFIG LOOKING LIKE I WANT IT
})
: file.match(matchFilesForFormConfigMap.get('PAGES'))
? parseCsv(file).then(parsedData => retConfig.pages = parsedData.data)
: console.log('there is an extra file: ' + file);
}
resolve(retConfig); // <- THIS RETURNS: {}
});
This is the code I'm using to call the function in hopes of getting my 'retConfig' filled with the CSV data.
getFilesFromDirectory(`${clOptions.directory}/**/*.csv`)
.then(async (files) => {
const config = await createFormConfig(files);
console.log(config);
})
.catch(err => console.error(err));
};

First, an async function returns a Promise, so you dont have to return one explicitely.Here is how you can simplify your code:
async function createFormConfig(files: string[]): Promise<object> {
// return new Promise(resolve => { <-- remove
const retConfig: any = {};
// ...
// The value returned by an async function is the one you get
// in the callback passed to the function `.then`
return retConfig;
// }); <-- remove
}
Then, your function createFormConfig returns the config before it has finished to compute it. Here is how you can have it computed before returning it:
async function createFormConfig(files: string[]): Promise<object> {
const retConfig: any = {};
// Return a Promise for each file that have to be parsed
const parsingCsv = files.map(async file => {
if (file.match(matchFilesForFormConfigMap.get('FIELD'))) {
const { data } = await parseCsv(file);
retConfig.fields = data;
} else if (file.match(matchFilesForFormConfigMap.get('FORM'))) {
const { data } = await parseCsv(file);
retConfig.formProperties = data[0];
} else if (file.match(matchFilesForFormConfigMap.get('PDF'))) {
const { data } = await parseCsv(file);
retConfig.jsPdfProperties = data[0];
} else if (file.match(matchFilesForFormConfigMap.get('META'))) {
const { data } = await parseCsv(file);
retConfig.name = data[0].name;
retConfig.imgType = data[0].imgType;
} else if (file.match(matchFilesForFormConfigMap.get('PAGES'))) {
const { data } = await parseCsv(file);
retConfig.pages = data;
} else {
console.log('there is an extra file: ' + file);
}
});
// Wait for the Promises to resolve
await Promise.all(parsingCsv)
return retConfig;
}

async functions already return promises, you don't need to wrap the code in a new one. Just return a value from the function and the caller will receive a promise that resolves to the returned value.
Also, you have made an async function, but you're not actually using await anywhere. So the for loop runs through the whole loop before any of your promises resolve. This is why none of the data is making it into your object.
It will really simplify your code to only use await and get rid of the then() calls. For example you can do this:
async function createFormConfig(files: string[]): Promise<object> {
const retConfig: any = {};
for (const file of files) {
if (file.match(matchFilesForFormConfigMap.get('FIELD')){
// no need for the then here
let parsedData = await parseCsv(file)
retConfig.field = parsedData.data
}
// ...etc
At the end you can just return the value:
return retConfig

fetch retry request (on failure)

I'm using browser's native fetch API for network requests. Also I am using the whatwg-fetch polyfill for unsupported browsers.
However I need to retry in case the request fails. Now there is this npm package whatwg-fetch-retry I found, but they haven't explained how to use it in their docs. Can somebody help me with this or suggest me an alternative?

From the fetch docs :
fetch('/users')
.then(checkStatus)
.then(parseJSON)
.then(function(data) {
console.log('succeeded', data)
}).catch(function(error) {
console.log('request failed', error)
})
See that catch? Will trigger when fetch fails, you can fetch again there.
Have a look at the Promise API.
Implementation example:
function wait(delay){
return new Promise((resolve) => setTimeout(resolve, delay));
}
function fetchRetry(url, delay, tries, fetchOptions = {}) {
function onError(err){
triesLeft = tries - 1;
if(!triesLeft){
throw err;
}
return wait(delay).then(() => fetchRetry(url, delay, triesLeft, fetchOptions));
}
return fetch(url,fetchOptions).catch(onError);
}
Edit 1: as suggested by golopot, p-retry is a nice option.
Edit 2: simplified example code.

I recommend using some library for promise retry, for example p-retry.
Example:
const pRetry = require('p-retry')
const fetch = require('node-fetch')
async function fetchPage () {
const response = await fetch('https://stackoverflow.com')
// Abort retrying if the resource doesn't exist
if (response.status === 404) {
throw new pRetry.AbortError(response.statusText)
}
return response.blob()
}
;(async () => {
console.log(await pRetry(fetchPage, {retries: 5}))
})()

I don't like recursion unless is really necessary. And managing an exploding number of dependencies is also an issue. Here is another alternative in typescript. Which is easy to translate to javascript.
interface retryPromiseOptions<T> {
retryCatchIf?:(response:T) => boolean,
retryIf?:(response:T) => boolean,
retries?:number
}
function retryPromise<T>(promise:() => Promise<T>, options:retryPromiseOptions<T>) {
const { retryIf = (_:T) => false, retryCatchIf= (_:T) => true, retries = 1} = options
let _promise = promise();
for (var i = 1; i < retries; i++)
_promise = _promise.catch((value) => retryCatchIf(value) ? promise() : Promise.reject(value))
.then((value) => retryIf(value) ? promise() : Promise.reject(value));
return _promise;
}
And use it this way...
retryPromise(() => fetch(url),{
retryIf: (response:Response) => true, // you could check before trying again
retries: 5
}).then( ... my favorite things ... )
I wrote this for the fetch API on the browser. Which does not issue a reject on a 500. And did I did not implement a wait. But, more importantly, the code shows how to use composition with promises to avoid recursion.
Javascript version:
function retryPromise(promise, options) {
const { retryIf, retryCatchIf, retries } = { retryIf: () => false, retryCatchIf: () => true, retries: 1, ...options};
let _promise = promise();
for (var i = 1; i < retries; i++)
_promise = _promise.catch((value) => retryCatchIf(value) ? promise() : Promise.reject(value))
.then((value) => retryIf(value) ? promise() : Promise.reject(value));
return _promise;
}
Javascript usage:
retryPromise(() => fetch(url),{
retryIf: (response) => true, // you could check before trying again
retries: 5
}).then( ... my favorite things ... )
EDITS: Added js version, added retryCatchIf, fixed the loop start.

One can easily wrap fetch(...) in a loop and catch potential errors (fetch only rejects the returning promise on network errors and the alike):
const RETRY_COUNT = 5;
async function fetchRetry(...args) {
let count = RETRY_COUNT;
while(count > 0) {
try {
return await fetch(...args);
} catch(error) {
// logging ?
}
// logging / waiting?
count -= 1;
}
throw new Error(`Too many retries`);
}