Develop Reference

Nested Async and Awaits resulting in errors... any ideas why? [duplicate]

Given the following code:
var arr = [1,2,3,4,5];
var results: number[] = await arr.map(async (item): Promise<number> => {
await callAsynchronousOperation(item);
return item + 1;
});
which produces the following error:
TS2322: Type 'Promise<number>[]' is not assignable to type 'number[]'.
Type 'Promise<number> is not assignable to type 'number'.
How can I fix it? How can I make async await and Array.map work together?

The problem here is that you are trying to await an array of promises rather than a Promise. This doesn't do what you expect.
When the object passed to await is not a Promise, await simply returns the value as-is immediately instead of trying to resolve it. So since you passed await an array (of Promise objects) here instead of a Promise, the value returned by await is simply that array, which is of type Promise<number>[].
What you probably want to do is call Promise.all on the array returned by map in order to convert it to a single Promise before awaiting it.
According to the MDN docs for Promise.all:
The Promise.all(iterable) method returns a promise that resolves
when all of the promises in the iterable argument have resolved, or
rejects with the reason of the first passed promise that rejects.
So in your case:
var arr = [1, 2, 3, 4, 5];
var results: number[] = await Promise.all(arr.map(async (item): Promise<number> => {
await callAsynchronousOperation(item);
return item + 1;
}));
This will resolve the specific error you are encountering here.
Depending on exactly what it is you're trying to do you may also consider using Promise.allSettled, Promise.any, or Promise.race instead of Promise.all, though in most situations (almost certainly including this one) Promise.all will be the one you want.

Solution below to properly use async await and Array.map together. Process all elements of the array in parallel, asynchronously AND preserve the order:
const arr = [1, 2, 3, 4, 5, 6, 7, 8];
const randomDelay = () => new Promise(resolve => setTimeout(resolve, Math.random() * 1000));
const calc = async n => {
await randomDelay();
return n * 2;
};
const asyncFunc = async() => {
const unresolvedPromises = arr.map(calc);
const results = await Promise.all(unresolvedPromises);
document.write(results);
};
document.write('calculating...');
asyncFunc();
Also codepen.
Notice we only "await" for Promise.all. We call calc without "await" multiple times, and we collect an array of unresolved promises right away. Then Promise.all waits for resolution of all of them and returns an array with the resolved values in order.

This is simplest way to do it.
await Promise.all(
arr.map(async (element) => {
....
})
)

There's another solution for it if you are not using native Promises but Bluebird.
You could also try using Promise.map(), mixing the array.map and Promise.all
In you case:
var arr = [1,2,3,4,5];
var results: number[] = await Promise.map(arr, async (item): Promise<number> => {
await callAsynchronousOperation(item);
return item + 1;
});

If you map to an array of Promises, you can then resolve them all to an array of numbers. See Promise.all.

You can use:
for await (let resolvedPromise of arrayOfPromises) {
console.log(resolvedPromise)
}
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for-await...of
If you wish to use Promise.all() instead you can go for Promise.allSettled()
So you can have better control over rejected promises.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/allSettled

I'd recommend using Promise.all as mentioned above, but if you really feel like avoiding that approach, you can do a for or any other loop:
const arr = [1,2,3,4,5];
let resultingArr = [];
for (let i in arr){
await callAsynchronousOperation(i);
resultingArr.push(i + 1)
}
FYI: If you want to iterate over items of an array, rather than indices (#ralfoide 's comment), use of instead of in inside let i in arr statement.

A solution using modern-async's map():
import { map } from 'modern-async'
...
const result = await map(myArray, async (v) => {
...
})
The advantage of using that library is that you can control the concurrency using mapLimit() or mapSeries().

I had a task on BE side to find all entities from a repo, and to add a new property url and to return to controller layer. This is how I achieved it (thanks to Ajedi32's response):
async findAll(): Promise<ImageResponse[]> {
const images = await this.imageRepository.find(); // This is an array of type Image (DB entity)
const host = this.request.get('host');
const mappedImages = await Promise.all(images.map(image => ({...image, url: `http://${host}/images/${image.id}`}))); // This is an array of type Object
return plainToClass(ImageResponse, mappedImages); // Result is an array of type ImageResponse
}
Note: Image (entity) doesn't have property url, but ImageResponse - has

How do I convert a JS promise to another object

So I have an asynchronous function that I need to call a number of times. Naturally, I would like to use Promise.all
const promises = [];
ids.forEach((id) => {
stockoutPromises.push(
asyncFunc(id),
});
});
results[] = Promise.all(promises);
Okay no problem there but how do I know which id goes with which result? Sure I could iterate through both arrays again, but is there another way to do this? Is there a way that I can create a promise that when resolved gives me an object that contains both id and result?
Thanks!

Chain a .then onto the call of asyncFunc so that the resulting item is not just of the asyncFunc result, but of it and the ID in an object:
ids.forEach((id) => {
stockoutPromises.push(
asyncFunc(id).then(result => ({ id, result }))
);
});
But it'd be better to use .map instead of .push in a loop:
const results = await Promise.all(
ids.map(id =>
asyncFunc(id).then(result => ({ id, result }))
)
);
const firstItem = results[0];
console.log(firstItem.result, firstItem.id);

The result of Promise.all will be in the order that they were in originally (in the array of promises). So you can easily reassociate just using the index:
// zip takes an array of keys, and an array of values, and creates an object:
const zip = (a, b) => Object.fromEntries(a.map((k, i) => [k, b[i]]));
// first id will resolve last, second will resolve next, etc.
const ids = [0, 1, 2, 3, 4];
const promises = ids.map((i) => new Promise((resolve) => {
setTimeout(() => resolve(i), 700 - (100 * i));
}));
// but we still get the correct item in each spot, because of how
// Promise.all works:
(async () => {
console.log(zip(ids, await Promise.all(promises)));
})()
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/all:
Returned values will be in order of the Promises passed, regardless of completion order.

Instead of using forEach, use map:
const promises = ids.map((id) => {
return asyncFunc(id)
});
const results = await Promise.all(promises);
map will return a new array of objects based on what the supplied function returns. In your case, you are calling an asyncFunc, which, I assume, returns a promise itself. So you can just return the results of map directlty without pushing to a new promises array.
Also, make sure to "await" the call to Promise.all;
You can check it out here in this jsfiddle that simply has the asyncFunc return a promise that doubles the id in the ids array.

Javascript async await not waiting as expected [duplicate]

Given the following code:
var arr = [1,2,3,4,5];
var results: number[] = await arr.map(async (item): Promise<number> => {
await callAsynchronousOperation(item);
return item + 1;
});
which produces the following error:
TS2322: Type 'Promise<number>[]' is not assignable to type 'number[]'.
Type 'Promise<number> is not assignable to type 'number'.
How can I fix it? How can I make async await and Array.map work together?

The problem here is that you are trying to await an array of promises rather than a Promise. This doesn't do what you expect.
When the object passed to await is not a Promise, await simply returns the value as-is immediately instead of trying to resolve it. So since you passed await an array (of Promise objects) here instead of a Promise, the value returned by await is simply that array, which is of type Promise<number>[].
What you probably want to do is call Promise.all on the array returned by map in order to convert it to a single Promise before awaiting it.
According to the MDN docs for Promise.all:
The Promise.all(iterable) method returns a promise that resolves
when all of the promises in the iterable argument have resolved, or
rejects with the reason of the first passed promise that rejects.
So in your case:
var arr = [1, 2, 3, 4, 5];
var results: number[] = await Promise.all(arr.map(async (item): Promise<number> => {
await callAsynchronousOperation(item);
return item + 1;
}));
This will resolve the specific error you are encountering here.
Depending on exactly what it is you're trying to do you may also consider using Promise.allSettled, Promise.any, or Promise.race instead of Promise.all, though in most situations (almost certainly including this one) Promise.all will be the one you want.

Solution below to properly use async await and Array.map together. Process all elements of the array in parallel, asynchronously AND preserve the order:
const arr = [1, 2, 3, 4, 5, 6, 7, 8];
const randomDelay = () => new Promise(resolve => setTimeout(resolve, Math.random() * 1000));
const calc = async n => {
await randomDelay();
return n * 2;
};
const asyncFunc = async() => {
const unresolvedPromises = arr.map(calc);
const results = await Promise.all(unresolvedPromises);
document.write(results);
};
document.write('calculating...');
asyncFunc();
Also codepen.
Notice we only "await" for Promise.all. We call calc without "await" multiple times, and we collect an array of unresolved promises right away. Then Promise.all waits for resolution of all of them and returns an array with the resolved values in order.

This is simplest way to do it.
await Promise.all(
arr.map(async (element) => {
....
})
)

There's another solution for it if you are not using native Promises but Bluebird.
You could also try using Promise.map(), mixing the array.map and Promise.all
In you case:
var arr = [1,2,3,4,5];
var results: number[] = await Promise.map(arr, async (item): Promise<number> => {
await callAsynchronousOperation(item);
return item + 1;
});

If you map to an array of Promises, you can then resolve them all to an array of numbers. See Promise.all.

You can use:
for await (let resolvedPromise of arrayOfPromises) {
console.log(resolvedPromise)
}
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for-await...of
If you wish to use Promise.all() instead you can go for Promise.allSettled()
So you can have better control over rejected promises.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/allSettled

I'd recommend using Promise.all as mentioned above, but if you really feel like avoiding that approach, you can do a for or any other loop:
const arr = [1,2,3,4,5];
let resultingArr = [];
for (let i in arr){
await callAsynchronousOperation(i);
resultingArr.push(i + 1)
}
FYI: If you want to iterate over items of an array, rather than indices (#ralfoide 's comment), use of instead of in inside let i in arr statement.

A solution using modern-async's map():
import { map } from 'modern-async'
...
const result = await map(myArray, async (v) => {
...
})
The advantage of using that library is that you can control the concurrency using mapLimit() or mapSeries().

I had a task on BE side to find all entities from a repo, and to add a new property url and to return to controller layer. This is how I achieved it (thanks to Ajedi32's response):
async findAll(): Promise<ImageResponse[]> {
const images = await this.imageRepository.find(); // This is an array of type Image (DB entity)
const host = this.request.get('host');
const mappedImages = await Promise.all(images.map(image => ({...image, url: `http://${host}/images/${image.id}`}))); // This is an array of type Object
return plainToClass(ImageResponse, mappedImages); // Result is an array of type ImageResponse
}
Note: Image (entity) doesn't have property url, but ImageResponse - has

Use async await with Array.map

Given the following code:
var arr = [1,2,3,4,5];
var results: number[] = await arr.map(async (item): Promise<number> => {
await callAsynchronousOperation(item);
return item + 1;
});
which produces the following error:
TS2322: Type 'Promise<number>[]' is not assignable to type 'number[]'.
Type 'Promise<number> is not assignable to type 'number'.
How can I fix it? How can I make async await and Array.map work together?

The problem here is that you are trying to await an array of promises rather than a Promise. This doesn't do what you expect.
When the object passed to await is not a Promise, await simply returns the value as-is immediately instead of trying to resolve it. So since you passed await an array (of Promise objects) here instead of a Promise, the value returned by await is simply that array, which is of type Promise<number>[].
What you probably want to do is call Promise.all on the array returned by map in order to convert it to a single Promise before awaiting it.
According to the MDN docs for Promise.all:
The Promise.all(iterable) method returns a promise that resolves
when all of the promises in the iterable argument have resolved, or
rejects with the reason of the first passed promise that rejects.
So in your case:
var arr = [1, 2, 3, 4, 5];
var results: number[] = await Promise.all(arr.map(async (item): Promise<number> => {
await callAsynchronousOperation(item);
return item + 1;
}));
This will resolve the specific error you are encountering here.
Depending on exactly what it is you're trying to do you may also consider using Promise.allSettled, Promise.any, or Promise.race instead of Promise.all, though in most situations (almost certainly including this one) Promise.all will be the one you want.

Solution below to properly use async await and Array.map together. Process all elements of the array in parallel, asynchronously AND preserve the order:
const arr = [1, 2, 3, 4, 5, 6, 7, 8];
const randomDelay = () => new Promise(resolve => setTimeout(resolve, Math.random() * 1000));
const calc = async n => {
await randomDelay();
return n * 2;
};
const asyncFunc = async() => {
const unresolvedPromises = arr.map(calc);
const results = await Promise.all(unresolvedPromises);
document.write(results);
};
document.write('calculating...');
asyncFunc();
Also codepen.
Notice we only "await" for Promise.all. We call calc without "await" multiple times, and we collect an array of unresolved promises right away. Then Promise.all waits for resolution of all of them and returns an array with the resolved values in order.

This is simplest way to do it.
await Promise.all(
arr.map(async (element) => {
....
})
)

There's another solution for it if you are not using native Promises but Bluebird.
You could also try using Promise.map(), mixing the array.map and Promise.all
In you case:
var arr = [1,2,3,4,5];
var results: number[] = await Promise.map(arr, async (item): Promise<number> => {
await callAsynchronousOperation(item);
return item + 1;
});

If you map to an array of Promises, you can then resolve them all to an array of numbers. See Promise.all.

You can use:
for await (let resolvedPromise of arrayOfPromises) {
console.log(resolvedPromise)
}
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for-await...of
If you wish to use Promise.all() instead you can go for Promise.allSettled()
So you can have better control over rejected promises.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/allSettled

I'd recommend using Promise.all as mentioned above, but if you really feel like avoiding that approach, you can do a for or any other loop:
const arr = [1,2,3,4,5];
let resultingArr = [];
for (let i in arr){
await callAsynchronousOperation(i);
resultingArr.push(i + 1)
}
FYI: If you want to iterate over items of an array, rather than indices (#ralfoide 's comment), use of instead of in inside let i in arr statement.

A solution using modern-async's map():
import { map } from 'modern-async'
...
const result = await map(myArray, async (v) => {
...
})
The advantage of using that library is that you can control the concurrency using mapLimit() or mapSeries().

I had a task on BE side to find all entities from a repo, and to add a new property url and to return to controller layer. This is how I achieved it (thanks to Ajedi32's response):
async findAll(): Promise<ImageResponse[]> {
const images = await this.imageRepository.find(); // This is an array of type Image (DB entity)
const host = this.request.get('host');
const mappedImages = await Promise.all(images.map(image => ({...image, url: `http://${host}/images/${image.id}`}))); // This is an array of type Object
return plainToClass(ImageResponse, mappedImages); // Result is an array of type ImageResponse
}
Note: Image (entity) doesn't have property url, but ImageResponse - has

Best es6 way to get name based results with Promise.all

By default the Promise.All([]) function returns a number based index array that contains the results of each promise.
var promises = [];
promises.push(myFuncAsync1()); //returns 1
promises.push(myFuncAsync1()); //returns 2
Promise.all(promises).then((results)=>{
//results = [0,1]
}
What is the best vanilla way to return a named index of results with Promise.all()?
I tried with a Map, but it returns results in an array this way:
[key1, value1, key2, value2]
UPDATE:
My questions seems unclear, here is why i don't like ordered based index:
it's crappy to maintain: if you add a promise in your code you may have to rewrite the whole results function because the index may have change.
it's awful to read: results[42] (can be fixed with jib's answer below)
Not really usable in a dynamic context:
var promises = [];
if(...)
promises.push(...);
else{
[...].forEach(... => {
if(...)
promises.push(...);
else
[...].forEach(... => {
promises.push(...);
});
});
}
Promise.all(promises).then((resultsArr)=>{
/*Here i am basically fucked without clear named results
that dont rely on promises' ordering in the array */
});

ES6 supports destructuring, so if you just want to name the results you can write:
var myFuncAsync1 = () => Promise.resolve(1);
var myFuncAsync2 = () => Promise.resolve(2);
Promise.all([myFuncAsync1(), myFuncAsync2()])
.then(([result1, result2]) => console.log(result1 +" and "+ result2)) //1 and 2
.catch(e => console.error(e));
Works in Firefox and Chrome now.

Is this the kind of thing?
var promises = [];
promises.push(myFuncAsync1().then(r => ({name : "func1", result : r})));
promises.push(myFuncAsync1().then(r => ({name : "func2", result : r})));
Promise.all(promises).then(results => {
var lookup = results.reduce((prev, curr) => {
prev[curr.name] = curr.result;
return prev;
}, {});
var firstResult = lookup["func1"];
var secondResult = lookup["func2"];
}

If you don't want to modify the format of result objects, here is a helper function that allows assigning a name to each entry to access it later.
const allNamed = (nameToPromise) => {
const entries = Object.entries(nameToPromise);
return Promise.all(entries.map(e => e[1]))
.then(results => {
const nameToResult = {};
for (let i = 0; i < results.length; ++i) {
const name = entries[i][0];
nameToResult[name] = results[i];
}
return nameToResult;
});
};
Usage:
var lookup = await allNamed({
rootStatus: fetch('https://stackoverflow.com/').then(rs => rs.status),
badRouteStatus: fetch('https://stackoverflow.com/badRoute').then(rs => rs.status),
});
var firstResult = lookup.rootStatus; // = 200
var secondResult = lookup.badRouteStatus; // = 404
If you are using typescript you can even specify relationship between input keys and results using keyof construct:
type ThenArg<T> = T extends PromiseLike<infer U> ? U : T;
export const allNamed = <
T extends Record<string, Promise<any>>,
TResolved extends {[P in keyof T]: ThenArg<T[P]>}
>(nameToPromise: T): Promise<TResolved> => {
const entries = Object.entries(nameToPromise);
return Promise.all(entries.map(e => e[1]))
.then(results => {
const nameToResult: TResolved = <any>{};
for (let i = 0; i < results.length; ++i) {
const name: keyof T = entries[i][0];
nameToResult[name] = results[i];
}
return nameToResult;
});
};

A great solution for this is to use async await. Not exactly ES6 like you asked, but ES8! But since Babel supports it fully, here we go:
You can avoid using only the array index by using async/await as follows.
This async function allows you to literally halt your code inside of it by allowing you to use the await keyword inside of the function, placing it before a promise. As as an async function encounters await on a promise that hasn't yet been resolved, the function immediately returns a pending promise. This returned promise resolves as soon as the function actually finishes later on. The function will only resume when the previously awaited promise is resolved, during which it will resolve the entire await Promise statement to the return value of that Promise, allowing you to put it inside of a variable. This effectively allows you to halt your code without blocking the thread. It's a great way to handle asynchronous stuff in JavaScript in general, because it makes your code more chronological and therefore easier to reason about:
async function resolvePromiseObject(promiseObject) {
await Promise.all(Object.values(promiseObject));
const ret = {};
for ([key, value] of Object.entries(promiseObject)) {
// All these resolve instantly due to the previous await
ret[key] = await value;
};
return ret;
}
As with anything above ES5: Please make sure that Babel is configured correctly so that users on older browsers can run your code without issue. You can make async await work flawlessly on even IE11, as long as your babel configuration is right.

in regards to #kragovip's answer, the reason you want to avoid that is shown here:
https://medium.com/front-end-weekly/async-await-is-not-about-making-asynchronous-code-synchronous-ba5937a0c11e
"...it’s really easy to get used to await all of your network and I/O calls.
However, you should be careful when using it multiple times in a row as the await keyword stops execution of all the code after it. (Exactly as it would be in synchronous code)"
Bad Example (DONT FOLLOW)
async function processData() {
const data1 = await downloadFromService1();
const data2 = await downloadFromService2();
const data3 = await downloadFromService3();
...
}
"There is also absolutely no need to wait for the completion of first request as none of other requests depend on its result.
We would like to have requests sent in parallel and wait for all of them to finish simultaneously. This is where the power of asynchronous event-driven programming lies.
To fix this we can use Promise.all() method. We save Promises from async function calls to variables, combine them to an array and await them all at once."
Instead
async function processData() {
const promise1 = downloadFromService1();
const promise2 = downloadFromService2();
const promise3 = downloadFromService3();
const allResults = await Promise.all([promise1, promise2, promise3]);