We Keep Coding

Why should we store Promise objects in variables?

Let us suppose that we've got a function that resolves a promise like below:
function timeoutPromise(interval) {
return new Promise((resolve, reject) => {
setTimeout(function(){
resolve("done");
}, interval);
});
};
and let us suppose that we call that function inside an asynchronous function in two different ways;
slow synchornous way:
async function timeTest() {
await timeoutPromise(3000);
await timeoutPromise(3000);
await timeoutPromise(3000);
}
Here we simply await all three timeoutPromise() calls directly. Each subsequent one is forced to wait until the last one finished, this will result in total run time of around 9 seconds.
and fast asynchronous way:
async function timeTest() {
const timeoutPromise1 = timeoutPromise(3000);
const timeoutPromise2 = timeoutPromise(3000);
const timeoutPromise3 = timeoutPromise(3000);
await timeoutPromise1;
await timeoutPromise2;
await timeoutPromise3;
}
Here we store the three Promise objects in variables, which has the effect of setting off their associated processes all running simultaneously. This will result in total run time of around 3 seconds.
But the question is, why storing Promise objects in variables has the effect setting off their associated processs running simultaneously? what happens under the hood?

await foo();
await bar();
will only call bar (and thus create the second promise) after the promise returned by foo is resolved.
var x = foo();
var y = bar();
await x;
calls bar (and thus creating the second promise) before the promise returned by foo is resolved.
That's what makes the promises "concurrent". If you add console.log in various places you will see the difference in execution:
function timeoutPromise(name, interval) {
return new Promise((resolve, reject) => {
console.log(`Promise ${name} created.`);
setTimeout(function(){
console.log(`Promise ${name} resolved.`);
resolve("done");
}, interval);
});
};
async function timeTest1() {
console.log('test 1');
await timeoutPromise(1, 3000);
console.log('between promise 1 and 2');
await timeoutPromise(2, 3000);
}
async function timeTest2() {
console.log('test 2');
const timeoutPromise1 = timeoutPromise(1, 3000);
console.log('between promise 1 and 2');
const timeoutPromise2 = timeoutPromise(2, 3000);
await timeoutPromise1;
console.log('between promise 1 and 2 with await');
await timeoutPromise2;
}
timeTest1().then(timeTest2);

Await not working even when whole stack is async [duplicate]

I wrote this code in lib/helper.js:
var myfunction = async function(x,y) {
....
return [variableA, variableB]
}
exports.myfunction = myfunction;
Then I tried to use it in another file :
var helper = require('./helper.js');
var start = function(a,b){
....
const result = await helper.myfunction('test','test');
}
exports.start = start;
I got an error:
await is only valid in async function
What is the issue?

The error is not refering to myfunction but to start.
async function start() {
....
const result = await helper.myfunction('test', 'test');
}
// My function
const myfunction = async function(x, y) {
return [
x,
y,
];
}
// Start function
const start = async function(a, b) {
const result = await myfunction('test', 'test');
console.log(result);
}
// Call start
start();
I use the opportunity of this question to advise you about an known anti pattern using await which is : return await.
WRONG
async function myfunction() {
console.log('Inside of myfunction');
}
// Here we wait for the myfunction to finish
// and then returns a promise that'll be waited for aswell
// It's useless to wait the myfunction to finish before to return
// we can simply returns a promise that will be resolved later
// useless async here
async function start() {
// useless await here
return await myfunction();
}
// Call start
(async() => {
console.log('before start');
await start();
console.log('after start');
})();
CORRECT
async function myfunction() {
console.log('Inside of myfunction');
}
// Here we wait for the myfunction to finish
// and then returns a promise that'll be waited for aswell
// It's useless to wait the myfunction to finish before to return
// we can simply returns a promise that will be resolved later
// Also point that we don't use async keyword on the function because
// we can simply returns the promise returned by myfunction
function start() {
return myfunction();
}
// Call start
(async() => {
console.log('before start');
await start();
console.log('after start');
})();
Also, know that there is a special case where return await is correct and important : (using try/catch)
Are there performance concerns with `return await`?

To use await, its executing context needs to be async in nature
As it said, you need to define the nature of your executing context where you are willing to await a task before anything.
Just put async before the fn declaration in which your async task will execute.
var start = async function(a, b) {
// Your async task will execute with await
await foo()
console.log('I will execute after foo get either resolved/rejected')
}
Explanation:
In your question, you are importing a method which is asynchronous in nature and will execute in parallel. But where you are trying to execute that async method is inside a different execution context which you need to define async to use await.
var helper = require('./helper.js');
var start = async function(a,b){
....
const result = await helper.myfunction('test','test');
}
exports.start = start;
Wondering what's going under the hood
await consumes promise/future / task-returning methods/functions and async marks a method/function as capable of using await.
Also if you are familiar with promises, await is actually doing the same process of promise/resolve. Creating a chain of promise and executes your next task in resolve callback.
For more info you can refer to MDN DOCS.

When I got this error, it turned out I had a call to the map function inside my "async" function, so this error message was actually referring to the map function not being marked as "async". I got around this issue by taking the "await" call out of the map function and coming up with some other way of getting the expected behavior.
var myfunction = async function(x,y) {
....
someArray.map(someVariable => { // <- This was the function giving the error
return await someFunction(someVariable);
});
}

I had the same problem and the following block of code was giving the same error message:
repositories.forEach( repo => {
const commits = await getCommits(repo);
displayCommit(commits);
});
The problem is that the method getCommits() was async but I was passing it the argument repo which was also produced by a Promise. So, I had to add the word async to it like this: async(repo) and it started working:
repositories.forEach( async(repo) => {
const commits = await getCommits(repo);
displayCommit(commits);
});

If you are writing a Chrome Extension and you get this error for your code at root, you can fix it using the following "workaround":
async function run() {
// Your async code here
const beers = await fetch("https://api.punkapi.com/v2/beers");
}
run();
Basically you have to wrap your async code in an async function and then call the function without awaiting it.

The current implementation of async / await only supports the await keyword inside of async functions Change your start function signature so you can use await inside start.
var start = async function(a, b) {
}
For those interested, the proposal for top-level await is currently in Stage 2: https://github.com/tc39/proposal-top-level-await

async/await is the mechanism of handling promise, two ways we can do it
functionWhichReturnsPromise()
.then(result => {
console.log(result);
})
.cathc(err => {
console.log(result);
});
or we can use await to wait for the promise to full-filed it first, which means either it is rejected or resolved.
Now if we want to use await (waiting for a promise to fulfil) inside a function, it's mandatory that the container function must be an async function because we are waiting for a promise to fulfiled asynchronously || make sense right?.
async function getRecipesAw(){
const IDs = await getIds; // returns promise
const recipe = await getRecipe(IDs[2]); // returns promise
return recipe; // returning a promise
}
getRecipesAw().then(result=>{
console.log(result);
}).catch(error=>{
console.log(error);
});

If you have called async function inside foreach update it to for loop

Found the code below in this nice article: HTTP requests in Node using Axios
const axios = require('axios')
const getBreeds = async () => {
try {
return await axios.get('https://dog.ceo/api/breeds/list/all')
} catch (error) {
console.error(error)
}
}
const countBreeds = async () => {
const breeds = await getBreeds()
if (breeds.data.message) {
console.log(`Got ${Object.entries(breeds.data.message).length} breeds`)
}
}
countBreeds()
Or using Promise:
const axios = require('axios')
const getBreeds = () => {
try {
return axios.get('https://dog.ceo/api/breeds/list/all')
} catch (error) {
console.error(error)
}
}
const countBreeds = async () => {
const breeds = getBreeds()
.then(response => {
if (response.data.message) {
console.log(
`Got ${Object.entries(response.data.message).length} breeds`
)
}
})
.catch(error => {
console.log(error)
})
}
countBreeds()

In later nodejs (>=14), top await is allowed with { "type": "module" } specified in package.json or with file extension .mjs.
https://www.stefanjudis.com/today-i-learned/top-level-await-is-available-in-node-js-modules/

This in one file works..
Looks like await only is applied to the local function which has to be async..
I also am struggling now with a more complex structure and in between different files. That's why I made this small test code.
edit: i forgot to say that I'm working with node.js.. sry. I don't have a clear question. Just thought it could be helpful with the discussion..
function helper(callback){
function doA(){
var array = ["a ","b ","c "];
var alphabet = "";
return new Promise(function (resolve, reject) {
array.forEach(function(key,index){
alphabet += key;
if (index == array.length - 1){
resolve(alphabet);
};
});
});
};
function doB(){
var a = "well done!";
return a;
};
async function make() {
var alphabet = await doA();
var appreciate = doB();
callback(alphabet+appreciate);
};
make();
};
helper(function(message){
console.log(message);
});

A common problem in Express:
The warning can refer to the function, or where you call it.
Express items tend to look like this:
app.post('/foo', ensureLoggedIn("/join"), (req, res) => {
const facts = await db.lookup(something)
res.redirect('/')
})
Notice the => arrow function syntax for the function.
The problem is NOT actually in the db.lookup call, but right here in the Express item.
Needs to be:
app.post('/foo', ensureLoggedIn("/join"), async function (req, res) {
const facts = await db.lookup(something)
res.redirect('/')
})
Basically, nix the => and add async function .

"await is only valid in async function"
But why? 'await' explicitly turns an async call into a synchronous call, and therefore the caller cannot be async (or asyncable) - at least, not because of the call being made at 'await'.

Yes, await / async was a great concept, but the implementation is completely broken.
For whatever reason, the await keyword has been implemented such that it can only be used within an async method. This is in fact a bug, though you will not see it referred to as such anywhere but right here. The fix for this bug would be to implement the await keyword such that it can only be used TO CALL an async function, regardless of whether the calling function is itself synchronous or asynchronous.
Due to this bug, if you use await to call a real asynchronous function somewhere in your code, then ALL of your functions must be marked as async and ALL of your function calls must use await.
This essentially means that you must add the overhead of promises to all of the functions in your entire application, most of which are not and never will be asynchronous.
If you actually think about it, using await in a function should require the function containing the await keyword TO NOT BE ASYNC - this is because the await keyword is going to pause processing in the function where the await keyword is found. If processing in that function is paused, then it is definitely NOT asynchronous.
So, to the developers of javascript and ECMAScript - please fix the await/async implementation as follows...
await can only be used to CALL async functions.
await can appear in any kind of function, synchronous or asynchronous.
Change the error message from "await is only valid in async function" to "await can only be used to call async functions".

How can I wait for 2 promises to complete, to run another?

I'm trying to run two promises with:
Promise.all([...])
But they both have their own .then:
Promise.all([promise1().then(...), promise2().then(...)])
And I wish to run another .then on the Promise.all, while also waiting for both .then to return if that makes any sense.
Here's a fiddle showing what I mean.

If you run
function get1() {
return new Promise((r)=>setTimeout(() => r(),3000))
}
function rejection() {/*Handle rejection*/}
function doAll(...ps) {
return Promise.all(ps.map(rejection))
}
(async () => {
var p1 = get1().then(()=>console.log("1"));
var p2 = get1().then(()=>console.log("2"));
Promise.all([p1, p2]).then(()=>{
console.log("3")
})
})()
then the result is the correct
1
2
3
If you run
function get1() {
return new Promise((r)=>setTimeout(() => r(),3000))
}
function rejection() {/*Handle rejection*/}
function doAll(...ps) {
return Promise.all(ps)
}
(async () => {
var p1 = get1().then(()=>console.log("1"));
var p2 = get1().then(()=>console.log("2"));
doAll(p1, p2).then(()=>{
console.log("3")
})
})()
then you again get the correct
1
2
3
As a result, the problem is with the part of ps.map(rejection). Let's see:
function get1() {
return new Promise((r)=>setTimeout(() => r(),3000))
}
function rejection() {/*Handle rejection*/}
function doAll(...ps) {
console.log(ps);
console.log(ps.map(rejection));
return Promise.all(ps.map(rejection));
}
(async () => {
var p1 = get1().then(()=>console.log("1"));
var p2 = get1().then(()=>console.log("2"));
doAll(p1, p2).then(()=>{
console.log("3")
})
})()
Output
The array of two elements, both being undefined is trivial to evaluate. Because ps.map(rejection) is an arrow function that names its parameter rejection and does not return anything.

if you want to work with async operation in javascript, there are 3 ways that you can use.
callback
Promise
async/await
In order to implement the exact thing that you want, the best and the optimized way is using the async/await method.
you can do it like this :
async function getAllData(){
const p1 = await promise1;
const p2 = await promise2;
}
now getAllData return Promise, you can use .then() to get the result and .catch() to get errors.
to read more about syntax and other features you can go to this site : explain about async and await

await is only valid in async function

I wrote this code in lib/helper.js:
var myfunction = async function(x,y) {
....
return [variableA, variableB]
}
exports.myfunction = myfunction;
Then I tried to use it in another file :
var helper = require('./helper.js');
var start = function(a,b){
....
const result = await helper.myfunction('test','test');
}
exports.start = start;
I got an error:
await is only valid in async function
What is the issue?

The error is not refering to myfunction but to start.
async function start() {
....
const result = await helper.myfunction('test', 'test');
}
// My function
const myfunction = async function(x, y) {
return [
x,
y,
];
}
// Start function
const start = async function(a, b) {
const result = await myfunction('test', 'test');
console.log(result);
}
// Call start
start();
I use the opportunity of this question to advise you about an known anti pattern using await which is : return await.
WRONG
async function myfunction() {
console.log('Inside of myfunction');
}
// Here we wait for the myfunction to finish
// and then returns a promise that'll be waited for aswell
// It's useless to wait the myfunction to finish before to return
// we can simply returns a promise that will be resolved later
// useless async here
async function start() {
// useless await here
return await myfunction();
}
// Call start
(async() => {
console.log('before start');
await start();
console.log('after start');
})();
CORRECT
async function myfunction() {
console.log('Inside of myfunction');
}
// Here we wait for the myfunction to finish
// and then returns a promise that'll be waited for aswell
// It's useless to wait the myfunction to finish before to return
// we can simply returns a promise that will be resolved later
// Also point that we don't use async keyword on the function because
// we can simply returns the promise returned by myfunction
function start() {
return myfunction();
}
// Call start
(async() => {
console.log('before start');
await start();
console.log('after start');
})();
Also, know that there is a special case where return await is correct and important : (using try/catch)
Are there performance concerns with `return await`?

To use await, its executing context needs to be async in nature
As it said, you need to define the nature of your executing context where you are willing to await a task before anything.
Just put async before the fn declaration in which your async task will execute.
var start = async function(a, b) {
// Your async task will execute with await
await foo()
console.log('I will execute after foo get either resolved/rejected')
}
Explanation:
In your question, you are importing a method which is asynchronous in nature and will execute in parallel. But where you are trying to execute that async method is inside a different execution context which you need to define async to use await.
var helper = require('./helper.js');
var start = async function(a,b){
....
const result = await helper.myfunction('test','test');
}
exports.start = start;
Wondering what's going under the hood
await consumes promise/future / task-returning methods/functions and async marks a method/function as capable of using await.
Also if you are familiar with promises, await is actually doing the same process of promise/resolve. Creating a chain of promise and executes your next task in resolve callback.
For more info you can refer to MDN DOCS.

When I got this error, it turned out I had a call to the map function inside my "async" function, so this error message was actually referring to the map function not being marked as "async". I got around this issue by taking the "await" call out of the map function and coming up with some other way of getting the expected behavior.
var myfunction = async function(x,y) {
....
someArray.map(someVariable => { // <- This was the function giving the error
return await someFunction(someVariable);
});
}

I had the same problem and the following block of code was giving the same error message:
repositories.forEach( repo => {
const commits = await getCommits(repo);
displayCommit(commits);
});
The problem is that the method getCommits() was async but I was passing it the argument repo which was also produced by a Promise. So, I had to add the word async to it like this: async(repo) and it started working:
repositories.forEach( async(repo) => {
const commits = await getCommits(repo);
displayCommit(commits);
});

If you are writing a Chrome Extension and you get this error for your code at root, you can fix it using the following "workaround":
async function run() {
// Your async code here
const beers = await fetch("https://api.punkapi.com/v2/beers");
}
run();
Basically you have to wrap your async code in an async function and then call the function without awaiting it.

The current implementation of async / await only supports the await keyword inside of async functions Change your start function signature so you can use await inside start.
var start = async function(a, b) {
}
For those interested, the proposal for top-level await is currently in Stage 2: https://github.com/tc39/proposal-top-level-await

async/await is the mechanism of handling promise, two ways we can do it
functionWhichReturnsPromise()
.then(result => {
console.log(result);
})
.cathc(err => {
console.log(result);
});
or we can use await to wait for the promise to full-filed it first, which means either it is rejected or resolved.
Now if we want to use await (waiting for a promise to fulfil) inside a function, it's mandatory that the container function must be an async function because we are waiting for a promise to fulfiled asynchronously || make sense right?.
async function getRecipesAw(){
const IDs = await getIds; // returns promise
const recipe = await getRecipe(IDs[2]); // returns promise
return recipe; // returning a promise
}
getRecipesAw().then(result=>{
console.log(result);
}).catch(error=>{
console.log(error);
});

If you have called async function inside foreach update it to for loop

Found the code below in this nice article: HTTP requests in Node using Axios
const axios = require('axios')
const getBreeds = async () => {
try {
return await axios.get('https://dog.ceo/api/breeds/list/all')
} catch (error) {
console.error(error)
}
}
const countBreeds = async () => {
const breeds = await getBreeds()
if (breeds.data.message) {
console.log(`Got ${Object.entries(breeds.data.message).length} breeds`)
}
}
countBreeds()
Or using Promise:
const axios = require('axios')
const getBreeds = () => {
try {
return axios.get('https://dog.ceo/api/breeds/list/all')
} catch (error) {
console.error(error)
}
}
const countBreeds = async () => {
const breeds = getBreeds()
.then(response => {
if (response.data.message) {
console.log(
`Got ${Object.entries(response.data.message).length} breeds`
)
}
})
.catch(error => {
console.log(error)
})
}
countBreeds()

In later nodejs (>=14), top await is allowed with { "type": "module" } specified in package.json or with file extension .mjs.
https://www.stefanjudis.com/today-i-learned/top-level-await-is-available-in-node-js-modules/

This in one file works..
Looks like await only is applied to the local function which has to be async..
I also am struggling now with a more complex structure and in between different files. That's why I made this small test code.
edit: i forgot to say that I'm working with node.js.. sry. I don't have a clear question. Just thought it could be helpful with the discussion..
function helper(callback){
function doA(){
var array = ["a ","b ","c "];
var alphabet = "";
return new Promise(function (resolve, reject) {
array.forEach(function(key,index){
alphabet += key;
if (index == array.length - 1){
resolve(alphabet);
};
});
});
};
function doB(){
var a = "well done!";
return a;
};
async function make() {
var alphabet = await doA();
var appreciate = doB();
callback(alphabet+appreciate);
};
make();
};
helper(function(message){
console.log(message);
});

A common problem in Express:
The warning can refer to the function, or where you call it.
Express items tend to look like this:
app.post('/foo', ensureLoggedIn("/join"), (req, res) => {
const facts = await db.lookup(something)
res.redirect('/')
})
Notice the => arrow function syntax for the function.
The problem is NOT actually in the db.lookup call, but right here in the Express item.
Needs to be:
app.post('/foo', ensureLoggedIn("/join"), async function (req, res) {
const facts = await db.lookup(something)
res.redirect('/')
})
Basically, nix the => and add async function .

"await is only valid in async function"
But why? 'await' explicitly turns an async call into a synchronous call, and therefore the caller cannot be async (or asyncable) - at least, not because of the call being made at 'await'.

Yes, await / async was a great concept, but the implementation is completely broken.
For whatever reason, the await keyword has been implemented such that it can only be used within an async method. This is in fact a bug, though you will not see it referred to as such anywhere but right here. The fix for this bug would be to implement the await keyword such that it can only be used TO CALL an async function, regardless of whether the calling function is itself synchronous or asynchronous.
Due to this bug, if you use await to call a real asynchronous function somewhere in your code, then ALL of your functions must be marked as async and ALL of your function calls must use await.
This essentially means that you must add the overhead of promises to all of the functions in your entire application, most of which are not and never will be asynchronous.
If you actually think about it, using await in a function should require the function containing the await keyword TO NOT BE ASYNC - this is because the await keyword is going to pause processing in the function where the await keyword is found. If processing in that function is paused, then it is definitely NOT asynchronous.
So, to the developers of javascript and ECMAScript - please fix the await/async implementation as follows...
await can only be used to CALL async functions.
await can appear in any kind of function, synchronous or asynchronous.
Change the error message from "await is only valid in async function" to "await can only be used to call async functions".

why Javascript async/await code run async in parellel

I saw this good article to introduce async/await from Google.
However, I couldn't understand why these code run in parallel
async function parallel() {
const wait1 = wait(500);
const wait2 = wait(500);
await wait1;
await wait2;
return "done!";
}
And this run in series
async function series() {
await wait(500);
await wait(500);
return "done!";
}
Why is the key difference between these two methods ?
In my opinion, both of them are await promise and should work the same result.
Here is my test code. Can run in browser console which support async/await.
function wait(){
return new Promise((res)=>{setTimeout(()=>{res()}, 2000)})
}
async function parallel() {
const wait1 = wait();
const wait2 = wait();
await wait1;
await wait2;
return "done!";
}
async function series() {
await wait();
await wait();
return "done!";
}
parallel().then(res => console.log("parallel!"))
series().then(res => console.log("series!"))
======
Thanks for the answers.
But I still have some question. Why exact the async/await means?
In my knowledge, constructing Promise instance would execute directly.
Here is my test code
function wait(){
return new Promise((res)=>{setTimeout(()=>{console.log("wait!");res();}, 2000)})
}
wait()
//Promise {<pending>}
//wait!
let w = wait()
//undefined
//wait!
let w = await wait()
//wait!
//undefined
async function test(){await wait()}
// undefined
test()
//Promise {<pending>}
//wait!
So why const wait1 = wait(); inside parallel function execute directly?
By the way, should I open another question to ask these question?

await doesn't cause the Promise or its setTimeout() to start, which seems to be what you're expecting. The call to wait() alone starts them immediately, whether there's an active await on the promise or not.
await only helps you know when the already on-going operation, tracked through the promise, has completed.
So, the difference is just due to when wait() is being called and starting each timeout:
parallel() calls wait() back-to-back as quickly as the engine can get from one to the next, before either are awaited, so the 2 timeouts begin/end at nearly the same time.
series() forces the 2nd wait() to not be called until after the 1st has completed, by having an await act in between them.
Regarding your edit, async and await are syntactic sugar. Using them, the engine will modify your function at runtime, inserting additional code needed to interact with the promises.
A (possible, but not precise) equivalent of series() without await and async might be:
function series() {
return Promise.resolve()
.then(function () { return wait(500) })
.then(function () { return wait(500) })
.then(function () { return "done!"; });
}
And for parallel():
function parallel() {
const wait1 = wait(500);
const wait2 = wait(500);
return Promise.resolve()
.then(wait1)
.then(wait2)
.then(function () { return "done!"; });
}

In parallel(), you call both methods and then await their results while in series() you await the result of the first wait() call before calling the second wait().
Why exact the async/await means? In my knowledge, constructing Promise instance would execute directly.
The Promise instance is returned immediately, i.e. synchronously. But the value of the Promise is evaluated asynchronously by calling the first parameter given to its constructor, a function usually called resolve. This is what you are awaiting for.

The difference is that in parallel, both of the promises are scheduled right after each other.
In series, you wait for the first wait to resolve and then schedule the second wait to be called.
If we expand the methods we'd get something similar to:
function parallel() {
const wait1 = wait();
const wait2 = wait();
// Both wait1 and wait2 timeouts are scheduled
return wait1
.then(() => wait2)
.then(() => "done");
}
function series() {
// The second wait happens _after_ the first has been *resolved*
return wait()
.then(() => wait())
.then(() => "done");
}