Related
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".
I have the following situation:
Why does the clients variable resolve in undefined when I call it below;
function getClientList() {
TeamSpeak.connect({
host: "localhost",
serverport: 9987,
nickname: "NodeJS Query Framework",
}).then(async (teamspeak) => {
const clients = await teamspeak.clientList({ clientType: 0 });
if (clients) return clients;
else return null;
});
}
When I call it like that it resolves as undefined
const clients = getClientList();
The short answer to your question is that you cannot use a resulting variable until the Promise resolves. Whether you use .then or await it's really the same mechanism and you can't access the result of your call until it's ready.
You can only get the results of it by waiting for the Promise to resolve. This can be done by using await if your function is marked as async, or by using the .then() function on the Promise since it will get called after the result is ready.
Think of the Promise as a black box that has a then function in it. Remember that each .then() function also returns as a Promise object which is why you can chain them as promise.then(...).then(...).then(...);. Your code then looks like this:
function getClientList() {
PROMISE1.then(someFunction);
}
const clients = getClientList();
Note that even though it spans multiple lines, your getClientList function is really just one line of code TeamSpeak.connect(...).then(...);
There are a few things wrong with your code. Let's go through them one by one:
The getClientList function doesn't return anything explicitly. In its most basic form it's function getClientList() { someCall(); } and that returns undefined by default. That's why clients is always undefined.
If you add a return statement to the function, it will return a Promise and might look like this:
function getClientList() {
return PROMISE1.then(someFunction);
}
const clients = getClientList();
This is better because clients is not undefined any more, but it will be the Promise that may not have finished its work yet. To see the result of the call you'd need to use the then function like clients.then(result => console.log(result)) to allow it to wait for the asynchronous work to complete.
You're using both .then and async/await. It will probably be cleaner and easier to work with if you go with one or the other (await is my preference). Your code can look like this:
async function getClientList() {
const teamspeak = await TeamSpeak.connect({
host: "localhost",
serverport: 9987,
nickname: "NodeJS Query Framework",
});
const clients = await teamspeak.clientList({ clientType: 0 });
return clients || null; // returns as a Promise resolving to clients or null
}
const clients = await getClientList(); // assuming you're in another async fn
There is no error handling. If any of the asynchronous calls fail then the Promise will either reject or cause an exception. You should handle that by either surrounding it with a try/catch block at some level, or by adding .catch() after the final then, or by adding a second function to the .then() call to handle a rejection.
So putting it all together your code could look like this:
async function getClientList() {
const teamspeak = await TeamSpeak.connect({
host: "localhost",
serverport: 9987,
nickname: "NodeJS Query Framework",
});
return await teamspeak.clientList({ clientType: 0 }) || null;
}
// Example calling getClientList using ".then"
getClientList()
.then(
client => {
/* use the client value in this scope */
},
reason => {/* report the reason for rejection */}
) // end of .then
.catch(err => {/* report the error */});
// Example calling getClientList using "await"
try {
const client = await getClientList();
/* use the client value in this scope */
} catch (err) {/* report the error */}
No matter how you make the call, you're only able to access the clients value (returned by calling the clientList function) within the scope of either the .then or after await.
When you call getClientList() the inner "connect" function is started instantly in your library while the "then" callback is left to be executed in a future you can't know (when your library has completed the task triggered in "connect").
Preparing this "then" callback is done instantly, so getClientList() ends without returning anything.
The "return" instructions inside "then" are not returns for getClientList() but for the one who executes the callback (the code in your library).
What you can do is promissify the process: Return a promise that can be waited for and will be solved in the future when callback has been executed.
Try it like this:
function getClientList() {
// Tell the function caller that he will have the result at some time in the future.
// In other words: return a promise.
return new Promise( (resolve,reject) => {
TeamSpeak.connect({
host: "localhost",
serverport: 9987,
nickname: "NodeJS Query Framework",
}).then(async (teamspeak) => {
const clients = await teamspeak.clientList({ clientType: 0 });
// Give the caller that is waiting by holding the promise the desired result.
if (clients) resolve(clients);
// Throw error to the caller that is waiting by holding the promise .
else reject('Connect failed');
});
});
}
But notice you must await for the promise inside an "async" function. Otherwise you would block the main thread.
async function main() {
const clients = await getClientList();
}
Do set a try/catch as the promise might throw an error if "reject" is executed:
async function main() {
try{
const clients = await getClientList();
} catch(err){
console.log(err);
// Will show:
// Connect failed
}
}
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 to use the asynchronous Await in Node.js by using these function and how
The request.get() function returns a Promise by which user will await...
I have tried the Below code so far and also gave the explanation below
async function fun1(req, res){
let response = await request.get('http://localhost:3000');
if (response.err) { console.log('error');}
else { console.log('fetched response');
}
The code above basically asks the javascript engine running the code to wait for the request.get() function to complete before moving on to the next line to execute it. The request.get() function returns a Promise for which user will await . Before async/await, if it needs to be made sure that the functions are running in the desired sequence, that is one after the another, chain them one after the another or register callbacks.
async function fun1(req, res){
let response = await request.get('http://localhost:3000');
if (response.err) { console.log('error');}
else { console.log('fetched response');
}
request package does not use return promise. Use the request-promise package which wraps the request with Promise.
You can use it like:
const rp = require('request-promise')
async function getSomeData() {
try {
const url = 'http://some.com'
// waits for promise to resolve
const data = await rp(url)
// data contains resolved value if successfull
// continue some other stuff
...
} catch (e) {
// handle error if error occurred
console.error(e)
}
}
I'm trying to write a node module, to handle my various db calls.
I want to use async/await where ever I can, but I'm having some issues with it.
I've been using promises a bit, and export those functions fine.
Example:
function GetUsernames() {
return new Promise(function (resolve, reject) {
sql.connect(config).then(function () {
new sql.Request()
.query("SELECT [UserName] FROM [Users] ORDER BY [LastLogin] ASC").then(function (recordset) {
resolve(recordset);
}).catch(function (err) {
reject(err);
});
});
});
}
And then I export in the following:
module.exports = {
GetUsernames: GetUsernames,
GetScopes: GetScopes,
UpdateToken: UpdateToken,
SetOwner: SetOwner
};
But, how should I do this, with an async function, to use the async/await that is available in node7?
Do I still just return a promise? I tried doing that, but when I then call it in my code, it doesn't work.
const db = require("dataprovider");
...
var result = await db.GetUsernames();
It gives me:
SyntaxError: Unexpected identifier
on the db name (works fine if I just use the promise functions, with then().)
Maybe my google skills are terrible, but I haven't managed to google anything I could use, on this issue.
How on earth do I make an async function, in my module, that I can await elsewhere?
To turn on the await keyword, you need to have it inside an async function.
const db = require("dataprovider");
...
let result = getUserNames();
async function getUserNames() {
return await db.GetUsernames();
}
See http://javascriptrambling.blogspot.com/2017/04/to-promised-land-with-asyncawait-and.html for more information.
Also, just as an FYI, it a code convention to start function with lowercase, unless you are returning a class from it.
async - await pattern really makes your code easier to read. But node do not allow for global awaits. You can only await an asynchronous flow. What you trying to do is to await outside async flow. This is not permitted. This is kind of anti-pattern for node application. When dealing with promises, what we actually do is generate an asynchronous flow in our program. Our program continue without waiting for promise. So you can export your functions as async but cannot await for them outside async flow. for example.
const db = require('dataprovider');
...
let result = (async () => await db.GetUserNames())();
console.log(result); // outputs: Promise { <pending> }
Thus, async-await pattern works for async flow. Thus use them inside async functions, so that node can execute them asynchronously. Also, you can await for Promises as well. for example.
let fn = async () => await new Promise( (resolve, reject)=>{...} );
fn().then(...);
Here you have created async function 'fn'. This function is thenable. also you can await for 'fn' inside another async function.
let anotherFn = async () => await fn();
anotherFn().then(...);
Here we are waiting for async function inside a async function. Async-Await pattern makes your code readable and concise. This is reflected in large projects.
lets say that you have below function in baseoperation.ts
async function GetAllwithFilter(keyword?: any){
//... async job here
}
then simply you can add below code on the end of baseoperation.ta
module.exports = { GetAllOrAnyName: GetAllwithFilter}
and in the required class you can call it using
const objname = require('./baseoperation');
and to call it in the api using express and router you can use route function with get because it's accept promise call direct
router.route('/').get((req,res) => { objname.GetUsernames().then(then(result => {
res.json(result[0]);
});