I am using async/await to return the Promise , to use it as promoise in node script. When I am trying to make use of the return value as Promise , its giving a error a.then is not a function
here is the sample code
function test () {
//do something .......
//....
return global.Promise;
}
(async ()=> {
let a = await test();
a.then(()=> { console.log('good ')}, (err)=> { console.log()});
})();
The Promise constructor function isn't a promise, it is a tool to make promises with.
Even if it was a promise, since you are awaiting the return value of test, it would have been resolved into a value before you try to call then on it. (The point of await is that it replaces the use of then() callbacks).
You can await a function that returns a promise like this:
function test() {
return new Promise((resolve, reject) => {
if (true) {
reject("Custom error message");
}
setTimeout(() => {
resolve(56)
}, 200);
})
}
async function main() {
try {
const a = await test();
console.log(a)
} catch (e) { // this handles the "reject"
console.log(e);
}
}
main();
If you change the true to false you can test the "resolve" case.
await retrieves the resolved value from a Promise
let a = await test(); // `a` is no longer a Promise
I've put together two ways of retrieving values from a Promise
using await
(async () => {
try {
let a = await test();
console.log('Good', a);
} catch(err) {
console.log(err);
}
})();
using .then()
test().then(a => {
console.log('Good', a);
}).catch(err => {
console.log(err);
});
Please note that, the async arrow function is removed because there is no await needed.
Related
I'm learning to use async/await and this is quite confused to me. As I know, async functions always return a promise. If the return value of an async function is not explicitly a promise, it will be implicitly wrapped in a promise.
async function foo() {
setTimeout(() => {
throw new Error('error');
}, 5000);
}
async function test() {
try {
await foo();
} catch (err) {
console.log(err);
}
console.log('not reached');
}
test();
Expected output: log the error after 5 secs, then log "not reach".
What actually happened: "not reach" gets logged right after I start the script, then the error gets logged after 5 seconds.
Attempts: the following code does exactly what I want, but I don't know what's the difference between those code.
Environment: nodejs 16.14.2
function foo() {
return new Promise((resolve, reject) => {
setTimeout(() => {
reject(new Error('foo'));
}, 5000);
});
}
async function test() {
try {
await foo();
} catch (err) {
console.log(err);
}
console.log('not reached');
}
test();
Thanks a lot!
foo needs to actually resolve
Your example doesn't work because foo doesn't actually care about the timeout. It just starts the timeout then returns immediately. What you need to do is make foo resolve when the timeout fires, which actually requires you to construct a Promise explicitly.
async function foo() {
return new Promise(resolve => {
setTimeout(resolve, 5000)
})
}
Since you return a Promise, you could technically not declare foo as async, but it doesn't matter, and (I think) it's still helpful for other developers to see that the function returns a Promise without having to look through the implementation.
In your second example, foo returns a promise, setTimeout needs to produce a promise for it to work in your example.
For example
async function foo() {
// promise returned
return new Promise(resolve => {
setTimeout(() => { resolve() } , 5000)
})
}
I'm using the async.eachLimit function to control the maximum number of operations at a time.
const { eachLimit } = require("async");
function myFunction() {
return new Promise(async (resolve, reject) => {
eachLimit((await getAsyncArray), 500, (item, callback) => {
// do other things that use native promises.
}, (error) => {
if (error) return reject(error);
// resolve here passing the next value.
});
});
}
As you can see, I can't declare the myFunction function as async because I don't have access to the value inside the second callback of the eachLimit function.
You're effectively using promises inside the promise constructor executor function, so this the Promise constructor anti-pattern.
Your code is a good example of the main risk: not propagating all errors safely. Read why there.
In addition, the use of async/await can make the same traps even more surprising. Compare:
let p = new Promise(resolve => {
""(); // TypeError
resolve();
});
(async () => {
await p;
})().catch(e => console.log("Caught: " + e)); // Catches it.
with a naive (wrong) async equivalent:
let p = new Promise(async resolve => {
""(); // TypeError
resolve();
});
(async () => {
await p;
})().catch(e => console.log("Caught: " + e)); // Doesn't catch it!
Look in your browser's web console for the last one.
The first one works because any immediate exception in a Promise constructor executor function conveniently rejects the newly constructed promise (but inside any .then you're on your own).
The second one doesn't work because any immediate exception in an async function rejects the implicit promise returned by the async function itself.
Since the return value of a promise constructor executor function is unused, that's bad news!
Your code
There's no reason you can't define myFunction as async:
async function myFunction() {
let array = await getAsyncArray();
return new Promise((resolve, reject) => {
eachLimit(array, 500, (item, callback) => {
// do other things that use native promises.
}, error => {
if (error) return reject(error);
// resolve here passing the next value.
});
});
}
Though why use outdated concurrency control libraries when you have await?
I agree with the answers given above and still, sometimes it's neater to have async inside your promise, especially if you want to chain several operations returning promises and avoid the then().then() hell. I would consider using something like this in that situation:
const operation1 = Promise.resolve(5)
const operation2 = Promise.resolve(15)
const publishResult = () => Promise.reject(`Can't publish`)
let p = new Promise((resolve, reject) => {
(async () => {
try {
const op1 = await operation1;
const op2 = await operation2;
if (op2 == null) {
throw new Error('Validation error');
}
const res = op1 + op2;
const result = await publishResult(res);
resolve(result)
} catch (err) {
reject(err)
}
})()
});
(async () => {
await p;
})().catch(e => console.log("Caught: " + e));
The function passed to Promise constructor is not async, so linters don't show errors.
All of the async functions can be called in sequential order using await.
Custom errors can be added to validate the results of async operations
The error is caught nicely eventually.
A drawback though is that you have to remember putting try/catch and attaching it to reject.
BELIEVING IN ANTI-PATTERNS IS AN ANTI-PATTERN
Throws within an async promise callback can easily be caught.
(async () => {
try {
await new Promise (async (FULFILL, BREAK) => {
try {
throw null;
}
catch (BALL) {
BREAK (BALL);
}
});
}
catch (BALL) {
console.log ("(A) BALL CAUGHT", BALL);
throw BALL;
}
}) ().
catch (BALL => {
console.log ("(B) BALL CAUGHT", BALL);
});
or even more simply,
(async () => {
await new Promise (async (FULFILL, BREAK) => {
try {
throw null;
}
catch (BALL) {
BREAK (BALL);
}
});
}) ().
catch (BALL => {
console.log ("(B) BALL CAUGHT", BALL);
});
I didn't realized it directly by reading the other answers, but what is important is to evaluate your async function to turn it into a Promise.
So if you define your async function using something like:
let f = async () => {
// ... You can use await, try/catch, throw syntax here (see answer of Vladyslav Zavalykhatko) ..
};
your turn it into a promise using:
let myPromise = f()
You can then manipulate is as a Promise, using for instance Promise.all([myPromise])...
Of course, you can turn it into a one liner using:
(async () => { code with await })()
static getPosts(){
return new Promise( (resolve, reject) =>{
try {
const res = axios.get(url);
const data = res.data;
resolve(
data.map(post => ({
...post,
createdAt: new Date(post.createdAt)
}))
)
} catch (err) {
reject(err);
}
})
}
remove await and async will solve this issue. because you have applied Promise object, that's enough.
I am having an issue with Mocha returning an incorrect result when testing a function that uses a promise. Not sure why it is returning "undefined" - the function being called (testFunction()) shouldn't be returning until the promise is completed.
Test:
it('test1', function() {
let testResult = testFunction();
assert.equal(testResult,'success');
});
Promise:
getPromise() {
return new Promise((resolve, reject) => {
resolve('success');
}
}
testFunction()
testFunction = () => {
getPromise().then(result => { return result}
).catch(error =>{ return error });
}
Result:
AssertionError [ERR_ASSERTION]: undefined == 'success'
testFunction does not have a return statement in it, and so it is returning undefined. The returns in the callbacks only determine what the callbacks return, not what testFunction returns.
Since you're dealing with promises, you will have to return a promise. You can't return a string, because that string doesn't exist yet. So, testFunction might look like this:
testFunction = () => {
return getPromise()
.then(result => { return result })
.catch(error => { return error });
}
But this .then is not doing anything, so unless there's some extra code you didn't mention, it can be removed:
testFunction = () => {
return getPromise()
.catch(error => { return error; });
}
Your test will need to be written to expect a promise to be returned. This is easiest if you use an async function, as in:
it('test1', async function () {
let testResult = await testFunction();
asset.equal(testResult, 'success');
});
Alternatively, you can use a done callback to tell the test when your async work is finished.
it('test1', function (done) {
let promise = testFunction();
promise.then(testResult => {
assert.equal(testResult, 'success');
done();
});
});
I'm trying to understand JavaScript async/await. How can I rewrite the below such that the output is "Hi" then "Bye" instead of "Bye" then "Hi":
JSFiddle
sayHi()
.then(sayBye);
async function sayHi() {
await setTimeout(function() {
$("#myOutput").text('hi');
}, 1000);
}
async function sayBye() {
$("#myOutput").text('bye');
}
In order to await setTimeout it needs to be wrapped into Promise. Then with async/await you can flatten your code write it without Promise then API:
(async () => { // await has to be inside async function, anonymous in this case
await sayHi()
sayBye()
})()
async function sayHi() {
return new Promise(function (resolve) {
$("#myOutput").text('hi');
setTimeout(function() {
resolve()
}, 1000)
});
}
async function sayBye() {
$("#myOutput").text('bye');
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="myOutput"></div>
setTimeout doesn't return a Promise. Create a helper function to wrap it in a Promise and then you can await it.
function delay(fn, t) {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(fn());
}, t);
});
}
sayHi()
.then(sayBye);
async function sayHi() {
await delay(() => {
//$("#myOutput").text('hi');
console.log("Hi");
}, 1000);
}
async function sayBye() {
//$("#myOutput").text('bye');
console.log("Bye");
}
Use the Promise way
sayHi()
.then(sayBye);
function sayHi() {
return new Promise(resolve => {
setTimeout(()=> {
$("#myOutput").text('hi'), resolve()
}, 1000);
})
}
async function sayBye() {
$("#myOutput").text('bye');
}
or the sayHi like this:
async function sayHi() {
await new Promise(resolve => {
setTimeout(()=> {
$("#myOutput").text('hi'), resolve()
}, 1000)
})
}
Using async/await is an excellent way to build acynchronous code in a quite controllable way. Promise based async function goes into microtasks depository, which event loop executes before events and methods contained in ordinary DOM refresh/web API depository (setTimeout() for example). However some versions of Opera and Firefox browsers set priority to setTimeout() over microtasks depository. Anyway, you can control order of execution if you combine Promise based function with async/await enabled function. For example:
// create function that returns Promise
let hi = () => {
return new Promise((resolve, reject) => {
setTimeout(_ => {
resolve('Hi '); // after 1500ms function is resolved and returns 'Hi '
}, 1500);
});
}
// create ordinary function that will return 'bye'
let sayBye = () => {
return 'Bye';
}
// create third function that will be async 'enabled',
// so it can use await keyword before Promise based functions
let sayHi = async () => {
let first = await hi(); // we store 'Hi ' into 'first' variable
console.log(first);
let second = sayBye(); // this execution will await until hi() is finished
console.log(second);
}
// execute async enabled function
sayHi();
We can add try / catch block inside sayHi() function for controlling error on promise reject(), but this is out of scope of your question.
Have a nice day!
You cannot use async/await for functions that are not returning Promise
When an async function is called, it returns a Promise. When the async function returns a value, the Promise will be resolved with the returned value. When the async function throws an exception or some value, the Promise will be rejected with the thrown value.
An async function can contain an await expression, that pauses the execution of the async function and waits for the passed Promise's resolution, and then resumes the async function's execution and returns the resolved value.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function
Usually it is used to handle data that comes from server, because when you have multiple queries it can override previous one and you will handle the wrong one.
Async/await lets you handle exactly data you are awaiting for.
I'm using the async.eachLimit function to control the maximum number of operations at a time.
const { eachLimit } = require("async");
function myFunction() {
return new Promise(async (resolve, reject) => {
eachLimit((await getAsyncArray), 500, (item, callback) => {
// do other things that use native promises.
}, (error) => {
if (error) return reject(error);
// resolve here passing the next value.
});
});
}
As you can see, I can't declare the myFunction function as async because I don't have access to the value inside the second callback of the eachLimit function.
You're effectively using promises inside the promise constructor executor function, so this the Promise constructor anti-pattern.
Your code is a good example of the main risk: not propagating all errors safely. Read why there.
In addition, the use of async/await can make the same traps even more surprising. Compare:
let p = new Promise(resolve => {
""(); // TypeError
resolve();
});
(async () => {
await p;
})().catch(e => console.log("Caught: " + e)); // Catches it.
with a naive (wrong) async equivalent:
let p = new Promise(async resolve => {
""(); // TypeError
resolve();
});
(async () => {
await p;
})().catch(e => console.log("Caught: " + e)); // Doesn't catch it!
Look in your browser's web console for the last one.
The first one works because any immediate exception in a Promise constructor executor function conveniently rejects the newly constructed promise (but inside any .then you're on your own).
The second one doesn't work because any immediate exception in an async function rejects the implicit promise returned by the async function itself.
Since the return value of a promise constructor executor function is unused, that's bad news!
Your code
There's no reason you can't define myFunction as async:
async function myFunction() {
let array = await getAsyncArray();
return new Promise((resolve, reject) => {
eachLimit(array, 500, (item, callback) => {
// do other things that use native promises.
}, error => {
if (error) return reject(error);
// resolve here passing the next value.
});
});
}
Though why use outdated concurrency control libraries when you have await?
I agree with the answers given above and still, sometimes it's neater to have async inside your promise, especially if you want to chain several operations returning promises and avoid the then().then() hell. I would consider using something like this in that situation:
const operation1 = Promise.resolve(5)
const operation2 = Promise.resolve(15)
const publishResult = () => Promise.reject(`Can't publish`)
let p = new Promise((resolve, reject) => {
(async () => {
try {
const op1 = await operation1;
const op2 = await operation2;
if (op2 == null) {
throw new Error('Validation error');
}
const res = op1 + op2;
const result = await publishResult(res);
resolve(result)
} catch (err) {
reject(err)
}
})()
});
(async () => {
await p;
})().catch(e => console.log("Caught: " + e));
The function passed to Promise constructor is not async, so linters don't show errors.
All of the async functions can be called in sequential order using await.
Custom errors can be added to validate the results of async operations
The error is caught nicely eventually.
A drawback though is that you have to remember putting try/catch and attaching it to reject.
BELIEVING IN ANTI-PATTERNS IS AN ANTI-PATTERN
Throws within an async promise callback can easily be caught.
(async () => {
try {
await new Promise (async (FULFILL, BREAK) => {
try {
throw null;
}
catch (BALL) {
BREAK (BALL);
}
});
}
catch (BALL) {
console.log ("(A) BALL CAUGHT", BALL);
throw BALL;
}
}) ().
catch (BALL => {
console.log ("(B) BALL CAUGHT", BALL);
});
or even more simply,
(async () => {
await new Promise (async (FULFILL, BREAK) => {
try {
throw null;
}
catch (BALL) {
BREAK (BALL);
}
});
}) ().
catch (BALL => {
console.log ("(B) BALL CAUGHT", BALL);
});
I didn't realized it directly by reading the other answers, but what is important is to evaluate your async function to turn it into a Promise.
So if you define your async function using something like:
let f = async () => {
// ... You can use await, try/catch, throw syntax here (see answer of Vladyslav Zavalykhatko) ..
};
your turn it into a promise using:
let myPromise = f()
You can then manipulate is as a Promise, using for instance Promise.all([myPromise])...
Of course, you can turn it into a one liner using:
(async () => { code with await })()
static getPosts(){
return new Promise( (resolve, reject) =>{
try {
const res = axios.get(url);
const data = res.data;
resolve(
data.map(post => ({
...post,
createdAt: new Date(post.createdAt)
}))
)
} catch (err) {
reject(err);
}
})
}
remove await and async will solve this issue. because you have applied Promise object, that's enough.