I need to make two post requests in succession. There is an async "post form" function which starts the request with the "await" keyword and the result is handled in the "then" block.
I try to do this post request by awaiting a promise, wherein I call the above mentioned async function. The problem is that it doesn't seem that the promise can be resolved inside the functions "then" block.
I have some example code to show the issue (look at start() function where it starts). See fiddle
//Returns a promise. It contains another async-await promise
function mainFunction() {
return new Promise( () => { //<-- this never resolves
async function postForm(){
//(Post a form)
await new Promise( (resolve, reject) => {
setTimeout(() => {
resolve('resolved');
}, 2000);
}).then( response => { // <-- how to resolve the first promise inside here?
console.log('promise 2 response: ',response)
//Try to resolve the 1st promise (doesn't work)
new Promise( (resolve,reject) => resolve('promise 1 resolved') );
}).catch( error => {
console.log(error)
})
}
postForm();
})
}
//Make two posts in succession
async function start(){
//Post 1 - (never resolves)
await mainFunction().then( response => {
//(Next: make post request 2)
}).catch( error => {
console.log(error)
})
}
start();
How can I resolve the first promise, or isn't it possible? Is there some other solution?
The idea is to make another post request when the first one is resolved.
If you use promises then you work with .then() and .catch() if you work with async/ await then you dont use .then() and .catch() because async / await is syntatic sugar so that we dont need to nest our responses over and over.
If you want to make 2 post request for example with axios or fetch then you can simply use async / await because they return an promise.
async function requests(){
//await makes the request wait till it gets some response
const response1 = await axios.post("url", {data});
const response2 = await axios.post("url2", {data});
}
Related
I have an isolated scene. And there is a promise in a separate file. But my scene does not want to wait for an answer. And continue to work. And continues to run, how do I make it wait for an answer, and continued to work
file: a
async function apiExchangerate(country, amount) {
return new Promise(function (resolve, reject) {
axios.get(``, {
headers: { "Accept-Encoding": "gzip,deflate,compress" },
}).then(
(response) => {
var result = String(response.data.result).replace(/\..*/, '');
console.log('Processing Request');
resolve(result);
},
(error) => {
reject(error);
}
);
});
}
module.exports = apiExchangerate
file: b
let request_currency = await apiExchangerate(country,amount) // you have to wait for a response, and then continue the work
I want my function to wait for a response and continue executing the script. On the Internet, I have not found an answer to my question.
P.s it doesn't work - What is the explicit promise construction antipattern and how do I avoid it?
You're wrapping Promises in Promises for no real reason. One of the reasons why this is an anti-pattern is because it's very easy to get confused by that and to mis-handle resolving/rejecting those Promises.
(My guess is that somewhere you're returning a Promise which resolves to... a Promise. And you're not double-awaiting it so you never resolve the actual Promise.)
Don't over-design it. Simplify the function. axios.get already returns a Promise, and the function is already async, use those:
async function apiExchangerate(country, amount) {
let response = await axios.get(``, { headers: { "Accept-Encoding": "gzip,deflate,compress" } });
let result = String(response.data.result).replace(/\..*/, '');
console.log('Processing Request');
return result;
}
Then what you have is a simple async function which will internally await its own operations. And you can await that function:
let request_currency = await apiExchangerate(country, amount);
I'm trying async/await functionality. I have such code imitating a request:
const getJSON = async () => {
const request = () => new Promise((resolve, reject) => (
setTimeout(() => resolve({ foo: 'bar'}), 2000)
));
const json = await request();
return json;
}
When I use the code in this way
console.log(getJSON()); // returns Promise
it returns a Promise
but when I call this line of code
getJSON().then(json => console.log(json)); // prints { foo: 'bar' }
it prints json as expected
Is it possible to use just code like console.log(getJSON())? What don't I understand?
Every async function returns a Promise object. The await statement operates on a Promise, waiting until the Promise resolves or rejects.
So no, you can't do console.log on the result of an async function directly, even if you use await. Using await will make your function wait and then return a Promise which resolves immediately, but it won't unwrap the Promise for you. You still need to unwrap the Promise returned by the async function, either using await or using .then().
When you use .then() instead of console.logging directly, the .then() method makes the result of the Promise available to you. But you can't get the result of the Promise from outside the Promise. That's part of the model of working with Promises.
A function defined with async always returns a Promise. If you return any other value that is not a Promise, it will be implicitly wrapped in a Promise. The statement const json = await request(); unwraps the Promise returned by request() to a plain object { foo: 'bar' }. This is then wrapped in a Promise before being returned from getJSON so a Promise is what you ultimately get when you call getJSON(). So to unwrap it, you can either call getJSON().then() like you've done or do await getJSON() to get the resolved value.
Return value of an async function will always be an AsyncFunction Object, which will return a Promise when called. You can not change that return type. The point of async/await is to easily wait for other async process to complete inside an async function.
const getResOrErr = () => {
const callAsyncCodeHere = async () => {
const request = () =>
new Promise((resolve, reject) =>
setTimeout(() => resolve({ foo: "bar" }), 2000)
);
const json = await request();
return json;
};
return callAsyncCodeHere()
.then(console.log)
.catch(console.log);
};
getResOrErr();
Try this. You can achieve making a function inside your main function and then put you promise code inside that function. Call it there and when you get the response or error just return it.
I have a async function that awaits a promise which resolves when it receives some 'data'. However, when I run the test, I get a Error: Timeout of 300000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.
Here is my code snippet, I am using this in truffle to test solidity contracts :
contract("Test", async (accounts) => {
it("test description", async () => {
let first = await getFirstEvent(oracle.LogResult({fromBlock:'latest'}));
let second = await getFirstEvent(oracle.LogResult({fromBlock:'latest'}));
Promise.all([first,second]);
//some assertion code
});
const getFirstEvent = (_event) => {
return new Promise((resolve, reject) => {
_event.once('data', resolve).once('error', reject)
});
}
});
Isn't the promise resolving ? I can see 'data' coming back in the callback because I am emitting the callback event in the solidity code I am testing.
I managed to resolve this issue, so posting it here so that others can use the approach.
I created a Promise that times out after a duration we can set :
const timeoutPromise = new Promise((_, reject) => {
setTimeout(() => {a
reject(new Error('Request timed out'));
}, 200000);
})
Then, I race the timeoutPromise with the Promise which is fetching data, like this for the case I posted :
Promise.race([getFirstEvent(oracle.LogResult({fromBlock:'latest'})), timeoutPromise]);
It looks to me like there's a few things wrong here.
First of all, your function isn't returning anything i.e. it should be return Promise.all([first, second]);.
Secondly, if the goal of the promise.all is to execute the promises in parallel, then that's not what it's doing here because you already have await statements on those function calls above. What you are looking for here would be:
return await Promise.all([
getFirstEvent(oracle.LogResult({fromBlock:'latest'}),
getFirstEvent(oracle.LogResult({fromBlock:'latest'})]);
Now in terms of the promise not resolving, I'm assuming the event is generated from oracle.LogResult(). In this case, what you'd want to do is setup your promises to listen for the event first, for example:
let first = getFirstEvent();
let second = getSecondEvent();
Now you have 2 promises that are listening for the events. Next, you generate the event:
oracle.LogResult({ fromBlock: 'latest' });
oracle.LogResult({ fromBlock: 'latest' });
Finally, you ensure you wait on the result of the promises:
return await Promise.all([first, second]);
Introduction
Hey there,
I am trying to pass out data from the mongoose withTransaction callback. Right now, I am using the following code which implements callbacks:
const transactionSession = await mongoose.startSession()
await transactionSession.withTransaction(async (tSession) => {
try {
// MARK Transaction writes & reads removed for brevity
console.log("Successfully performed transaction!")
cb(null, "Any test data")
return Promise.resolve()
} catch (error) {
console.log("Transaction aborted due to error:", error)
cb(error)
return Promise.reject()
}
})
} catch (error) {
console.log(error)
return cb(error)
}
A more detailed snippet of the withTransaction helper in use can be found here.
A link to the official Mongoose documentation regarding the withTransaction helper can be found here.
At the moment, I am using a callback to pass out data from the withTransactioncallback:
cb(null, "Any test data")
However, the problem is that naturally the callback is executed first, before the Promise.resolve() is returned. This means, that (in my case) a success response is sent back to the client before any necessary database writes are committed:
// this is executed first - the callback will send back a response to the client
cb(null, "Any test data")
// only now, after the response already got sent to the client, the transaction is committed.
return Promise.resolve()
Why I think this is a problem:
Honestly, I am not sure. It just doesn't feel right to send back a success-response to the client, if there hasn't been any database write at that time. Does anybody know the appropriate way to deal with this specific use-case?
I thought about passing data out of the withTransaction helper using something like this:
const transactionResult = await transactionSession.withTransaction({...})
I've tried it, and the response is a CommandResult of MongoDB, which does not include any of the data I included in the resolved promise.
Summary
Is it a problem, if a success response is sent back to the client before the transaction is committed? If so, what is the appropriate way to pass out data from the withTransaction helper and thereby committing the transaction before sending back a response?
I would be thankful for any advice I get.
It looks like there is some confusion here as to how to correctly use Promises, on several levels.
Callback and Promise are being used incorrectly
If the function is supposed to accept a callback, don't return a Promise. If the function is supposed to return a Promise, use the callback given by the Promise:
const transactionSession = await mongoose.startSession()
await transactionSession.withTransaction( (tSession) => {
return new Promise( (resolve, reject) => {
//using Node-style callback
doSomethingAsync( (err, testData) => {
if(err) {
reject(err);
} else {
resolve(testData); //this is the equivalent of cb(null, "Any test data")
}
});
})
Let's look at this in more detail:
return new Promise( (resolve, reject) => { This creates a new Promise, and the Promise is giving you two callbacks to use. resolve is a callback to indicate success. You pass it the object you'd like to return. Note that I've removed the async keyword (more on this later).
For example:
const a = new Promise( (resolve, reject) => resolve(5) );
a.then( (result) => result == 5 ); //true
(err, testData) => { This function is used to map the Node-style cb(err, result) to the Promise's callbacks.
Try/catch are being used incorrectly.
Try/catch can only be used for synchronous statements. Let's compare a synchronous call, a Node-style (i.e. cb(err, result)) asynchronous callback, a Promise, and using await:
Synchronous:
try {
let a = doSomethingSync();
} catch(err) {
handle(err);
}
Async:
doSomethingAsync( (err, result) => {
if (err) {
handle(err);
} else {
let a = result;
}
});
Promise:
doSomethingPromisified()
.then( (result) => {
let a = result;
})
.catch( (err) => {
handle(err);
});
Await. Await can be used with any function that returns a Promise, and lets you handle the code as if it were synchronous:
try {
let a = await doSomethingPromisified();
} catch(err) {
handle(err);
}
Additional Info
Promise.resolve()
Promise.resolve() creates a new Promise and resolves that Promise with an undefined value. This is shorthand for:
new Promise( (resolve, reject) => resolve(undefined) );
The callback equivalent of this would be:
cb(err, undefined);
async
async goes with await. If you are using await in a function, that function must be declared to be async.
Just as await unwraps a Promise (resolve into a value, and reject into an exception), async wraps code into a Promise. A return value statement gets translated into Promise.resolve(value), and a thrown exception throw e gets translated into Promise.reject(e).
Consider the following code
async () => {
return doSomethingSync();
}
The code above is equivalent to this:
() => {
const p = new Promise(resolve, reject);
try {
const value = doSomethingSync();
p.resolve(value);
} catch(e) {
p.reject(e);
}
return p;
}
If you call either of the above functions without await, you will get back a Promise. If you await either of them, you will be returned a value, or an exception will be thrown.
I'm working on Ionic v4 with Angular.
In my project i use the BLE to communicate with a raspberry.
I have several step :
Search Device around me
Connect to this device
Activate Notification
Send Messages
Currently i have something like :
this.ble.scan().subscribe(result => {
if (device === theDeviceIWant) {
this.ble.connect(device.id).subscribe(result => {
this.ble.startNotification(infosaboutDevice).subscribe(result => {
// Message 1
this.ble.writeWithoutResponse(infos, message).then(result => {
// Message 2
this.ble.writeWithoutResponse(infos, message).then(result => {
// Message 3
this.ble.writeWithoutResponse(infos, message).then(result => {
// Message X
this.ble.writeWithoutResponse(infos, message).then(result => {
})
})
})
})
})
})
})
}
I want to do something like that :
this.myScan();
this.myConnect();
this.myNotification();
this.myMessage('Text 1');
this.myMessage('Text 2');
this.myMessage('Text X');
The probleme : My function ‘myConnect‘ don't wait the end of ‘myScan‘ to start. So somme stuff needed by ‘myConnect‘ is do in ‘myScan‘.
I already try to use ‘async/await‘ but does not work. I think i don't use it correctly :
await this.myConnect().then(async () => {
await this.myNotification().then(async () => {
await this.myMessage('03020000').then(async () => {
await this.myMessage('010100').then(async () => {
await this.myMessage('020200' + this.random.toString(16));
});
});
});
});
Help me to understand how to create a function who wait the end of the before one to start :D
Just use async/await OR then
await this.myConnect(); // this awaits the Promise returned by myConnect to be resolved
await this.myNotification(); // same for this Promise
await this.myMessage('03020000'); // and so on...
await this.myMessage('010100');
await this.myMessage('020200' + this.random.toString(16));
The keyword await makes JavaScript wait until that promise settles and
returns its result.
So you dont need to use then in await this.myConnect().then(()=>{});
use await this.myConnect();
Below is example which help you understand better
function SignalOne() {
return new Promise((resolve, reject) => {
setTimeout(()=>{
resolve('Hello iam signal one');
}, 2000);
});
}
function SignalTwo() {
return new Promise((resolve, reject) => {
setTimeout(()=>{
resolve('Hello iam signal Two');
}, 1000);
});
}
async function sendSignal() {
let one = await SignalOne();
let two = await SignalTwo();
console.log(one);
console.log(two);
}
sendSignal();
Try this:
async myScan() {
// do things
}
ngOnInit() {
const scan = this.myScan(); // myScan doesn't actually have to return here
await scan;
const connect = this.myConnect();
await connect;
// more stuff
}
This is essentially what Promises are made for.
A Promise is an object representing the eventual completion or failure
of an asynchronous operation.
You can read up about Promises here. Once you read thru that, I left an example for you below to demonstrate how to use a Promise:
//Wrap the operation you want to wait for in a Promise (in this case: setTimeout)
const promise = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('3 seconds have passed');
}, 3000);
});
//Once the operation is resolved the callback in the .then will be called
promise.then(val => document.querySelector('#target').innerHTML = val);
<div id="target">This message will change once the operation is resolved in 3 seconds.</div>
I would embrace Observables. Looking at what you want..
Search Device around me
Connect to this device
Activate Notification
Send Messages
1 and 2 would be chained with switchMap, as responses depend on each other. Then 3 and 4 could be performed in order, but not dependent on each other, therefore we could use concat with those. (If this is not correct flow, adjust accordingly with these two operators).
So I suggest the following:
import { never } from 'rxjs';
import { switchMap, concat } from 'rxjs/operators';
// ...
this.ble.scan().pipe(
switchMap((device) => {
if (device === theDeviceIWant) {
return this.ble.connect(device.id)
}
// terminates rest of code
return never();
}),
concat(
this.ble.startNotification(...),
this.ble.writeWithoutResponse(...)
)
).subscribe(data => console.log(data))
You're so close! Rather than using .then and async just use one or the other. Here are a few ways to accomplish what you are trying to do:
Using .then:
This is your typical chaining syntax. Promises can be chained using .then() and passing in a function. If the return value is a value (not a Promise) then it will resolve to that value. But if it did return a Promise then it will chain together and your next .then() will resolve to the "inner" async call result.
// Message 1
return this.ble.writeWithoutResponse(infos, message).then(result1 => {
// Message 2
return this.ble.writeWithoutResponse(infos, message);
}).then(result2 => {
// Message 3
return this.ble.writeWithoutResponse(infos, message);
)}.then(result3 => {
// Message X
return this.ble.writeWithoutResponse(infos, message);
}).then(result4 => { })
Using async/await
This approach achieves the same result but uses special keywords to automatically chain promises together. async/await allows you to skip the .then() and return calls so you can invoke your async functions as if they were synchronous.
// Message 1
let result1 = await this.ble.writeWithoutResponse(infos, message)
// Message 2
let result2 = await this.ble.writeWithoutResponse(infos, message);
// Message 3
let result3 = await this.ble.writeWithoutResponse(infos, message);
// Message X
let result4 = await this.ble.writeWithoutResponse(infos, message);
To learn more about Promise's and async javascript, check out these resources:
Promises on MDN
Promises on Google Web Fundamentals
Video on Async/Await