Summary: creating my own API that returns epoch time, and it involves using an express.js server, but it's running res.send() before the function call. I referenced this page, but it didn't help. Here's what I have:
app.get('/timestampAPI', async (req, res,) => {
try {
let finalResult = await getTimeStamp();
res.send({ something: finalResult });
} catch (error) {
console.log(error);
}
});
It'll start to run the function getTimeStamp(), and before that function finishes, it runs the res.send() function which shows up as '{}' because finalResult doesn't have a value. getTimeStamp() is an async function. I'm unsure of what I'm doing wrong.
Edit:
getTimeStamp() function:
async function getTimeStamp() {
await axios.get('https://showcase.api.linx.twenty57.net/UnixTime/tounixtimestamp?datetime=now')
.then(response => {
// also used console.log(response.data.UnixTimeStamp), which returns the timestamp
return response.data;
})
.catch(error => {
var errorMessage = error.response.statusText;
console.log(errorMessage);
});
}
Another edit: yes, the API referenced above does return the current epoch time, but CORS is blocking my other site from accessing it directly, so I can't use it on that site, which is why I'm using node.js for it so that I can allow myself to access it through my node.js program. Couldn't think of another way
returning value of the then method does not return from getTimeStamp function you should write you code in resolve pattern or using await like below
try this, make sure you write correct field name in response object
async function getTimeStamp() {
try{
const res = await axios.get('https://showcase.api.linx.twenty57.net/UnixTime/tounixtimestamp?datetime=now')
return res.data
}catch(error){
throw error
}
As an alternative to Mohammad's answer you can also use returning getTimeStamp function's result as a promise and it can solve your problem.
async function getTimeStamp() {
return new Promise((resolve, reject) => {
axios.get('https://showcase.api.linx.twenty57.net/UnixTime/tounixtimestamp?datetime=now')
.then(response => {
// also used console.log(response.data.UnixTimeStamp), which returns the timestamp
resolve(response.data);
})
.catch(error => {
var errorMessage = error.response.statusText;
console.log(errorMessage);
reject(error);
});
})
}
Or you would also replace await with return in getTimeStamp function in your code if you don't want to return promise.(Which is not I recommend.). You should also throw the error in catch block which is generated in getTimeStamp function for catching the error in try-catch block that you use to call app.get(...).
Related
I have a function that is supposed to return a user's email from using the Firebase admin API getUser(uid) function,
function getEmail(uid){
var email;
admin.auth().getUser(uid)
.then((userRecord) => {
return userRecord.email;
})
.catch((error) => {
console.log('Error fetching user data:', error);
});
}
But in my other function when I make a variable that calls the function,
email = getEmail(uid);
the value in email is undefined, because the getUser function has returned a promise. How do I make the function getEmail wait to get the value of userRecord before returning?
I've tried adding await statements in different parts of the function but I'm not sure how to do it correctly. I'm a beginner in using the Google API.
Return the Promise that you get from admin.auth().getUser(uid) in your getEmail function.
function getEmail(uid) {
return admin.auth().getUser(uid)
.then((userRecord) => {
return userRecord.email;
})
.catch((error) => {
console.log('Error fetching user data:', error);
});
}
Then either use the returned Promise to chain a then callback.
getEmail(uid).then(email => {
console.log(email);
});
Or await it in an async function.
async function main() {
const email = await getEmail(uid);
console.log(email);
}
main();
Edit
As you explained in your comment below:
basically, I just want the code to run as if it were all synchronous/ line-by-line.
It's impossible to make asynchronous code synchronous, but it is possible to make it run line-by-line with async / await syntax. Promises that are being awaited in an async function will run line by line as you're telling the code to wait for it to resolve before going to the next line.
I've modified the code from your CodePen to include the aforementioned syntax. We're still using the same getEmail function that returns the promise from admin.auth().getUser(uid) and just wait for it to finish before doing something with the result.
function getEmail(uid) {
return admin.auth().getUser(uid)
.then((userRecord) => {
return userRecord.email;
})
.catch((error) => {
console.log('Error fetching user data:', error);
});
}
app.post("/sendx", async (req, res) => {
const uid = req.body.x.user.uid;
const email = await getEmail(uid);
let verified = false;
/*console.log(email.substring(email.indexOf('#')));
if (email.substring(email.indexOf('#')) == "#xxx.edu") {
verified = true;
}*/
res.send({ verified });
});
I hope this clears it up.
I have three API calls which should be dependent on one another. The second API call should trigger only when the first succeeds.
With my current implementation, I'm getting a CORS error when the first API call is made and was able to catch the error in the catch block. However, I'm seeing that the second and third APIs calls are made irrespective of the error that got caught in the first API call.
Could anyone please advise?
const firstApiCall = async() => {
try {
await axios.post(
process.env.FIRST_API,
payload
);
]
} catch (err) {
console.log(`err`, err);
}
};
const secondApiCall = async() => {
try {
await axios.post(
process.env.SECOND_API,
payload
}
} catch (err) {
console.log(`err`, err);
}
};
const thirdApiCall = async() => {
try {
await axiosInstance.patch(
process.env.THIRD_API,
payload
);
} catch (err) {
console.log('err', err);
}
};
firstApiCall();
secondApiCall();
thirdApiCall();
You're calling the functions synchronously when you need to do it asynchronously:
async function performTasks() {
await firstApiCall();
await secondApiCall();
await thirdApiCall();
}
performTasks();
You can use the ES6 Promise implementation approacg. Therefore you should take a look to this ressource : [Promise][1]
With the promise approach you can react at each step / each API call.
[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/then
async await functions work only in their local scope.
For example:
const myFunc = async() => {
try{
//...
await foo();
//All the code below will be blocked till
//the promise return by foo function is resolved
}
catch{
//...
}
}
const main = () => {
myFunc();
otherFunc();
//Other function calls
//Regardless of using async await in myFunc,
//the code bellow myFunc will be executed as
//async await will work only in myFunc block scope
}
main()
What you can do is, use async await inside the main function, so that the functions would be called in an order
const main = async () => {
await myFunc();
await otherFunc();
}
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.
Take the following contrived example:
const housekeepingStuff = async function (data) {
const result = await notImportant(data);
result.more = 'yawn';
storeInDatabase(result);
};
const getStuff = async function () {
try {
const data = await getData();
data.extra = 'wow';
housekeepingStuff(data); // <---- don't want to await... but need to for error catching
return Promise.resolve(data);
} catch (err) {
return Promise.reject(err);
}
};
try {
const myData = await doSomeStuff();
res.send(myData);
} catch (err) {
console.log(err);
res.sendStatus(400);
}
I want to return the data from getStuff () ASAP without waiting for housekeepingStuff() but if I don't await that function then I have an uncaught error.
I could call housekeepingStuff() outside the getStuff() function, after getting and sending the data to whoever wants it:
try {
const myData = await doSomeStuff();
res.send(myData);
await housekeepingStuff(data); // <---- am awaiting but who cares because nothing follows
} catch (err) {
console.log(err);
res.sendStatus(400);
}
But that doesn't seem right because I don't want to have to remember to call housekeepingStuff() every time I call doSomeStuff()... it should ideally be handled "internally".
What is the correct approach here?
A promise (or async) function has 2 possible outcomes:
A successful outcome
An error outcome
To get either outcome, you must wait for it. You can't wait for 1 condition and not for the other, because the entire thing needs to execute so you can find out what the outcome was.
Otherwise you're really asking the javascript engine: Please predict for me if the function will fail, and if it does, await it.
The correct approach therefore is to just await it.
However, if you don't care about either successful or failed outcomes of this function, just call the function via another async function that eats all the errors:
async function doSomeStuffAndIgnoreError() {
try {
await doSomeStuff();
} catch (e) {
console.error(e);
}
}
So what I want to achieve is that when a function returns an empty Object from the Promise, the Await function must not be executed and the rest of the Application must carry on executing other tasks. As the object that is returned maybe not always be available but should be returned when available.
function getData(Data) : Promise<Object> {
return new Promise((resolve, reject) => {
request({
// Method
}, (err, resp, file)=> {
if (err) {
reject(err);
} else {
resolve({
// Return object infomation
});
}
});
});
}
let someData = await Promise.all(data.map(getData));
// This should have a part that ignores if getData is empty and this await function ignored.
The rest of the application should be able to run as normal. I have tried to use:
.catch(error => { });
But didn't work the way I wanted it to work
There may be a better way, but what solved my issue was to pass an empty array to the data
if (isNullOrUndefined(data)) {
data = [];
}
In this way the await function now works the way I want it to work and does not throw error:
TypeError: Cannot read property 'map' of undefined