Reexecuting await/async/promise.all function with a new value - javascript

I am basically trying to send multiple API queries at once and wait for the result. The code works well for the first query, but I am actually struggling to reset the value of the prior async return. After the first execution is completed, it keeps returning the very first values. The goal is to assign random href(URL) with every function's execution and recall the API once again, so it can call the API once again with a newly randomized href and return new values. Here is the code, would be great is someone could help:
async function fetchData() {
const [test0_array_response,
test1_array_response,
test2_array_response,
] = await Promise.all([
fetch(random_href+'/test0/'),
fetch(random_href+'/test1/'),
fetch(random_href+'/test2/'),
]);
const test0_array = await test0_array_response.json();
const test1_array= await test1_array_response.json();
const test2_array = await test2_array_response.json();
return [test0_array, test1_array, test2_array];
}
function callApi() {
console.log(fetchData()) // returns a promise
};

From what I can see random_href is part of the closure of fetchData, so you need to pass it as argument inside callApi or wherever you use fetchData in order to use his correct value.
Take a look here for learn what I means with closure.
You should do something like this:
async function fetchData(random_href) {
const [test0_array_response,
test1_array_response,
test2_array_response,
] = await Promise.all([
fetch(random_href+'/test0/'),
fetch(random_href+'/test1/'),
fetch(random_href+'/test2/'),
]);
const test0_array = await test0_array_response.json();
const test1_array= await test1_array_response.json();
const test2_array = await test2_array_response.json();
return [test0_array, test1_array, test2_array];
}
function callApi(random_href) {
console.log(fetchData(random_href)) // returns a promise
};
Another approach could be to make an object random_container = { href: 'VALUE'} and use it's reference inside the fetchData function. So, for example, fetch(random_container.href+'/test0/'). In this way random_container will be part of the closure of fetchData but you'll be able to read the last value of random_container.href.

Related

How to structure a recursive async/await function for firestore in node.js

What I am trying to do:
I am trying to retrieve all comments and their replies from firestore (firebase database) using a recursive method. Here is the structure of the data:
What is the problem
The parent asynchronous function does not wait for the nested asynchronous function to complete.
getThread = async (req, res) => {
// Getting comments belonging to thread
const thread_document = await db.doc(`/Threads/${req.params.threadid}`).get()
threadData = thread_document.data()
threadData.threadid = thread_document.id
const comment_query = await db.collection('Comments').where('threadid', '==', threadData.threadid).get()
// Getting replies belonging to comments
for (document of comment_query){
let commentData = await getReplies(document.id)
threadData.comments.push(commentData )
}
return res.json(threadData)
}
//Recursive function to retrieve replies
getReplies = async (id) => {
let comment = await db.doc(`/Comments/${id}`).get()
let commentData = comment.data()
commentData.comment_replies = commentData.replies.map(idx => {
// The parent async function does not wait for the the async function here to finish.
// Placing a await keyword here will raise the error 'await is only valid in async functions and the top level bodies of modules'
return getReplies(idx)
})
console.log(commentData)
return commentData
}
Given the following example,
Since the parent async function does not wait for the nested async function, the order of execution now is A -> B -> a, and a fails to be mapped into commentData and commentData for comment A would end up empty. Hence, I want to program to do A -> a -> B. To do so, I would like to place a await keyword just before the getReplies like
return await getReplies(idx)
but it will raise the error,
await is only valid in async functions and the top level bodies of modules.
Which is confusing as getReplies is already a async function. I've looked into other solutions in stackoverflow but I am not able to get the recursive function working. Any insights would be appreciated, thank you.
commentData.comment_replies = commentData.replies.map(idx => {
// ...
return getReplies(idx)
})
This map statement is going to create an array of promises, but not wait for those promises to finish. You should use Promise.all to combine them into a single promise, and then await that promise to get the array of comment replies:
const promises = commentData.replies.map(idx => {
return getReplies(idx);
});
commentData.comment_replies = await Promise.all(promises);
Which is confusing as getReplies is already a async function.
You were getting that error because the function you're inside is idx => { return getReplies(idx) }, which is not an async function. But putting an await in there is not a solution to your problem anyway.

How to consume a promise and use the result later on with my code?

I'm new to async operations and js. Here is my question.
I have a Person class. I want to init Person instance using data that I get from an API call.
class Person {
constructor(data) {
this.data = data;
}
}
I am making an API call using Axios. I get a response and want to use it in my class.
const res = axios.get('https://findperson.com/api/david');
const david = new Person(res);
I understand that res is a promise at this stage, and that I need to consume it.
How should I do it?
How can I take the response and use it properly?
axios.get() return a promise of an object which contains the returned data, status, headers, etc...
async function getPerson() {
try {
const res = await axios.get('https://findperson.com/api/david');
const david = new Person(res.data);
// do something with david
} catch (error) {
console.log(error)
}
}
or
function getPerson() {
axios
.get('https://findperson.com/api/david')
.then(res => {
const david = new Person(res.data)
// do something with david
})
.catch(console.log)
}
Inside another async function, or at the top level of a module or at the REPL (in node 16.6+ or some earlier versions with the --experimental-repl-await feature enabled), you can just use await.
const res = await axios.get('https://findperson.com/api/david');
That will wait for the promise to be resolved and unwrap it to store the contained value in res.
If you want to get the value out of the world of async and into synchro-land, you have to do something with it via a callback function:
axios.get('https://findperson.com/api/david').then(
res => {
// do stuff with res here
});
... but don't be fooled; without an await, any code that comes after that axios.get call will run immediately without waiting for the callback. So you can't do something like copy res to a global var in the callback and then expect it to be set in subsequent code; it has to be callbacks all the way down.
You can do this:
axios.get('https://findperson.com/api/david').then(res => {
const david = new Person(res);
});
Or in async function: (See async await for javascript)
const res = await axios.get('https://findperson.com/api/david');
const david = new Person(res);

getting data from js promise

I am doing an api call that returns a promise. The call works fine but I want to do treat data contained within the promise. Here is My call:
let promiseArray = this.get('store').query('member', {query: term, option: this.get('option')});
promiseArray.then(members => {console.log(members);
});
let var= members;
console.log(var);
My problem is that this doesn't return an array of my model i.e members, also the second display of members display undefined , it return an object containing a lot of meta data, also the array but inside some meta data.
How could I get simply the array ?
You can use async await for your purpose.
const promiseFunc = () => {
// Return the promise and await this inside a async function
return this.get('store').query('member', {query: term, option: this.get('option')});
}
const asyncFunc = async () => {
const value = await promiseFunc();
console.log(value);
}
asyncFunc();

Extracting JSON from Fetch API

I realise very similar questions have been answered before, but I'm still finding it very confusing as to how this works...
From my understanding promises are used to deal with asyc requests - these promises essentially send back the state or a "promise" that at some point later a JSON body (or other object) will be delivered.
What I'm trying to understand is how I properly handle these requests so that the function doesn't return until the JSON body is ready to be parsed.
Below I'm trying to simply extract the key "result" (which returns a string "result") and parse it to another variable that can be stored and then later used somewhere else in my code. Unfortunately, my code always returns a [Object Promise], rather than the extracted JSON. I believe this is because response.json is also a promise... however, I don't understand how I get out of the "chain of promises" and return a value that I can actually do something with.
Thanks for any advice,
async function name() {
const response = await fetch('https://xxxxx.herokuapp.com/timespent', {});
const json = await response.json();
return json.result;
}
let varr = name();
console.log(varr)
Since your function is async it always return a promise. You need to use await for result.
read more about async here
async function name() {
const response = await fetch('https://mautargets.herokuapp.com/timespent', {});
const json = await response.json();
return json.result;
}
async function result(){
//await can only be called from inside of async function. So we need async function for await name()
let varr = await name();
console.log(varr) // Success
}
result()
In your example code, name function is declared async, so it returns a promise.
Inside that function body, you correctly handle async calls like fetch or the JSON transformation.
What you need now is either use await to wait for the function to "resolve", or use the "older" then/catch promises methods. Note that you cannot always use await outside an async function so you may need to wrap it.
Example :
async function name() {
const response = await fetch('https://mautargets.herokuapp.com/timespent', {});
const json = await response.json();
return json.result;
}
// using promise.then
name().then(result => console.log(result));
// wrapping await
(async function test() {
try{
console.log(await name());
}catch(error) {
// error goes here if promise got rejected
}
})()
You could have a callback in the function declaration, and use '.then(...)' and call it when the promise has been resolved:
async function name(cb) {
const response = await
fetch('https://mautargets.herokuapp.com/timespent', {});
const json = response.json();
json.then(x => cb(x))
}
name(console.log)
This is because you're using an Async function, which will return a promise.
Or if you would like the method to return, you could either call it in another Asynchronous context and utilize await again:
// Assume no callback: code just as you had it.
async function wrapper() {
console.log(await name())
}
Or you could do name().then(...) as specified before:
// Assume no callback: code just as you had it.
name().then(console.log)
Hope this helps!
I'm actually looking for the answer(same as yours), so I found this way.
I. ASK/REQUEST for data
async function fetchMyJson() {
const response = await fetch('https://1stAPI.devdeveloper1.repl.co/fiveD');
const myData = await response.json();
return myData;
}
II.GET Extract data
fetchMyJson().then(myData => {
let myData_output = myData.USD[0].rate; // fetched or Get OUTPUT data
console.log(myData_output);
document.body.innerHTML = `<div>${myData_output}</div>`; //make sure you add ${} for output
});
async function fetchMyJson() {
const response = await fetch('https://1stAPI.devdeveloper1.repl.co/fiveD');
const myData = await response.json();
return myData;
}
//GET Extract data
fetchMyJson().then(myData => {
let myData_output = myData.USD[0].rate; // fetched or Get OUTPUT data
console.log(myData_output);
document.body.innerHTML = `<div>${myData_output}</div>`; //make sure you add ${} for output
});
It is correct that you await fetch and .json since they are async.
async function name() {
const response = await fetch('http://blah.com/api', {});
const json = await response.json();
return json.result;
}
However, async and Promises inside function name make it async too. So the return value of name is a Promise that you should await it, or .then it, like:
// Old style .then
name().then(result => console.log(result))
// Modern style await
async function main() {
const result = await name()
console.log(result)
}

How to return a Promise from async function?

When I try to return a promise from an async function, it's impossible to distinguish the status of the returned Promise and the function.
I think, the simplest solution is to put the promise to be returned in an array. The following is a stupid example, but I hope it demonstrates the problem:
function loadMetaData(id) {/*...*/} // Returns Promise<MetaData>
function loadSingleData(name) {/*...*/} // Returns Promise<SingleData>
async function startLoadingSingleData(id, object) {
const metaData = object.metaData = await loadMetadata(id);
const singleDataPromise = loadSingleData(metaData.dataToLoad);
singleDataPromise.then(metaData => object.metaData = metaData);
return [singleDataPromise];
}
async function logData(id) {
const object = new SomeUsefulClassWhatTakesAdvantageOfMetadataProp();
somePromise = (await startLoadingSingleData(id))[0];
// Now metadata surely loaded, do something with it
console.log(object.metaData);
// But somedata will be loaded only in future
somePromise.then(singleData => console.log(singleData));
// And maybe: (depends of use-case)
await somePromise;
}
When executing logData(/*...*/), first the metaData of the given ID of the given data after a short period, and after a little waiting the full singleData is expected.
But this is kinda hackish.
What is the intended way to overcome this situation?
PS.:
This problem occurs too, when I try to return a Promise which resolves with the promise.
Yes, unfortunately JS promises are not algebraic and you cannot fulfill a promise with another promise. There's no way around that (other than not using native promises, and not using async/await).
The easiest and most common solution is indeed using a wrapper object. It comes naturally to your problem:
// takes an id, returns a Promise<{metaData: Data, singleDataPromise: Promise<Data>}>
async function startLoadingSingleData(id) {
const object = new SomeUsefulClassWhatTakesAdvantageOfMetadataProp();
object.metaData = await loadMetadata(id);
// ^^^^^
object.singleDataPromise = loadSingleData(object.metaData.dataToLoad);
// ^ no await here
return object;
}
async function logData(id) {
const object = await startLoadingSingleData(id));
// Now metadata surely loaded, do something with it
console.log(object.metaData);
// But some singleData will be loaded only in future
const singleData = await object.singleDataPromise;
console.log(singleData);
}
Notice that this potentially leads to problems with unhandled rejections if there is an exception in your code and you never get to await the singleDataPromise.
The (probably much better) alternative is to restructure your functions so that you don't create any promises before using (i.e. awaiting) them, like #Paulpro also suggested. So you'd just write a single strictly sequential function
async function logData(id) {
const object = new SomeUsefulClassWhatTakesAdvantageOfMetadataProp();
object.metaData = await loadMetadata(id);
// Now metadata surely loaded, do something with it
console.log(object.metaData);
// But some singleData will be loaded only in future
object.singleData = await loadSingleData(object.metaData.dataToLoad);
console.log(object.singleData);
}
Your problem is that startLoadingSingleData has too many responsibilities. It is responsible for both loading the metadata and triggering loading of singledata.
Your logData function uses await startLoadingSingleData(id) as a way to make sure that metadata is available, which does not seem very intuituve. It is not obvious that startLoadingSingleData(id) returns a Promise that resolves when the metadata has loaded, and would be quite confusing for a coder looking at it for the first time (or yourself after a few months). Code should be self-documenting as much as possible so that you don't need to explain every line with comments.
My recommendation is to completely remove the startLoadingSingleData function and just do this inside logData:
async function logData(id) {
const metaData = await loadMetadata(id);
console.log(metaData);
const singleData = await loadSingleData(metaData.name);
console.log(singleData);
}
or if you don't want logData to await the SingleData Promise:
async function logData(id) {
const metaData = await loadMetadata(id);
console.log(metaData);
loadSingleData(metaData.name).then( singleData => {
console.log(singleData);
} );
}
If you really want to keep using the function startLoadingSingleData instead then I think you should make it return an array or an object containing two Promises:
function startLoadingSingleData(id) {
const metaDataLoaded = loadMetadata(id);
const singleDataLoaded = metaDataLoaded.then(
metaData => loadSingleData(metaData.dataToLoad)
);
return { metaDataLoaded, singleDataLoaded };
}
Then your usage would look something like:
async function logData(id) {
const { metaDataLoaded, singleDataLoaded } = startLoadingSingleData(id);
const metaData = await metaDataLoaded;
console.log(metaData);
const singleData = await singleDataLoaded;
console.log(singleData);
}

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