How can I build a function which gets some data asynchronously then uses that data to get more asynchronous data?
I am using Dexie.js (indexedDB wrapper) to store data about a direct message. One thing I store in the object is the user id which I'm going to be sending messages to. To build a better UI I'm also getting some information about that user such as the profile picture, username, and display name which is stored on a remote rdbms. To build a complete link component in need data from both databases (local indexedDB and remote rdbms).
My solution returns an empty array. It is being computed when logging it in Google Chrome and I do see my data. However because this is not being computed at render time the array is always empty and therefor I can't iterate over it to build a component.
const [conversations, setConversations] = useState<IConversation[]>()
const [receivers, setReceivers] = useState<Profile[]>()
useEffect(() => {
messagesDatabase.conversations.toArray().then(result => {
setConversations(result)
})
}, [])
useEffect(() => {
if (conversations) {
const getReceivers = async () => {
let receivers: Profile[] = []
await conversations.forEach(async (element) => {
const receiver = await getProfileById(element.conversationWith, token)
// the above await is a javascript fetch call to my backend that returns json about the user values I mentioned
receivers.push(receiver)
})
return receivers
}
getReceivers().then(receivers => {
setReceivers(receivers)
})
}
}, [conversations])
/*
The below log logs an array with a length of 0; receivers.length -> 0
but when clicking the log in Chrome I see:
[
0: {
avatarURL: "https://lh3.googleusercontent.com/..."
displayName: "Cool guy"
userId: "1234"
username: "cool_guy"
}
1: ...
]
*/
console.log(receivers)
My plan is to then iterate over this array using map
{
receivers && conversations
? receivers.map((element, index) => {
return <ChatLink
path={conversations[index].path}
lastMessage={conversations[index].last_message}
displayName={element.displayName}
username={element.username}
avatarURL={element.avatarURL}
key={index}
/>
})
: null
}
How can I write this to not return a empty array?
Here's a SO question related to what I'm experiencing here
I believe your issue is related to you second useEffect hook when you attempt to do the following:
const getReceivers = async () => {
let receivers: Profile[] = []
await conversations.forEach(async (element) => {
const receiver = await getProfileById(element.conversationWith, token)
receivers.push(receiver)
})
return receivers
}
getReceivers().then(receivers => {
setReceivers(receivers)
})
}
Unfortunately, this won't work because async/await doesn't work with forEach. You either need to use for...of or Promise.all() to properly iterate through all conversations, call your API, and then set the state once it's all done.
Here's is a solution using Promise.all():
function App() {
const [conversations, setConversations] = useState<IConversation[]>([]);
const [receivers, setReceivers] = useState<Profile[]>([]);
useEffect(() => {
messagesDatabase.conversations.toArray().then(result => {
setConversations(result);
});
}, []);
useEffect(() => {
if (conversations.length === 0) {
return;
}
async function getReceivers() {
const receivers: Profile[] = await Promise.all(
conversations.map(conversation =>
getProfileById(element.conversationWith, token)
)
);
setReceivers(receivers);
}
getReceivers()
}, [conversations]);
// NOTE: You don't have to do the `receivers && conversations`
// check, and since both are arrays, you should check whether
// `receivers.length !== 0` and `conversations.length !== 0`
// if you want to render something conditionally, but since your
// initial `receivers` state is an empty array, you could just
// render that instead and you won't be seeing anything until
// that array is populated with some data after all fetching is
// done, however, for a better UX, you should probably indicate
// that things are loading and show something rather than returning
// an empty array or null
return receivers.map((receiver, idx) => <ChatLink />)
// or, alternatively
return receivers.length !== 0 ? (
receivers.map((receiver, idx) => <ChatLink />)
) : (
<p>Loading...</p>
);
}
Alternatively, using for...of, you could do the following:
function App() {
const [conversations, setConversations] = useState<IConversation[]>([]);
const [receivers, setReceivers] = useState<Profile[]>([]);
useEffect(() => {
messagesDatabase.conversations.toArray().then(result => {
setConversations(result);
});
}, []);
useEffect(() => {
if (conversations.length === 0) {
return;
}
async function getReceivers() {
let receivers: Profile[] = [];
const profiles = conversations.map(conversation =>
getProfileById(conversation.conversationWith, token)
);
for (const profile of profiles) {
const receiver = await profile;
receivers.push(receiver);
}
return receivers;
}
getReceivers().then(receivers => {
setReceivers(receivers);
});
}, [conversations]);
return receivers.map((receiver, idx) => <ChatLink />);
}
i think it is happening because for getReceivers() function is asynchronous. it waits for the response, in that meantime your state renders with empty array.
you can display spinner untill the response received.
like
const[isLoading,setLoading]= useState(true)
useEffect(()=>{
getReceivers().then(()=>{setLoading(false)}).catch(..)
} )
return {isLoading ? <spinner/> : <yourdata/>}
Please set receivers initial value as array
const [receivers, setReceivers] = useState<Profile[]>([])
Also foreach will not wait as you expect use for loop instead of foreach
I am not sure it is solution for your question
but it could help you to solve your error
Related
function UserTransactionsComponent1() {
const [accounts, setAccounts] = useState();
useEffect(() => {
async function fetchData() {
const res = await fetch(
'https://proton.api.atomicassets.io/atomicassets/v1/accounts'
);
const { data } = await res.json();
setAccounts(data);
}
fetchData();
}, []);
accounts.map((result) => {
const { account } = result;
});
return <PageLayout>Hi! {account}</PageLayout>;
}
export default UserTransactionsComponent1;
I console.log(accounts) right before I map it and all the properties are there. The issue is that the account in the acounts.map is showing greyed out on VSCode. It's not being picked up on the return. This is causing me to receive the following error: TypeError: Cannot read properties of undefined (reading 'map'). What's the reason for this?
The return statement is outside the variable (account) scope.
function UserTransactionsComponent1() {
const [accounts, setAccounts] = useState();
useEffect(() => {
async function fetchData() {
const res = await fetch(
"https://proton.api.atomicassets.io/atomicassets/v1/accounts"
);
const { data } = await res.json();
setAccounts(data);
}
fetchData();
}, []);
const getAccounts = () => {
if (accounts)
return accounts?.map((result) => {
const { account } = result;
return account;
})
}
return (
<PageLayout>
Hi!{" "}
{getAccounts()}
</PageLayout>
);
}
export default UserTransactionsComponent1;
The problem is that your map function is running before your fetch has completed, so accounts is still undefined when you try mapping.
There's a few ways to solve this. One options is just to use .then(). So put your map function inside of .then, inside your useEffect.
.then(() => accounts.map( // insert rest of function here ))
This tells the code to run the map function only after the fetch completes
accounts is not defined until the fetch is complete, so you need to map it in an effect, which waits for the state of accounts to be set:
useEffect(() => {
accounts.map(...);
}, [accounts]);
On top of that, when you return, account will be undefined. You can create a loading screen or something while the data is fetching, then re-render with the data:
return (
<PageLayout>{accounts ? accounts : "Loading..."}</PageLayout>
);
I'm not sure what you're trying to do in your map function; you're not specifying specifically which account in the array you want; you'll need another state.
I am creating a simple quiz with persisting response state and i am currently stuck trying to figure out why responseState is undefined in my handleAnswerClick function, this function is triggered much later if at all per click. By then the States should all be set.
const Question: React.FC<IProps> = (props) => {
const [responseState, setResponseState] = useState([]);
const [question, setQuestion] = useState<IStateProps>(props.id);
const [answers, setAnswers] = useState([]);
const [choice, setChoice] = useState();
const initialResponseState = () => {
const data = {
duration: '00:01:00',
examTakerProgress: 1,
question: props.id,
answer: '1',
}
axios.post(`${host}/api/responseState/`, data)
.then(() => {
console.log('Created the initial Response State');
})
.catch((err) => {
console.log(err);
})
}
const getData = async () => {
await axios.all([
axios.get(`${host}/api/question/${question}`),
axios.get(`${host}/api/responseState/examTakerProgress/1/question/${props.id}`)
])
.then(axios.spread((questionData, responseData) => {
if (!responseData.data.length > 0) {
initialResponseState();
}
setResponseState(responseData.data);
setQuestion(questionData.data);
setAnswers(questionData.data.answers);
setChoice(responseData.data.length > 0 ? responseData.data[0].answer : '');
setTimeout(() => {
props.toggleLoading();
}, 50);
}))
.catch(err => console.error(err));
};
useEffect(() => {
getData()
}, [])
const handleAnswerClick = async (number: number) => {
setChoice(number);
const data = {
duration: '00:01:00',
examTakerProgress: 1,
question: props.id,
answer: number,
}
await axios.put(`${host}/api/responseState/${responseState[0].id}/`, data)
.then((data) => {
console.log(data);
console.log('choice changed to :' + number);
})
.catch((err) => {
console.log(err);
})
}
if (props.loading) {
return <Loader/>
}
return (
<React.Fragment>
<h3>{question.label}</h3>
<p className='mb-3'>{question.description}</p>
<ListGroup as="ul">
{answers.map((answer, index) => {
if (answer.id === choice) {
return <ListGroup.Item key={answer.id} action active> <Answer key={index}
answer={answer}/></ListGroup.Item>
} else {
return <ListGroup.Item key={answer.id} action onClick={() => {
handleAnswerClick(answer.id)
}}><Answer key={index}
answer={answer}/></ListGroup.Item>
}
}
)}
</ListGroup>
</React.Fragment>
)};
Can someone explain me the reason why this is happening?
Based on this line
const [responseState, setResponseState] = useState({});,
the responseState is an object.
but in the handleAnswerClick function you are using it as an array ${responseState[0].id}.
Note that this actually works if the object has a 0 key but since you're getting undefined that is not the case.
Reasons why responseState is undefined.
The most obvious is that you set responseState to undefined
The only place you call setResponseState is in getData where you have setResponseState(responseData.data);, so please check if responseData.data isn't undefined.
You never get to call setResponseState and the initial state is an object but you try to get responseState[0].id
I'm not sure what data type you are handling, but if you defined const [responseState, setResponseState] = useState({}); with the default value of useState as {} and then try to access responseState[0].id, there is two things happening.
Either you have an object with number keys or you have an array, but if you have an array, you should declare the initial value as an array.
But this isn't the problem, the problem is that you might never get to the part where you call setResponseState(responseData.data); and then you will have responseState to be {} and when trying to do responseState[0] it will be undefined, and when calling responseState[0].id you will get an error saying that you cannot read id of undefined.
Ways to fix this
Debug your code, make sure that the api call returns what you are expecting and setResponseState(responseData.data); is called.
Check if responseState[0] exists before try to access responseState[0].id
I would be able to help more, but you didn't add more information, so it's hard to help, but above is what I think it's the problem
Basically i am creating a responseState when a question loads for the first time, if a question is afterwards reloaded the State is already there since it is persisted. This means i also have to make sure to call setResponseState for the first case where the ResponseState still does not exist.
const initialResponseState = () => {
const data = {
duration: '00:01:00',
examTakerProgress: 1,
question: props.id,
answer: '1',
}
axios.post(`${host}/api/responseState/`, data)
.then(() => {
setResponseState(res.data)
console.log('Created the initial Response State');
})
.catch((err) => {
console.log(err);
})}
I am performing pagination inside and Observable stream.
The pagination is implemented with a cursor and a total count using recursion.
I am able to emit the every page using the following code observer.next(searches);, by the way I would like to use just observable and no promises but I cannot express recursion using RxJs operators.
Any suggestions?
const search = id =>
new Observable(observer => { recursePages(id, observer) })
const recursePages = (id, observer, processed, searchAfter) => {
httpService.post(
"http://service.com/search",
{
size: 50,
...searchAfter ? { search_after: searchAfter } : null,
id,
})
.toPromise() // httpService.post returns an Observable<AxiosResponse>
.then(res => {
const body = res.data;
const searches = body.data.hits.map(search => ({ data: search.data, cursor: search.id }));
observer.next(searches);
const totalProcessed = processed + searches.length;
if (totalProcessed < body.data.total) {
return recursePages(id, observer, totalProcessed, searches[searches.length - 1].cursor);
}
observer.complete();
})
}
// General Observer
incomingMessages.pipe(
flatMap(msg => search(JSON.parse(msg.content.toString()))),
concatAll(),
).subscribe(console.log),
these methods will recursively gather all the pages and emit them in an array. the pages can then be streamed with from as shown:
// break this out to clean up functions
const performSearch = (id, searchAfter?) => {
return httpService.post(
"http://service.com/search",
{
size: 50,
...searchAfter ? { search_after: searchAfter } : null,
id,
});
}
// main recursion
const _search = (id, processed, searchAfter?) => {
return performSearch(id, searchAfter).pipe( // get page
switchMap(res => {
const body = res.data;
const searches = body.data.hits.map(search => ({ data: search.data, cursor: search.id }));
const totalProcessed = processed + searches.length;
if (totalProcessed < body.total) {
// if not done, recurse and get next page
return _search(id, totalProcessed, searches[searches.length - 1].cursor).pipe(
// attach recursed pages
map(nextPages => [searches].concat(nextPages)
);
}
// if we're done just return the page
return of([searches]);
})
)
}
// entry point
// switch into from to emit pages one by one
const search = id => _search(id, 0).pipe(switchMap(pages => from(pages))
if what you really need is all of the pages to emit one by one before they're all fetched, for instance so you can show page 1 as soon as it's available rather than wait on page 2+, then that can be done with some tweaking. let me know.
EDIT: this method will emit one by one
const _search = (id, processed, searchAfter?) => {
return performSearch(id, searchAfter).pipe( // get page
switchMap(res => {
const body = res.data;
const searches = body.data.hits.map(search => ({ data: search.data, cursor: search.id }));
const totalProcessed = processed + searches.length;
if (totalProcessed < body.total) {
// if not done, concat current page with recursive call for next page
return concat(
of(searches),
_search(id, totalProcessed, searches[searches.length - 1].cursor)
);
}
// if we're done just return the page
return of(searches);
})
)
}
const search = id => _search(id, 0)
you end up with an observable structure like:
concat(
post$(page1),
concat(
post$(page2),
concat(
post$(page3),
post$(page4)
)
)
)
and since nested concat() operations reduce to a flattened structure, this structure would reduce to:
concat(post$(page1), post$(page2), post$(page3), post$(page4))
which is what you're after and the requests run sequentially.
it also seems like expand might do the trick as per #NickL 's comment, soemthing like:
search = (id) => {
let totalProcessed = 0;
return performSearch(id).pipe(
expand(res => {
const body = res.data;
const searches = body.data.hits.map(search => ({ data: search.data, cursor: search.id }));
totalProcessed += searches.length;
if (totalProcessed < body.data.total) {
// not done, keep expanding
return performSearch(id, searches[searches.length - 1].cursor);
}
return EMPTY; // break with EMPTY
})
)
}
though I've never used expand before and this is based off some very limited testing of it, but I am pretty certain this works.
both of these methods could use the reduce (or scan) operator to gather results if you ever wanted:
search(id).pipe(reduce((all, page) => all.concat(page), []))
This is my used solution combining the expand and reduce operator
searchUsers(cursor?: string) {
return from(
this.slackService.app.client.users.list({
token: this.configService.get('SLACK_BOT_TOKEN'),
limit: 1,
...(cursor && { cursor }),
}),
);
}
Usage
.......
this.searchUsers()
.pipe(
expand((res) => {
if (!!res.response_metadata.next_cursor) {
return this.searchUsers(res.response_metadata.next_cursor);
}
return EMPTY;
}),
reduce((acc, val) => {
return [...acc, ...val.members];
}, []),
)
.subscribe((users) => {
console.log(JSON.stringify(users));
});
....
I am trying to fetch the data from firebase and push the data to the array one by one in a loop. But array of the fetched data doesn't show up on the React component.
I console logged and I found that as the picture below strangely shows, console.log(array.length) is 0, but console.log(array.length) show the contents of array.
code is as follows.
const getThread = (threadID) => {
return db.ref(`threads/${threadID}`).once('value')
.then((snapshot)=>{
return snapshot.val()
})
}
const getThreadList = (id) => {
let threadList = []
return db.ref(`users/${id}/friends`).once('value')
.then((snapshot)=>{
let friends = snapshot.val()
for(let key in friends){
if(friends.hasOwnProperty(key)) {
getThread(friends[key].threadID)
.then((thread)=>{
threadList.push(thread)
})
}
}
return threadList
})
}
class SomeComponent　extends Component{
componentDidMount(){
let { id } = this.props
getThreadList(id)
.then(threadList => {
this.setState(() => ({ threadList }))
})
}
render(){
//value of threadList doses't appear
I suspect this is due to the misuse of Promise so console.log shows value but React component doesn't show anything. But I also suspect the React side and couldn't locate exactly where the problem is.
It would be much appreciated if you suggest any clue on this problem.
You are not waiting for the nested promises to resolve. You could use Promise.all to accomplish the task.
const getThreadList = (id) => {
return db.ref(`users/${id}/friends`).once('value')
.then((snapshot)=>{
let friends = snapshot.val()
const threads = []
for(let key in friends){
if(friends.hasOwnProperty(key)) {
threads.push(getThread(friends[key].threadID))
}
}
return Promise.all(threads)
})
}
Or using Object.values
const getThreadList = id => {
return db
.ref(`users/${id}/friends`)
.once("value")
.then(snapshot => {
const loadingThreads = Object.values(snapshot.val()).map(friend =>
getThread(friend.threadID)
);
return Promise.all(loadingThreads);
});
};
I'm using latest react and very basic app which calls 3rd party service API which actually is not well designed in meaning of following.
I have to execute one call which return list and then have to iterate and call other end point to get data for item from list and then again in data have new list for which I have to call 3rd API end point.
After I receive all data I combined it to one items array and place it in state in componentDidMount function but this final step works only if I surround it with setTimeout.
Is there some elegant way to do that?
I'm using fetch and really pure react components, have my own simple service from where I call API, here is some code parts...
items[tag].sensors = [];
API.getObjects(sessionData, userDetails, tag).then(links => {
Object.keys(links.link).forEach(link => {
API.getObjects(sessionData, userDetails, link).then(objLink => {
Object.keys(objLink.link).forEach(function (key) {
let obj = objLink.link[key];
if (obj && obj.type === 'sensor') {
API.getSensorNames(sessionData, key).then(response => {
const sensor = response.sensor;
// some sensor calculations....
items[tag].sensors.push(sensor);
});
}
});
});
});
});
// this part only works if it's surrounded with timeout
setTimeout(function() {
let processedItems = [];
for (var key in items) {
if (items.hasOwnProperty(key)) {
processedItems.push(items[key]);
}
}
self.setState({
items: processedItems
});
}, 1000);
Thanks in advance.
Simply, You can use Promise to wait until you get values from the API call, therefore you will put your code in function like this
function prepareItems() {
items[tag].sensors = [];
return new Promise((resolve, reject) => {
API.getObjects(sessionData, userDetails, tag).then(links => {
Object.keys(links.link).forEach(link => {
API.getObjects(sessionData, userDetails, link).then(objLink => {
Object.keys(objLink.link).forEach(function(key) {
let obj = objLink.link[key];
if (obj && obj.type === "sensor") {
API.getSensorNames(sessionData, key).then(response => {
const sensor = response.sensor;
// some sensor calculations....
items[tag].sensors.push(sensor);
// whenever you set resolve it will end the promise
//and pass the result it to the then function
resolve(items)
});
}
});
});
});
});
});
}
and use then to get the result from the prepareItems function after its resolved
prepareItems().then(items => {
//Do what ever you want with prepared item
})
What about using async/await operators.
These operators allows you to wait until the response is ready.
You can use this kind of helper function.
getItems = async (...) => {
...
items[tag].sensors = []
const links = await API.getObjects(sessionData, userDetails, tag)
Object.keys(links.link).forEach(async (link) => {
const objLink = await API.getObjects(sessionData, userDetails, link)
Object.keys(objLink.link).forEach(async (key) => {
let obj = objLink.link[key]
if (obj && obj.type === 'sensor') {
const response = await API.getSensorNames(sessionData, key)
const sensor = response.sensor
items[tag].sensors.push(sensor)
}
})
})
this.setState({ items })
}
Also you can see this great documentation.