JavaScript doesn't work with permission ()
I am having a problem with a promise in JavaScript. So what I'm trying to do is what I wanted to get from firebase and then store the entire result returned into an array. After that, I will do some sorting on the array. Here is my code:
let promise = new Promise((resolve, reject) => {
var query = firebase.database().ref('');
query.once( 'value', data => {
data.forEach(subtypeSnapshot => {
var itemData = ;
var query = firebase.database().ref('').child(itemKey);
query.once( 'value', data => {
var itemDetail = ;
datasetarr.push();
});
});
resolve(datasetarr);
});
});
With this set of code, from the first one console.log
inside the promises, I managed to get them in my console:
With these words, it means that there is nothing wrong with my extraction from the base. Afterwhich, I wanted to save each one to an array, and this is the part:
datasetarr.push({type: subtype, quantity: quantity});
After everything is done and I have resolved the promise, when the promise is made, I then print the items in the array. However, nothing is printed in the for loop internally .then()
. Any ideas?
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As mentioned: your promise is resolved too early.
You can use Promise.all to wait for all promises to be resolved before resolving the packing promise. However, I put together a simple example due to the lack of firebase database, I just use functions that return Promises: https://jsfiddle.net/57b0gkLt/
According to the firebase documentation , query.once('value')
returns a Promise, so this should work.
EDIT : Like this one
var datasetarr = [];
let promiseItemDataList = new Promise((resolve, reject) => {
var query = firebase.database().ref('receiptItemIDsByCategory').child(category);
query.once( 'value', data => {
var promises = []; // NEW LINE
data.forEach(subtypeSnapshot => {
var itemData = subtypeSnapshot.val();
var itemKey = subtypeSnapshot.key;
var query = firebase.database().ref('receiptItems').child(itemKey);
var promise = query.once('value'); // NEW LINE
promises.push(promise); // NEW LINE
promise.then(data => { // .then instead of a callback
var itemDetail = data.val();
var subtype = itemDetail.type;
var quantity = itemDetail.quantity;
console.log('inside promise ' + subtype + ' ' + quantity);
datasetarr.push({type: subtype, quantity: quantity});
});
});
Promise.all(promises).then(() => resolve(datasetarr)); // NEW LINE
});
});
promiseItemDataList.then((arr) => {
for(var i = 0; i < arr.length; i++){
console.log('outside promise ' + arr[i].type + ' ' + arr[i].quantity);
}
});
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Your first call async
to retrieve the original dataset was processed correctly, but not subsequent calls caused by the loop .forEach
.
query.once( 'value', data => {
var itemDetail = data.val();
var subtype = itemDetail.type;
var quantity = itemDetail.quantity;
console.log('inside promise ' + subtype + ' ' + quantity);
datasetarr.push({type: subtype, quantity: quantity});
});
The problem is you are making a resolving
promise before returning another async call.
I'm not sure exactly how it query.once
handles callback
. It doesn't look like it does promises and the traditional callback function
way.
What you could do like work-around
is to wrap the calls forEach.async
in an object Promise
and then run a collection of promises with Promise.all([list_of_promises])
to make sure every single call was returned before the resolving
main promise.
Pseudocode:
var datasetarr = [];
let promiseItemDataList = new Promise((resolve, reject) => {
var query = firebase.database().ref('receiptItemIDsByCategory').child(category);
query.once( 'value', data => {
// Collect a list of promises for the sub item async calls
var get_data_promises = [];
data.forEach(subtypeSnapshot => {
get_data_promises.push(new Promise(function(resolve) {
var itemData = subtypeSnapshot.val();
var itemKey = subtypeSnapshot.key;
var query = firebase.database().ref('receiptItems').child(itemKey);
query.once( 'value', data => {
var itemDetail = data.val();
var subtype = itemDetail.type;
var quantity = itemDetail.quantity;
console.log('inside promise ' + subtype + ' ' + quantity);
datasetarr.push({type: subtype, quantity: quantity});
resolve("Done");
});
}))
// Fire them all - Once done resolve the main promise.
Promise.all(get_data_promises).done(function() { resolve(datasetarr); });
});
});
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var query = firebase.database().ref('receiptItemIDsByCategory').child(category);
query.once('value')
.then((data) => {
promises =[]
data.forEach( subtypeSnapshot => {
var itemData = subtypeSnapshot.val();
var itemKey = subtypeSnapshot.key;
var query = firebase.database().ref('receiptItems').child(itemKey);
p = query.once( 'value', data => {
var itemDetail = data.val();
var subtype = itemDetail.type;
var quantity = itemDetail.quantity;
console.log('inside promise ' + subtype + ' ' + quantity);
});
promises.push(p)
})
return promises
})
.then ((arrayofpromises) => {
Promise.all(arrayofpromises)
.then((results)=>{
console.log(results)
})
})
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