JavaScript承诺不与resolve（）一起工作

问题描述

我在JavaScript中遇到了一些问题。所以我想做的是我想从firebase中检索，然后将所有返回的结果存储到数组中。之后，我将对数组执行一些排序。这里是我的代码：

pre $ 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）;
}）;
}） ;


使用这组代码，从第一个 console.log 里面的承诺，我设法得到这些在我的控制台：

这意味着我的firebase检索没有任何问题。在这之后，我想把它们存储到数组中，这就是这个部分：

$ $ $ $ $ $ $> datasetarr.push（{type：subtype，数量：数量}）;

完成所有工作后，我解决了承诺，承诺完成后，阵列。但是，在 .then（）内的for循环中没有任何东西被打印出来。任何想法？

解决方案

正如已经提到：您的承诺太早解决。

您可以使用

根据， query.once（'value'）返回一个Promise， 。
$ b 编辑：像这样

  var datasetarr = []; 
 let promiseItemDataList = new Promise（（resolve，reject）=> {
 var query = firebase.database（）。ref（'receiptItemIDsByCategory'）。child（category）; 
 query。一次（'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 => {//。然后代替回调
 var itemDetail = data.val（）; 
 var subtype = itemDetail.type; 
 var quantity = itemDetail.quantity; 
 console.log（'inside promise'+ subtype +''+ quantity）; 
 datasetarr.push（{type：子类型，数量：数量}）; 
}）; 
}）; $（b）b 
 $ b Promise.all（promises）.then（（）=> resolve（datasetarr））; // NEW LINE 
}）; 
}）; （（arr）=> {
 for（var i = 0; i< arr.length; i ++）{
 console.log（'outside promise'+ arr [i] .type +''+ arr [i] .quantity）; 
} 
}）; 
  

I was having some problem with promise in JavaScript. So what I am trying to do is I wanted to retrieve from firebase, then store all the result returned into array. After that I will perform 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 console.log inside the promise, I managed to get these in my console:

With these, it means there is nothing wrong with my firebase retrieval. Afterwhich, I wanted to store each of them into array and this is the part:

datasetarr.push({type: subtype, quantity: quantity});

After done everything and I resolved the promise, when promise is done, I then print out the items in array. However, nothing is printed out at the for loop inside .then(). Any ideas?

解决方案

As already mentioned: Your Promise is resolved too early.

You can use Promise.all to wait for all promises to resolve before resolving the wrapping Promise. I put together a simple example, however, because of the lack of a firebase database, I just use functions returning Promises: https://jsfiddle.net/57b0gkLt/

According to the firebase documentation, query.once('value') returns a Promise, so this should work.

EDIT: Like this

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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