开发者问题收集

未捕获（在承诺中）TypeError：无法读取promiseKey.then中未定义的属性“length”

2017-08-16
10750
javascript asynchronous promise resolve

我尝试从承诺中返回一个推送数据数组，然后循环多次以填充所有数据。例如，我得到了一个品牌列表和一个函数，用于接收品牌参数并返回一个推送数据数组。 

var datasetarr = [];
brandlist = ['Bh', 'Ruffles'];

let promiseKey = new Promise((resolve, reject) => {
    for(var i = 0; i < brandlist.length; i++){
        datasetarr = populateData(brandlist[i], datasetarr);
    }
    resolve(datasetarr);
});

promiseKey.then((arr) => {
    console.log('complete promise');
    for(var i = 0; i < arr.length; i++){
        console.log(arr[i].date + ' ' + arr[i].total);
    }
});

错误消息是 

Uncaught (in promise) TypeError: Cannot read property 'length' of undefined
    at promiseKey.then

我的数据获取没有问题，因为我设法打印出了详细信息。这意味着承诺没有正确解决。这是从承诺中返回数组的正确方法吗？我不确定哪部分是错的。

2个回答

首先，您的 populateData 需要返回一个 Promise - 在这种情况下，它将是在 data.forEach 中创建的已解析的承诺数组 

var brandlist = ['Bh', 'Ruffles'];

let promiseKey = Promise.all(brandlist.map(brand => populateData(brand)))
.then(results => [].concat(...results)); // flatten the array of arrays

promiseKey.then((arr) => {
    console.log('complete promise');
    for(var i = 0; i < arr.length; i++){
        console.log(arr[i].date + ' ' + arr[i].total);
    }
});

function populateData(brand, datasetarr) {
    console.log('go in');
    var query;// = // query by brand parameter
    return query.once('value').then(data => {
        var promises = [];
        data.forEach(snapshot => {
            // get each receipt item details

            // get receipt details by receipt ID
            var query;// = // query receipts
            promises.push(query.once('value').then(data => { 
                // removed code
                // managed to print out here so data fetching is not a problem
                console.log(brand + ' ' + date + ' ' + itemTotal);
                return {date: date, total: itemTotal};
            })); 
        }); 
        return Promise.all(promises);
    });
}

或者，使用我在一周前的 这个答案 中提供的 snapshotToArray 函数 

const snapshotToArray = snapshot => {
    const ret = [];
    snapshot.forEach(childSnapshot => {
        ret.push(childSnapshot);
    });
    return ret;
};

function populateData(brand, datasetarr) {
    console.log('go in');
    var query;// = // query by brand parameter
    return query.once('value').then(data => Promise.all(snapshotToArray(data).map(snapshot => {
        // get each receipt item details

        // get receipt details by receipt ID
        var query;// = // query receipts
        return query.once('value').then(data => { 
            // removed code
            // managed to print out here so data fetching is not a problem
            console.log(brand + ' ' + date + ' ' + itemTotal);
            return {date: date, total: itemTotal};
        }); 
    }))); 
}
Jaromanda X
2017-08-16

虽然可以使用全局 datasetarray 变量并从任何地方将其 push 到该变量，但我不建议这样做。相反，编写一个方法 getData 来返回一个数组（一个承诺），并在多次调用该方法（每个品牌一次）后将它们连接在一起。 

const brandlist = ['Bh', 'Ruffles'];
const promiseKey = Promise.all(brandlist.map(getData)).then(arrays => [].concat(...arrays));
promiseKey.then(arr => {
    console.log('complete promise');
    for (const item of arr)
        console.log(item.date + ' ' + item.total);
});

function getData(brand) { // no array parameter!
    console.log('go in');
    const query = …; // query by brand parameter
    return query.once('value').then(data => {
        const promises = toArray(data).map(snapshot => {
        const query = …; // get receipt item details by receipt ID
        return query.once('value').then(data => { 
            …
            return {date: date, total: itemTotal}; // don't push, just return the result
        });
        return Promise.all(promises); // resolves with an array of results
    }); // resolves with that same result array
}
function toArray(forEachable) {
    const arr = [];
    forEachable.forEach(x => { arr.push(x); });
    return arr;
}
Bergi
2017-08-16