为什么我的承诺没有返回任何价值? [英] Why is no value returned from my Promise?

问题描述

在此稍有减少的代码中，每周一次从API下载一次或多个星期的观测值，并将其汇总到rows中并导出到CSV.至少那是个主意.实际上发生的是在exportToCsv函数(未显示)中引发了Uncaught (in promise) TypeError: Cannot read property 'toString' of undefined，因为从promiseArray推送到rows的_promise会以undefined的形式出现.我想念什么?

In this slightly reduced code, one or more weeks of observations are downloaded a week at a time from an API, and aggregated into rows and exported to CSV. At least that is the idea. What is actually happening is that an Uncaught (in promise) TypeError: Cannot read property 'toString' of undefined is raised in the (not shown) exportToCsv function because the _promise pushed from promiseArray to rows is coming out as undefined. What am I missing?

$("#downloadBtn").click(function() {
    weeks = getWeeks(startDate.val(), endDate.val());  // array like [[startDay1, endDay1], [startDay2, endDay2], ...]
    // start downloading the data
    var promiseArray = [];
    for (i=0; i< weeks.length; i++) {
        var _promise = Q.defer();
        fetchDataWeek( weeks[i][0], weeks[i][1], _promise );
        promiseArray.push(_promise)  // Push this promise into the array
    }
    Promise.all(promiseArray).then( function () { // Wait for all promises to resolve
        var rows = [headers];
        for (i=0; i < promiseArray.length; i++) {
            rows.push(promiseArray[i]);
        }
        exportToCsv( fileName, rows );
    })
});

function fetchDataWeek( startDay, endDay, _promise ) {
    url = "https://api" + startDay + endDay + ".json";
    $.ajax({
        url: url,
        success: function(result){
            var weekRows = parseHistory(result);
            _promise.resolve(weekRows);
        },
        error: function (error) {
            _promise.reject(error) // rejecting it in case of error
        }
    });            
}

// Extract all data from a query response
function parseHistory(data) {
    var weekRows = [];
    var days = data.history.days;
    for (var i = 0; i < days.length; i++) {
        dayRows = formatDay( days[i] );
        for (var j= 0; j < dayRows.length; j++) {
            weekRows.push(dayRows[j]);
        }
    }
    return weekRows;
}

推荐答案

promise不是一个神奇的对象，它在解决时会变成"一个不同的值.在执行rows.push(promiseArray[i]);时，您是在收集承诺对象而不是它们包含的结果.

A promise is not a magical object that "becomes" a different value when it is resolved. When you were doing rows.push(promiseArray[i]);, you were collecting promise objects not the results that they contained.

要访问已兑现或将要兑现的结果，您需要将.then(…)回调链接到该回调，在其中可以将结果作为参数进行访问.要从数组中的所有promise收集结果，请使用Promise.all，它会返回另一个promise，该promise不仅要等待所有输入的promise，而且还要用其结果值数组来实现.

To access the result that a promise was or will be fulfilled with, you need to chain a .then(…) callback to it in which you can access the result as the parameter. To collect the results from all the promises in the array, you use Promise.all, which returns another promise that not only waits for all the input promises but also fulfills with an array of their result values.

$("#downloadBtn").click(function() {
    var weeks = getWeeks(startDate.val(), endDate.val());
    // start downloading the data
    var promiseArray = weeks.map(function(week) { // map is simpler than a loop with `push`
        return fetchDataWeek( week[0], week[1] );
    })
    Promise.all(promiseArray).then( function(results) { // Wait for all promises to resolve
        var rows = [headers].concat(results);
        exportToCsv( fileName, rows );
    })
});

function fetchDataWeek( startDay, endDay, _promise ) {
    var url = "https://api" + startDay + endDay + ".json";
    var jQpromise = $.ajax({
        url: url
    });
    var qPromise = Q(jQpromise);
    return qPromise.then(parseHistory);
}

function parseHistory(data) {
    var weekRows = [];
    var days = data.history.days;
    for (var i = 0; i < days.length; i++) {
        var dayRows = formatDay( days[i] );
        for (var j= 0; j < dayRows.length; j++) {
            weekRows.push(dayRows[j]);
        }
    }
    return weekRows;
}

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