为什么我的 Promise 没有返回值？

Question

在这个略微简化的代码中，每周从 API 下载一周或多周的观察结果，然后汇总到 rows 并导出为 CSV。至少那是想法。实际发生的是 Uncaught (in promise) TypeError: Cannot read property 'toString' of undefined 在（未显示）exportToCsv 函数中被引发，因为从 promiseArray 推到 rows 的 _promise 作为undefined。我错过了什么？

$("#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;
}

Answer 1

您可以在 then 处理程序中收到您承诺的结果：

Promise.all(promiseArray).then( function (results) { // Wait for all promises to resolve
    var rows = [headers].concat(results);
    exportToCsv( fileName, rows );
  })

Answer 2

Promise 不是一个神奇的对象，它在解析时 "becomes" 具有不同的值。当您执行 rows.push(promiseArray[i]); 时，您收集的是 promise 对象而不是它们包含的结果。

要访问承诺已经或将要实现的结果，您需要将 .then(…) 回调链接到它，您可以在其中将结果作为参数访问。要从数组中的所有承诺中收集结果，您可以使用 Promise.all，这是另一个承诺，它 returns 不仅等待所有输入承诺，而且还用其结果值的数组来实现。

$("#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;
}

为什么我的 Promise 没有返回值？

Why is no value returned from my Promise?

javascript

jquery

promise

q