[英]Why a promise with a value undefined is returned from “findLocation()”?
[英]Why is no value returned from my Promise?
在这个稍微减少的代码中,每周一次从API下载一次或多个星期的观察值,并将其汇总rows
并导出到CSV。 至少那是主意。 实际发生的是,在(未显示) exportToCsv
函数中Uncaught (in promise) TypeError: Cannot read property 'toString' of undefined
了Uncaught (in promise) TypeError: Cannot read property 'toString' of undefined
因为从_promise
推promiseArray
各rows
promiseArray
是undefined
。 我想念什么?
$("#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;
}
一个诺言不是一个神奇的对象,它在解决时会“变成”一个不同的值。 当你做rows.push(promiseArray[i]);
,您是在收集承诺对象,而不是它们包含的结果。
要访问已兑现或将要兑现的结果,您需要将.then(…)
回调链接到其,在其中您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,可以使用Promise.all
,它返回另一个promise,该promise不但要等待所有输入promise,而且要用其结果值数组来实现。
$("#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;
}
您可以在then
处理程序中收到诺言的结果:
Promise.all(promiseArray).then( function (results) { // Wait for all promises to resolve
var rows = [headers].concat(results);
exportToCsv( fileName, rows );
})
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