循环反转数组的Javascript

Javascript for loop reversing array

本文关键字：Javascript 数组 循环      更新时间：2024-01-27

我在for循环中填充两个下拉控件，这两个控件都是包含相同数据的telerik下拉控件（月或季度）。奇怪的是，这个代码的结果是，一个控件是从Jan-Dec开始订购的，但另一个控件被颠倒了，Dec-Jan（monthToDropdown）。我已经修复了第二块代码的问题，但我想了解为什么会发生这种情况，某种参考问题？

这是产生交替下降的代码：

 function PeriodChoiceIndexChange(sender, args) {
    var itemText = args.get_item()._text;
    var monthToDropDown,  monthFromDropDown ;
    monthToDropDown = $find("<%= ToMonthDropDown.ClientID %>");
    monthFromDropDown = $find("<%= FromMonthDropDown.ClientID %>");
    var months = new Array();
    months[0] = "Jan";
    months[1] = "Feb";
    months[2] = "Mar";
    months[3] = "Apr";
    months[4] = "May";
    months[5] = "Jun";
    months[6] = "Jul";
    months[7] = "Aug";
    months[8] = "Sep";
    months[9] = "Oct";
    months[10] = "Nov";
    months[11] = "Dec";
    var quarters = new Array();
    quarters[0] = "Q1";
    quarters[1] = "Q2";
    quarters[2] = "Q3";
    quarters[3] = "Q4";
    monthToDropDown.trackChanges();
    monthToDropDown.clearItems();
    monthToDropDown.commitChanges();
    monthFromDropDown.trackChanges();
    monthFromDropDown.clearItems();
    monthFromDropDown.commitChanges();
    if (itemText == "Quarterly") {
        for (var i = 0; i < quarters.length; i++) {
            var comboItem = new Telerik.Web.UI.RadComboBoxItem();
            comboItem.set_text(quarters[i]);
            comboItem.set_value(quarters[i]);
            monthToDropDown.trackChanges();
            monthToDropDown.get_items().add(comboItem);
            monthToDropDown.commitChanges();
            monthFromDropDown.trackChanges();
            monthFromDropDown.get_items().add(comboItem);
            monthFromDropDown.commitChanges();
        }
    } else {
        for (var i = 0; i < months.length; i++) {
            var comboItem = new Telerik.Web.UI.RadComboBoxItem();
            comboItem.set_text(months[i]);
            comboItem.set_value(months[i]);
            monthToDropDown.trackChanges();
            monthToDropDown.get_items().add(comboItem);
            monthToDropDown.commitChanges();
            monthFromDropDown.trackChanges();
            monthFromDropDown.get_items().add(comboItem);
            monthFromDropDown.commitChanges();
        }
    }
    return false;
}

以下是按相同顺序填充两个控件的代码：

    if (itemText == "Quarterly") {
        for (var i = 0; i < quarters.length; i++) {
            var comboItem = new Telerik.Web.UI.RadComboBoxItem();
            comboItem.set_text(quarters[i]);
            comboItem.set_value(quarters[i]);
            var comboItem1 = new Telerik.Web.UI.RadComboBoxItem();
            comboItem1.set_text(quarters[i]);
            comboItem1.set_value(quarters[i]);
            monthToDropDown.trackChanges();
            monthToDropDown.get_items().add(comboItem);
            monthToDropDown.commitChanges();
            monthFromDropDown.trackChanges();
            monthFromDropDown.get_items().add(comboItem1);
            monthFromDropDown.commitChanges();
        }
    } else {
        for (var i = 0; i < months.length; i++) {
            var comboItem = new Telerik.Web.UI.RadComboBoxItem();
            comboItem.set_text(months[i]);
            comboItem.set_value(months[i]);
            var comboItem1 = new Telerik.Web.UI.RadComboBoxItem();
            comboItem1.set_text(months[i]);
            comboItem1.set_value(months[i]);
            monthToDropDown.trackChanges();
            monthToDropDown.get_items().add(comboItem);
            monthToDropDown.commitChanges();
            monthFromDropDown.trackChanges();
            monthFromDropDown.get_items().add(comboItem1);
            monthFromDropDown.commitChanges();
        }
    }

正如你所看到的，当我已经解决了这个问题时，我想了解这个反转背后的理论

@nnnnnn上面说的是正确的。在循环中，我们将元素添加到第一个下拉列表，然后尝试将其添加到第二个下拉列表。但是，将相同的元素添加到第二个下拉列表将从第一个下拉列表中删除。

我创建了一个SSCCE来说明实际发生了什么。

function PeriodChoiceIndexChange(sender, args) {
    var monthToDropDown = $find("ToMonthDropDown"),
        monthFromDropDown = $find("FromMonthDropDown");
    var months = new Array();
    months[0] = "Jan";
    months[1] = "Feb";
    monthFromDropDown.trackChanges();
    monthFromDropDown.clearItems();
    monthFromDropDown.commitChanges();
    monthToDropDown.trackChanges();
    monthToDropDown.clearItems();
    monthToDropDown.commitChanges();
    for (var i = 0; i < months.length; i++) {
        var comboItem = new Telerik.Web.UI.RadComboBoxItem();
        comboItem.set_text(months[i]);
        comboItem.set_value(months[i]);
        alert("[" + i + "][0] From Count = " + monthFromDropDown.get_items().get_count() + "; To Count = " + monthToDropDown.get_items().get_count());
        monthFromDropDown.get_items().add(comboItem);
        alert("[" + i + "][1] From Count = " + monthFromDropDown.get_items().get_count() + "; To Count = " + monthToDropDown.get_items().get_count());
        monthToDropDown.get_items().add(comboItem);
        alert("[" + i + "][2] From Count = " + monthFromDropDown.get_items().get_count() + "; To Count = " + monthToDropDown.get_items().get_count());
    }
    alert("[PeriodChoiceIndexChange] End");
    return false;
}

当你运行这个程序时，你会得到以下输出：

回路警报t0警报t1警报20 0，0 1，0 0，11 0，1 1，1 0，2

要修复当前的方法，只需更改最后一个块即可重新实例化项目，如下所示：

var comboItem = new Telerik.Web.UI.RadComboBoxItem();
comboItem.set_text(quarters[i]);
comboItem.set_value(quarters[i]);
monthToDropDown.trackChanges();
monthToDropDown.get_items().add(comboItem);
monthToDropDown.commitChanges();
comboItem = new Telerik.Web.UI.RadComboBoxItem();
comboItem.set_text(quarters[i]);
comboItem.set_value(quarters[i]);
monthFromDropDown.trackChanges();
monthFromDropDown.get_items().add(comboItem);
monthFromDropDown.commitChanges();
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