javascript - 找不到为什么要列出最近 12 个月的这个 js/jQuery 生成 "undefined"警告

Question

我无法摆脱在 ul 标记内和第一个 li 之前的 html 中显示的“未定义”消息。

HTML:

<ul id="monthFilters" class="dropdown-menu pull-right" role="menu" aria-labelledby="dropdownMenu1"></ul>

js/jQuery:

$(document).ready(function(){  
    var theMonths = new Array("01", "02", "03", "04", "05", "06", "07", "08", "09", "10", "11", "12");
    var theMonthNames = new Array("January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December");
    var today = new Date();

    var aMonth = today.getMonth();
    var aYear = today.getFullYear();

    var i;
    var monthList;

    for (i=0; i<12; i++) {

        monthList += '<li role="presentation"><a role="menuitem" tabindex="-1" href="?addedin=' + aYear + '-' + theMonths[aMonth] + '">'+theMonthNames[aMonth]+'</a></li>';

        aMonth--;

        if (aMonth < 0) {
            aMonth = 11;
            aYear--;
        } 
    }

    monthList += '<li role="presentation"><a role="menuitem" tabindex="-1" href="?all">Show All</a></li>';

    $("#monthFilters").append(monthList);

});

请参阅此 codepen 中的错误.控制台中未显示任何错误。我是不是太累了，错过了一些简单/愚蠢的东西？

Answer 1

您没有将 monthList 定义为字符串，因此您尝试向 undefined 添加一个字符串。只需在循环之前执行此操作

  var monthList = "";