我有一个目前相当不正常的 Javascript 程序，它给我带来了很多问题。但是，它抛出了一个我无法理解的错误：

TypeError: 'undefined' is not an object (evaluating 'sub.from.length')

正如您可能猜到的那样，我正在尝试做的是检查 sub 字典中某个“ from ”数组的 length 。这是 整个函数的源代码 ，这是我认为导致错误的循环代码：

console.log(afcHelper_ffuSubmissions.length); // just for debugging, returns the correct number
for (var i = 0; i < afcHelper_ffuSubmissions.length; i++) { // this whole section works fine
    var sub = afcHelper_ffuSubmissions[i];
    //console.log("THIS IS BROKEN DOWN BY LINK",afcHelper_Submissions[i]);
    if (pagetext.indexOf(afcHelper_ffuSections[sub.section]) == -1) {
        // Someone has modified the section in the mean time. Skip.
        document.getElementById('afcHelper_status').innerHTML += '<li>Skipping ' + sub.title + ': Cannot find section. Perhaps it was modified in the mean time?</li>';
        continue;
    }
    var text = afcHelper_ffuSections[sub.section];
    var startindex = pagetext.indexOf(afcHelper_ffuSections[sub.section]);
    var endindex = startindex + text.length;

    console.log(sub); 
    if (typeof(sub.from) != 'undefined' && sub.from.length > 0) { // ** problem spot?? this is the code i recently added.
        for (var i = 0; i < sub.from.length; i++) {
            mainid = sub.from[i]['id'];
            var sub = afcHelper_Submissions[mainid]; // and then it goes on from here...

任何想法都很好。坦率地说，我只是不明白为什么我会收到有关我已经明确检查过类型（ typeof(sub.from) ）的 TypeError ...