如何用javascript获取getElementsByTagName的长度

Question

使用一些示例代码对中的列表进行排序：

function sortList() {
  var list, i, switching, b, shouldSwitch;
  list = document.getElementById("timelineul");
  switching = true;
  /*Make a loop that will continue until
  no switching has been done:*/
  while (switching) {
    //start by saying: no switching is done:
    switching = false;
    b = list.getElementsByTagName("figcaption");
    //Loop through all list items:
    console.log(b);
    console.log(b.length);
    for (i = 0; i < (b.length - 1); i++) {
      //start by saying there should be no switching:
      shouldSwitch = false;
      /*check if the next item should
      switch place with the current item:*/
      console.log(b[i].innerHTML);
      console.log(b[i+1].innerHTML);
      if (b[i].innerHTML.toLowerCase() > b[i + 1].innerHTML.toLowerCase()) {
        /*if next item is alphabetically lower than current item,
        mark as a switch and break the loop:*/
        shouldSwitch= true;
        break;
      }
    }
    if (shouldSwitch) {
      /*If a switch has been marked, make the switch
      and mark the switch as done:*/
      b[i].parentNode.insertBefore(b[i + 1], b[i]);
      switching = true;
    }
  }
}

console.log(b)；产生以下结果：

​

​

但是，console.log(b.length)；结果为0

我怎样才能得到b中的项数，这样我就可以遍历它们，按照字幕对我的图形进行排序？

​

​

Answer 1

当您的javascript在html加载之前开始执行时，就会发生这种情况。如果是这种情况，则可能必须将对sortList()函数的调用包装在DomContentLoaded事件中，如下所示：

document.addEventListener("DOMContentLoaded", function(e) {
    sortList();
});

Answer 2

我同意Bala's answer的观点，主要是因为我无法重现您的问题：

​

var div = document.getElementById('a');
var spans = div.getElementsByTagName('span');

console.log(spans.length);

<div id="a">
  <ul>
    <li><span>hi</span></li>
    <li><span>yo</span></li>
    <li><span>dude</span></li>
  </ul>
</div>

​