如何獲得的getElementsByTagName的長度與JavaScript

Question

通過一些示例代碼工作進行排序內的列表：

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; 
    } 
    } 
} 

的console.log（B）;產生這樣的：

但是， 的console.log（b.length個）;結果爲0

如何獲取b中的項目數量，以便我可以遍歷它們以按照圖形對我的數字進行排序？

Answer 1

這種情況時，加載HTML之前，你的JavaScript開始執行情況。如果是這樣的話，那麼你可能需要調用換到sortlist中的（）函數，就像這樣DomContentLoaded事件中：

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

Answer 2

我同意Bala's answer，主要是因爲我無法重現您的問題：

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>