javascript - 无法删除类(class)

Question

我正尝试在我的 HTML 页面上调试下一个 JavaScript 代码:

var langs = document.getElementsByClassName('ss-code-language');
for (var i in langs)
{
    langs[i].classList.remove('ss-highlighted');
    document.getElementById('assa').classList.remove('ss-highlighted');
}

它工作正常。但是当我删除下一行时:

var langs = document.getElementsByClassName('ss-code-language');
for (var i in langs)
{
    langs[i].classList.remove('ss-highlighted');
}

我明白了

Uncaught TypeError: Cannot call method 'remove' of undefined.

但我在 Watch 中看到该项目已定义。下一行代码如何影响上一行代码。

Answer 1

document.getElementsByClassName('ss-code-language')返回类数组对象而不是数组。

elements is a HTMLCollection of found elements.

在 for (var i in langs) 中，您实际上是在遍历对象的所有属性。

遍历 HTMLCollection:

 collection = document.getElementsByClassName('ss-code-language');
 var len = collection.length //see below - accessing length is expensive!
 for(var i=0; i<len; i++){
      collection.item(i) /* ... */
 }

修复:使用 HTMLCollection 时，建议缓存 length 属性。查看更多:

• https://blogs.oracle.com/greimer/entry/best_way_to_code_a
• http://oreilly.com/server-administration/excerpts/even-faster-websites/writing-efficient-javascript.html

(感谢@Benjamin Gruenbaum)