按类名获取并添加类

Question

The first one, I can use document.getElementById to get the object, but on some websites, like the second, there is no id in element, but I also want to get the object with class name and add other class. How can I do this ? And do not use JQuery, Need If one class exist add other class to other element

  if (document.getElementsByClassName('class7') !== null) {
  document.querySelectorAll('class2 class4').className += " classtobeadded";
}

Answer 1

Assuming you want to select all elements that have class 'class1', and add class 'class2' to them, this will do it:

document.querySelectorAll('.class1').forEach(item => {
    item.classList.add('class2');
});

Note that when using querySelector() or querySelectorAll() , you should use CSS selectors like '.' before a class and '#' before an ID.