I've learned how to add a class name to a div within a specific div class here
If a specific class name is on the same page as shown below
Is there a way to add it to a div class within another div class?
As an image, only when the class name "box1" div is in the same page
I want to add box2_del to the class name "box2_in".
(Added for htmlPage1, not added for htmlPage2)
var box = document.getElementsByClassName ('box1'); var boxin = box.querySelector (". box2_in"); boxin.classList.add ('box2_del');
Answer # 1
getElementsByClassName is supposed to return multiple elements, so it returns something like an array called HTMLCollection.
var box = document.getElementsByClassName ('box1'); var boxin = box  .querySelector (". box2_in"); // ↑ When processing only the first element boxin.classList.add ('box2_del');
[Document.getElementsByClassName ()-Web API | MDN]
[Difference between the return value of getElementsByClassName and getElementById-Qiita]
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