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javascript - add a class name only if there is a specific class name

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');
javascript html div
  • 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 [0] .querySelector (". box2_in");
// ↑ When processing only the first element
boxin.classList.add ('box2_del');

    [Document.getElementsByClassName ()-Web API | MDN]
    https://developer.mozilla.org/en/docs/Web/API/Document/getElementsByClassName

    [Difference between the return value of getElementsByClassName and getElementById-Qiita]
    https://qiita.com/rindarinda5/items/c26dc81fe8cd98992dc1

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