how to change a different div on mouseenter?

Question

I have a series of divs within the divs. HTML: I have 6 sets like this:

<div class="example" id="example1">
<div class="note">
<div class="backgr_color"></div>
<div class="note_text">
<h2>Text</h2>
<h3>Text</h3>
<div class="note_comment">
<p>Text</p>
</div>
<div class="note_bottom">
<p>text</p>
</div>
</div>
</div>
</div>

The "note" divs are squares, and the "backgr_color" divs are small dots inside the squares. I want to change the size and the color of the "backgr_color" dot when I mouseenter the corresponding "note" div.How can I do it using querySelectorAll instead of getElementById 6 times? I understand that I should iterate through both sets and find corresponding divs (note[2] = backgr_color[2]) but I cannot figure out how to do it? I don't know JQuery yet so if there is a solution in JS, that would be great!

I understand how to iterate through notes_animation (notes) and affect the notes themselves but I don't understand how to find the respective backgr_color dots?

let notes_animation = document.querySelectorAll(".note");
let backgr_color = document.querySelectorAll(".backgr_color");

for (i = 0; i < notes_animation.length; i++) {
    notes_animation[i].addEventListener('mouseenter', function () {
        this.style.backgroundColor = "red";
        });
    }

Answer 1

JS isn't the right tool for this. CSS is a better approach as it's designed to set the UI properties, and is also hardware accelerated so will perform better than JS's event-based model.

You can achieve your goal by using the :hover pseudo-selector:

.note {
  width: 200px;
  height: 200px;
  position: relative;
  background-color: #C00;
} 

.note:hover .backgr_color {
  background-color: #FFF;
} 

.note .backgr_color {
  width: 20px;
  height: 20px;
  background-color: #000;
  position: absolute;
  top: 90px;
  left: 90px;
}

<div class="example" id="example1">
  <div class="note">
    <div class="backgr_color"></div>
    <div class="note_text">
      <h2>Text</h2>
      <h3>Text</h3>
      <div class="note_comment">
        <p>Text</p>
      </div>
      <div class="note_bottom">
        <p>text</p>
      </div>
    </div>
  </div>
</div>

<div class="example" id="example1">
  <div class="note">
    <div class="backgr_color"></div>
    <div class="note_text">
      <h2>Text</h2>
      <h3>Text</h3>
      <div class="note_comment">
        <p>Text</p>
      </div>
      <div class="note_bottom">
        <p>text</p>
      </div>
    </div>
  </div>
</div>

If you really wanted to do this using JS then you can hook the event to all the .note elements in a loop, then use querySelector() to find the .backgr_color element within the .note that raised the event:

const toggleNoteClass = e => e.target.querySelector('.backgr_color').classList.toggle('hover');

document.querySelectorAll('.note').forEach(note => {
  note.addEventListener('mouseenter', toggleNoteClass);
  note.addEventListener('mouseleave', toggleNoteClass);
});

.note {
  width: 200px;
  height: 200px;
  position: relative;
  background-color: #C00;
}

.note .backgr_color {
  width: 20px;
  height: 20px;
  background-color: #000;
  position: absolute;
  top: 90px;
  left: 90px;
}

.note .backgr_color.hover {
  background-color: #FFF;
}

<div class="example" id="example1">
  <div class="note">
    <div class="backgr_color"></div>
    <div class="note_text">
      <h2>Text</h2>
      <h3>Text</h3>
      <div class="note_comment">
        <p>Text</p>
      </div>
      <div class="note_bottom">
        <p>text</p>
      </div>
    </div>
  </div>
</div>

<div class="example" id="example1">
  <div class="note">
    <div class="backgr_color"></div>
    <div class="note_text">
      <h2>Text</h2>
      <h3>Text</h3>
      <div class="note_comment">
        <p>Text</p>
      </div>
      <div class="note_bottom">
        <p>text</p>
      </div>
    </div>
  </div>
</div>

Answer 2

It seems like you are very close. I would think you would want to add/remove a class to one of the html elements involved and handle the different styles via css. This is the approach I would suggest. If you want to remove the class on mouseleave though, you'll have to add another event listener.

let notes_animation = document.querySelectorAll(".note");
let backgr_color = document.querySelectorAll(".backgr_color");

for (i = 0; i < notes_animation.length; i++) {
    notes_animation[i].addEventListener('mouseenter', function (event) {
        event.target.querySelector('.backgr_color').classList.add('foo');
    });
}

.note {
  display: flex;
}

.backgr_color {
  background-color: tomato;
  height: 10px;
  width: 10px;
}

.backgr_color.foo {
  background-color: yellow;
}

<div class="example" id="example1">
  <div class="note">
    <div class="backgr_color"></div>
    <div class="note_text">
      <h2>Text</h2>
      <h3>Text</h3>
      <div class="note_comment">
        <p>Text</p>
      </div>
      <div class="note_bottom">
        <p>text</p>
      </div>
    </div>
  </div>
</div>