Animate toggle height from 0 to auto or 100%

Please help, I want the toggle buttons to only show and hide the paragraphs but the other contents inside the div should remain always visible. Also, the CSS is the same and I am trying to smoothen the transition of the DIV's height when showing and hiding the paragraphs

const toggleButtons = document.querySelectorAll('.toggleButton');
const divs = document.querySelectorAll('div');
const paragraphs = document.querySelectorAll('.paragraph');

toggleButtons.forEach((button, index) => {
  button.addEventListener('click', () => {
    const paragraph = paragraphs[index];
    paragraph.classList.toggle('hidden');
    paragraph.classList.toggle('visible');

    if (paragraph.classList.contains('visible')) {
      divs[index].style.height = paragraph.scrollHeight + 'px';
    } else {
      divs[index].style.height = '0';
    }
  });
});

div {
  height: 0;
  overflow: hidden;
  transition: height 0.5s;
  background: blue;
}

.paragraph {
  opacity: 0;
  transition: opacity 0.5s;
}

.visible {
  opacity: 1;
  display: block;
}

.hidden {
  opacity: 0;
  display: none;
}

<button class="toggleButton">Toggle 1</button>
<div> HI
  <p class="paragraph hidden">Paragraph 1.</p>
</div>

<button class="toggleButton">Toggle 2</button>
<div> Hi
  <p class="paragraph hidden">Paragraph 2.</p>
</div>

<button class="toggleButton">Toggle 3</button>
<div> Hi
  <p class="paragraph hidden">Paragraph 3.</p>
</div>

Solution

While waiting for all browsers to support

height: calc-size(auto);
/* Currently in Canary - Experimental features */

Here are several ways to toggle-animate an element's height

Toggle height using interpolate-size: allow-keywords

Currently supported in Chromium

.content {
  interpolate-size: allow-keywords;
  transition: height 0.6s;
  overflow: hidden;
  height: 0;
}

button:focus + * > .content {
  height: auto;
}

<button type="button">Toggle 1</button>
<div>
  <div>Short intro 1</div>
  <div class="content" id="content_1">
    1. Lorem ipsum dolor sit amet, consectetur adipisicing elit. Alias labore fuga quia, iste rerum nulla, reprehenderit at quasi id omnis dolorem. Iusto repellendus nemo reiciendis rerum facere maxime dicta. Fugit.
  </div>
</div>

<button type="button">Toggle 2</button>
<div>
  <div>Short intro 2</div>
  <div class="content" id="content_2">
    2. Lorem ipsum dolor sit ametIusto repellendus nemo reiciendis rerum facere maxime dicta. Fugit.
  </div>
</div>

<button type="button">Toggle 3</button>
<div>
  <div>Short intro 3</div>
  <div class="content" id="content_3">
    3. Lorem ipsum dolor sit amet, consectetur adipisicing elit. Voluptatibus id similique laboriosam eos perspiciatis aliquam corporis esse veniam ab aspernatur debitis, provident rerum nemo blanditiis! Beatae delectus, provident totam iusto.
  </div>
</div>

Toggle height using grid-template-columns, rows

The display property cannot be transitioned (animated). Also, the trick to transition max-height is fundamentally broken since it does not work well when collapsing during a defined amount of time; makes use of magic numbers to transition to: max-height: "big-magic-number"unit;) causing animation gaps or unwanted scrollbars when using values like 100%|vh.

Solution:

transition (animate) the carousel element using CSS grid and grid-template-rows from 0fr to 1fr

document.querySelectorAll('[data-toggle]').forEach(elBtn => {
  elBtn.addEventListener("click", () => {
    document.querySelector(elBtn.dataset.toggle).classList.toggle("open");
  });
});

.content {
  display: grid;
  grid-template-rows: 0fr;
  transition: grid-template-rows 0.5s ease-in-out;
  
  &.open {
    grid-template-rows: 1fr;
  }
  
  .content-inner {
    overflow: hidden;
  }
}

<button type="button" data-toggle="#content_1">Toggle 1</button>
<div> HI
  <div class="content" id="content_1">
    <div class="content-inner">
      1. Lorem ipsum dolor sit amet, consectetur adipisicing elit. Alias labore fuga quia, iste rerum nulla, reprehenderit at quasi id omnis dolorem. Iusto repellendus nemo reiciendis rerum facere maxime dicta. Fugit.
    </div>
  </div>
</div>

<button type="button" data-toggle="#content_2">Toggle 2</button>
<div> HI
  <div class="content" id="content_2">
    <div class="content-inner">
      2. Lorem ipsum dolor sit ametIusto repellendus nemo reiciendis rerum facere maxime dicta. Fugit.
    </div>
  </div>
</div>

<button type="button" data-toggle="#content_3">Toggle 3</button>
<div> HI
  <div class="content" id="content_3">
    <div class="content-inner">
      3. Lorem ipsum dolor sit amet, consectetur adipisicing elit. Voluptatibus id similique laboriosam eos perspiciatis aliquam corporis esse veniam ab aspernatur debitis, provident rerum nemo blanditiis! Beatae delectus, provident totam iusto.
    </div>
  </div>
</div>

<hr>

<button type="button" data-toggle="#content_3">I also toggle 3.</button>

Also, don't use <p> as your carousel collapsible element. Animate display-block elements like DIV. <p> might be indeed a display-block element, but historically served the purpose as a styled content wrapper, and should be used for content and semantics, not as an architectural block, that animates.

Accessibility
For simplification-of-answer I missed to, but don't forget to add the needed A11Y Aria attributes and the needed JS code to toggle the aria-expanded="true" "false" values on click - an example on how to achieve that can be found on this related Answer: Toggle Animate Details Height from 0 to auto

Acknowledgments: