JSFiddle - React, Tailwind, and code Playground

by gleezer

HTML

<button id="starter"> Calculate heights </button>

<!-- hiddenParent is between two other divs but takes no space in the document as it's display:none -->
<div id="hiddenParent" style="display: none;">
  <div class="childDiv">child div 1</div>
  <div class="childDiv">child div 2</div>
  <div class="childDiv">child div 3</div>
</div>


<div class="another-div">
    here's another div
</div>

CSS

#hiddenParent, .another-div{
    background: teal;
    width: 40%;
    padding: 20px;
    text-align: center;
    color: white;
    font-family: Arial;
    text-transform: uppercase;
    color: #ccc;
}
.childDiv{
    background: purple;
    padding: 10px;
    margin-bottom: 10px;
}

.another-div{
    background: orange;
    color: #1F9091;
}

JavaScript

var par = document.getElementById('hiddenParent'),
    cd = par.querySelectorAll('.childDiv'),
    len,
    heights = [],
    i;

function getHeight(){
    len = cd.length;
  // 1.we hide the parent
    par.style.visibility = 'hidden';
  // 2. we set its position to absolute, so it does not 
  // take space inside the window
    par.style.position = 'absolute';
  // 3. we set its display to block so it will gain back its natural height
    par.style.display = 'block';   

  // 4. we start looping over its children
    while(len--){
  // we get the height of each children... here we're just storing them in array,
  // which we will alert later
      heights[len] = window.getComputedStyle( cd[len] ).getPropertyValue('height');
    }
  // 5. Job is done, we can bring back everything to normal (if needed)
    par.cssText = 'display: none';   
}

document.getElementById('starter').addEventListener('click',function(){
   getHeight();
   alert(heights);
});