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HTML

CSS

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HTML

              
                <main>
  <h2>Using <code>window.getComputedStyle()</code> along with <code>getPropertyValue()</code></h2>
  
  <div class="parent">
    <p>Parent element</p>
    <div class="child">
      <p>Child element</p>
    </div>
  </div>
  
  <p><output></output></p>
  
  <p><strong>Resize the parent element</strong>. The width of both elements will be printed on the page on each resize. Neither the parent or child element has a width or height defined in the CSS, but <code>window.getComputedStyle()</code> along with <code>getPropertyValue()</code> will still return the actual width or height of the element.</p>

</main>
              
            
!

CSS

              
                * {
  box-sizing: border-box;
}

body {
  padding: 0 20px;
  line-height: 1.4;
  font-size: 1.3em;
  font-family: Helvetica, Arial, sans-serif;
}

main {
  max-width: 700px;
  margin: 0 auto;
}

code, output {
  color: firebrick;
  font-family: Consolas, monospace;
  word-wrap: break-word;
}

.parent {
  overflow: auto;
  border: solid 1px #ccc;
  resize: both;
  padding: 10px;
}

.child {
  border: solid 1px pink;
}
              
            
!

JS

              
                let parent = document.querySelector('.parent'),
    child = document.querySelector('.child'),
    op = document.querySelector('output');

parent.addEventListener('mouseup', function () {
  op.innerHTML = `Parent width is: ${window.getComputedStyle(parent).getPropertyValue('width')}<br><br>Child width is: ${window.getComputedStyle(child).getPropertyValue('width')}`;
}, false);
              
            
!
999px

Console