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HTML

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CSS

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CSS Base

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Vendor Prefixing

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JavaScript

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JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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HTML 

              
                <div class="wrapper">
  <div class="heightKepper"></div>
  <textarea class="textarea">This Textarea is autosizing
    
The neccessary height is stored in the textarea.scrollHeight.
But to get it, the height must be set  to inherit.
While setting the height of the element to inherit, some browsers set it to the 
realy small initial height and shortens the document body by that way.

The browser scrolls in a bad way.

To prevent that, the textarea gets wrapped in the flexbox with an second and invisible 
Element side by side. The Heightkepper does its job an sets the height of the flexbox.</textarea>
</div>
!

CSS 

              
                .wrapper {
  display: flex;
  flex-direction: row;
  align-items: flex-sart;
}

.heightKepper {
  width: 1px;
  margin-right: -1px;
}

.textarea {
  overflow: hidden;
  flex-grow: 1; 
  resize: none;
}
!

JS 

              
                const textarea = document.querySelector(".textarea")
const heightKeeper = document.querySelector(".heightKeeper")

function setHeight() {
  // The neccessary height is stored in the scrollHeight
  // but to get it, the height must be set  to inherit
  textarea.style.height = "inherit"  
  // update the height
  textarea.style.height = textarea.scrollHeight+"px"
  
  // While setting the height of the element to inherit, some browsers set it to the 
  // realy small initial height and shortens the document body by that way.
  // The browser scrolls in a bad way.
  // To prevent that, the textarea gets wrapped in the flexbox with an second and invisible 
  // Element side by side. The Heightkepper does its job an sets the height of the flexbox..
  heightKeeper.style.height = textarea.scrollHeight+"px"
}

textarea.addEventListener("input", setHeight)
window.addEventListener("resize", setHeight)
setHeight()
!
999px

Console