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var textContainer, textareaSize, input;
var autoSize = function () {
  textareaSize.innerHTML = input.value + '\n';
};

document.addEventListener('DOMContentLoaded', function() {
  textContainer = document.querySelector('.textarea-container');
  textareaSize = textContainer.querySelector('.textarea-size');
  input = textContainer.querySelector('textarea');
  
  autoSize();
  input.addEventListener('input', autoSize);
});

<div class="textarea-container">
  <textarea></textarea>
  <div class="textarea-size"></div>
</div>

.textarea-container {
  position: relative;
  width: 50%;
}

textarea, .textarea-size {
  min-height: 25px;
  font-family: sans-serif;
  font-size: 14px;
  box-sizing: border-box;
  padding: 4px;
  border: 1px solid;
  overflow: hidden;
  width: 100%;
}

textarea {
  height: 100%;
  position: absolute;
  resize:none;
  white-space: normal;
}

.textarea-size {
  visibility: hidden;
  white-space: pre-wrap;
  word-wrap: break-word;
  overflow-wrap: break-word;
}