CodePen

HTML

            
              <form action="#" method="post">
  <textarea onkeypress="checkTextValue()" onkeyup="checkTextValue()" onchange="checkTextValue()" id="text"></textarea>
  <input id="submit" type="submit" value="Submit">
</form>
            
          
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CSS

            
                #text {
  width: 540px;
  height: 50px;
  overflow: hidden;
  }

  #submit {
  display: none;
  }
            
          
!
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JS

            
              var textInput = document.getElementById('text')
  , submitButton = document.getElementById('submit');
function checkTextValue() {
  if (textInput.value !== '') {
    submitButton.style.display = 'block';
  } else {
    submitButton.style.display = 'none';
  }
}
            
          
!
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