Edit in JSFiddle

var copyTextareaBtn = document.querySelector('.js-textareacopybtn');

copyTextareaBtn.addEventListener('click', function(event) {
  var copyTextarea = document.querySelector('.js-copytextarea');
  copyTextarea.select();

  try {
    var successful = document.execCommand('copy');
    var msg = successful ? 'successful' : 'unsuccessful';
    console.log('Copying text command was ' + msg);
  } catch (err) {
    console.log('Oops, unable to copy');
  }
});

  <textarea class="js-copytextarea" style="width:100%;" rows="5">This is the text that will be selected. Note that you can hide this and implement a secret copy feature.</textarea>
<p> You can hide this textare in your document and change the text of the textarea and copy it when you want.
</p>

<p>
  <button class="js-textareacopybtn">Copy Textarea text</button>
</p>