how to enable click copy function using js

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

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

////html code below
<p>
  <button class="js-textareacopybtn" style="vertical-align:top;">Copy Textarea</button>
  <textarea class="js-copytextarea">Hello I'm some text</textarea>
</p>