<div class="wrap">
  <p>會上他表格在會過心設異不。充言交才各難；發車的！比我更怕它教時只發便的不式：是看了事，首法可成十裝客的會情容。</p>
  <button class="btnCopy">copy</button>
</div>

  

.wrap {
  margin-bottom: 10px;
}

console.clear();


const btnCopy = document.querySelector('.btnCopy');

btnCopy.addEventListener('click', function() {
  document.execCommand('copy');
  const selection = window.getSelection();
  alert(selection.toString());
});