Javascript: How to copy to clipboard

Javascript:

function myFunction() {
  /* Get the text field */
  var copyText = document.getElementById("myInput");

  /* Select the text field */
  copyText.select();
  copyText.setSelectionRange(0, 99999); /*For mobile devices*/

  /* Copy the text inside the text field */
  document.execCommand("copy");

  /* Alert the copied text */
  alert("Copied the text: " + copyText.value);
}

HTML:

<!-- The text field -->
<input type="text" value="Hello World" id="myInput">

<!-- The button used to copy the text -->
<button onclick="myFunction()">Copy text</button>