copyToClipboard.html

<script>
// A simple function to copy a string to clipboard. See https://github.com/lgarron/clipboard.js for a full solution.
var copyToClipboard = (function() {
  var _dataString = null;

  document.addEventListener("copy", function(e){
    if (_dataString !== null) {
      try {
        e.clipboardData.setData("text/plain", _dataString);
        e.preventDefault();
      } finally {
        _dataString = null;
      }
    }
  });

  return function(data) {
    _dataString = data;
    document.execCommand("copy");
  };
})();
</script>

<button onclick="copyToClipboard(this.textContent)">Copy me!</button>