Simple `navigator.clipboard.writeText()` polyfill.

copyToClipboard.html

<script>
// A minimal polyfill for `navigator.clipboard.writeText()` that works most of the time in most modern browsers.
// Note that:
// - In Edge, this may call `resolve()` even if copying failed.
// - In Safari, this may fail if there is nothing selected on the page.
// See https://github.com/lgarron/clipboard-polyfill for a more robust solution.
// License: public domain
function writeText(str) {
  return new Promise(function(resolve, reject) {
    var success = false;
    function listener(e) {
      e.clipboardData.setData("text/plain", str);
      e.preventDefault();
      success = true;
    }
    document.addEventListener("copy", listener);
    document.execCommand("copy");
    document.removeEventListener("copy", listener);
    success ? resolve(): reject();
  });
};
</script>

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