lesspass/sandbox/copy-paste-async.html

<button id="copyButton" onclick="Run()">Generate</button>
<script>
  function GetButton() {
    return document.getElementById("copyButton");
  }
  function UpdateButtonText(text) {
    GetButton().innerText = text;
  }
  function DisableButton() {
    GetButton().disabled = true;
  }
  function GeneratePassword() {
    UpdateButtonText("Generating");
    setTimeout(() => UpdateButtonText("Generating."), 1000);
    setTimeout(() => UpdateButtonText("Generating.."), 2000);
    setTimeout(() => UpdateButtonText("Generating..."), 3000);
    return new Promise(resolve => {
      setTimeout(resolve, 4000, 'm<=u6T8`hM"nxwuY');
    });
  }
  function CopyToClipboard(password) {
    return navigator.clipboard.writeText(password).then(() => {
      UpdateButtonText("Copied in clipboard!");
      DisableButton();
    });
  }
  function Run() {
    GeneratePassword().then(password => {
      CopyToClipboard(password).catch(error => {
        UpdateButtonText("Click to copy!");
        GetButton().onclick = () => CopyToClipboard(password);
      });
    });
  }
</script>