jarv.is/assets/js/src/clipboard.js

import ClipboardJS from "clipboard";
import trimNewlines from "trim-newlines";

// the default text of the copy button:
const copyTerm = "Copy";

// immediately give up if not supported
if (ClipboardJS.isSupported()) {
  // loop through each code fence on page (if any)
  document.querySelectorAll("div.highlight").forEach((highlightDiv) => {
    const button = document.createElement("button");
    button.className = "copy-button";
    button.innerText = copyTerm;

    // insert button as a sibling to Hugo's code fence
    highlightDiv.insertBefore(button, highlightDiv.firstChild);
  });

  new ClipboardJS("button.copy-button", {
    text: (trigger) => {
      // actual code element will (should) have class "language-*", even if plaintext
      const fenceElement = trigger.parentElement.querySelector('code[class^="language-"]'); // eslint-disable-line quotes

      return fenceElement ? trimNewlines(fenceElement.innerText) : false;
    },
  }).on("success", (e) => {
    // show a subtle indication of success
    e.trigger.innerText = "✓";

    // reset button to original text after 2 seconds
    setTimeout(() => {
      e.trigger.innerText = copyTerm;
    }, 2000);

    // text needed to be auto-selected to copy, unselect immediately
    e.clearSelection();
  });
}