add copy-to-clipboard button to code fences

assets/js/src/clipboard.js

@@ -0,0 +1,33 @@
+import ClipboardJS from "clipboard";
+
+// 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, {
+      // actual code element will have class "language-*", even if plaintext
+      text: (trigger) => trigger.parentElement.querySelector('code[class^="language-"]').innerText, // eslint-disable-line quotes
+    }).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();
+    });
+  });
+}