HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>createElement options.is</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} pre{border:1px solid #ccc;padding:0.75rem;white-space:pre-wrap;}</style>
</head>
<body>
  <button id="btn" type="button">Create expanding-list ul</button>
  <div id="mount"></div>
  <pre id="out">Click the button (needs customized built-in support)</pre>
  <script>
    class ExpandingList extends HTMLUListElement {
      constructor() {
        super();
        this.dataset.ready = "true";
      }
    }

    if (!customElements.get("expanding-list")) {
      try {
        customElements.define("expanding-list", ExpandingList, { extends: "ul" });
      } catch (err) {
        document.getElementById("out").textContent = "define failed: " + err.message;
      }
    }

    document.getElementById("btn").addEventListener("click", function () {
      try {
        var expandingList = document.createElement("ul", { is: "expanding-list" });
        var li = document.createElement("li");
        li.textContent = "Item one";
        expandingList.appendChild(li);
        var mount = document.getElementById("mount");
        mount.innerHTML = "";
        mount.appendChild(expandingList);
        document.getElementById("out").textContent = [
          "is attribute: " + expandingList.getAttribute("is"),
          "instanceof ExpandingList: " + (expandingList instanceof ExpandingList),
          "outerHTML: " + expandingList.outerHTML
        ].join("\n");
      } catch (err) {
        document.getElementById("out").textContent = "createElement failed: " + err.message;
      }
    });
  </script>
</body>
</html>

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Document.createElement — options.is (Try It)