HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>part tab switch</title>
  <style>
    .tab { display: inline-block; padding: 8px 16px; cursor: pointer; border: 1px solid #ccc; }
  </style>
</head>
<body>
  <div id="tabs">
    <span class="tab" part="tab active">Tab 1</span>
    <span class="tab" part="tab">Tab 2</span>
    <span class="tab" part="tab">Tab 3</span>
  </div>
  <pre id="out"></pre>
  <script>
    var tabs = document.querySelectorAll("#tabs .tab");
    tabs.forEach(function(tab) {
      tab.addEventListener("click", function(e) {
        tabs.forEach(function(t) { t.part = "tab"; });
        e.target.part = "tab active";
        var lines = [];
        tabs.forEach(function(t) {
          lines.push(t.textContent + ": " + t.part.value);
        });
        document.getElementById("out").textContent = lines.join("\n");
      });
    });
    document.getElementById("out").textContent = "Click a tab above";
  </script>
</body>
</html>

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Element.part — tab switch (Try It)