HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>getElementsByClassName scoped</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} .test{font-weight:700;} pre{border:1px solid #ccc;padding:0.75rem;}</style>
</head>
<body>
  <p class="test">outside parent</p>
  <div id="parent-id">
    <p>hello world 1</p>
    <p class="test">hello world 2</p>
    <p>hello world 3</p>
  </div>
  <button id="btn" type="button">Search inside parent</button>
  <pre id="out">Click the button</pre>
  <script>
    document.getElementById("btn").addEventListener("click", function () {
      var parentDOM = document.getElementById("parent-id");
      var test = parentDOM.getElementsByClassName("test");
      var testTarget = test[0];
      document.getElementById("out").textContent =
        "length: " + test.length + "\n" +
        "first: " + (testTarget && testTarget.textContent);
    });
  </script>
</body>
</html>

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Document.getElementsByClassName — scoped search (Try It)