HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>getElementsByClassName basic</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} .test{color:#0b6;} pre{border:1px solid #ccc;padding:0.75rem;}</style>
</head>
<body>
  <p>hello world 1</p>
  <p class="test">hello world 2</p>
  <p class="test">hello world 3</p>
  <button id="btn" type="button">Run</button>
  <pre id="out">Click Run</pre>
  <script>
    document.getElementById("btn").addEventListener("click", function () {
      var list = document.getElementsByClassName("test");
      document.getElementById("out").textContent =
        "length: " + list.length + "\n" +
        "first: " + (list[0] && list[0].textContent);
    });
  </script>
</body>
</html>

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Document.getElementsByClassName — one class (Try It)