HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>getElementsByTagName basic</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;border:solid green 3px;} pre{border:1px solid #ccc;padding:0.75rem;}</style>
</head>
<body>
  <p>Some outer text</p>
  <p>Some outer text</p>
  <div id="div1">
    <p>Some div1 text</p>
    <p>Some div1 text</p>
    <p>Some div1 text</p>
    <div id="div2">
      <p>Some div2 text</p>
      <p>Some div2 text</p>
    </div>
  </div>
  <p>Some outer text</p>
  <p>Some outer text</p>
  <button id="btn1" type="button">Show all p elements in document</button>
  <pre id="out">Click the button</pre>
  <script>
    document.getElementById("btn1").addEventListener("click", function () {
      var allParas = document.getElementsByTagName("p");
      document.getElementById("out").textContent =
        "There are " + allParas.length + " paragraph in this document";
    });
  </script>
</body>
</html>

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Document.getElementsByTagName — count paragraphs (Try It)