HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>getElementsByTagNameNS scoped</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;border:solid green 3px;} #div1{border:solid blue 3px;padding:0.5rem;margin:0.5rem 0;} #div2{border:solid red 3px;padding:0.5rem;} button{display:block;margin:0.35rem 0;} pre{border:1px solid #ccc;padding:0.75rem;}</style>
</head>
<body>
  <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>
  <button id="btn2" type="button">Count p in div1 (NS)</button>
  <button id="btn3" type="button">Count p in div2 (NS)</button>
  <pre id="out">Click a button</pre>
  <script>
    var XHTML_NS = "http://www.w3.org/1999/xhtml";
    document.getElementById("btn2").addEventListener("click", function () {
      var div1 = document.getElementById("div1");
      var div1Paras = div1.getElementsByTagNameNS(XHTML_NS, "p");
      document.getElementById("out").textContent =
        "There are " + div1Paras.length + " <p> elements in div1 element";
    });
    document.getElementById("btn3").addEventListener("click", function () {
      var div2 = document.getElementById("div2");
      var div2Paras = div2.getElementsByTagNameNS(XHTML_NS, "p");
      document.getElementById("out").textContent =
        "There are " + div2Paras.length + " <p> elements in div2 element";
    });
  </script>
</body>
</html>

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Document.getElementsByTagNameNS — scoped parent (Try It)