HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>querySelector null</title>
</head>
<body>
  <div class="card"><p>Hello</p></div>
  <pre id="out"></pre>
  <script>
    var card = document.querySelector(".card");
    var ghost = card.querySelector(".does-not-exist");
    var lines = [String(ghost)];
    if (ghost) {
      lines.push("found");
    } else {
      lines.push("No match found");
    }
    document.getElementById("out").textContent = lines.join("\n");
  </script>
</body>
</html>

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Element.querySelector — null when no match (Try It)