HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>opacity cssNumber</title>
  <style>
    body { font-family: sans-serif; padding: 16px; }
    #box {
      width: 140px; height: 90px; background: #3b82f6; color: #fff;
      display: flex; align-items: center; justify-content: center;
      margin: 12px 0;
    }
    button { margin: 4px 6px 4px 0; padding: 8px 12px; cursor: pointer; }
    #out { margin-top: 12px; font-weight: bold; color: #1d4ed8; }
  </style>
</head>
<body>
  <div id="box">Opacity</div>
  <button type="button" id="half">.css("opacity", 0.5)</button>
  <button type="button" id="full">.css("opacity", 1)</button>
  <div id="out">opacity is listed in $.cssNumber — jQuery does not append px.</div>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    $("#half").on("click", function() {
      $("#box").css("opacity", 0.5);
      $("#out").text("Set opacity to 0.5 (not 0.5px). Current: " + $("#box").css("opacity"));
    });
    $("#full").on("click", function() {
      $("#box").css("opacity", 1);
      $("#out").text("Reset opacity to 1.");
    });
  </script>
</body>
</html>

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jQuery.cssNumber — opacity without px (Try It)