HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>keydown keyup pair</title>
  <style>
    body { font-family: sans-serif; padding: 16px; max-width: 420px; }
    label { display: block; margin-bottom: 6px; font-weight: 600; }
    input { width: 100%; padding: 10px 12px; border: 2px solid #cbd5e1; border-radius: 6px; font-size: 16px; box-sizing: border-box; margin-bottom: 16px; }
    .row { font-family: monospace; font-size: 14px; margin-bottom: 8px; color: #334155; }
    #hint { color: #64748b; font-size: 14px; margin-top: 10px; }
  </style>
</head>
<body>
  <label for="target">Hold a key</label>
  <input type="text" id="target" placeholder="Hold Space or a letter, then release">
  <div class="row" id="down">Keys held: none</div>
  <div class="row" id="up">Last released: —</div>
  <p id="hint">keydown adds to map; keyup removes — accurate held-key state.</p>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    var keysDown = {};
    $("#target").on("keydown", function(event) {
      keysDown[event.which] = true;
      $("#down").text("Keys held: " + Object.keys(keysDown).join(", "));
    }).on("keyup", function(event) {
      delete keysDown[event.which];
      $("#up").text("Last released: " + event.which);
      $("#down").text("Keys held: " + (Object.keys(keysDown).length ? Object.keys(keysDown).join(", ") : "none"));
    });
  </script>
</body>
</html>

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jQuery keydown + keyup — held key detection (Try It)