HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>mousedown drag start</title>
  <style>
    #handle {
      width: 120px; height: 48px; border: 2px solid #333;
      display: grid; place-items: center; user-select: none; cursor: grab;
    }
    #handle.is-dragging { background: #e3f2fd; cursor: grabbing; }
  </style>
</head>
<body>
  <div id="handle">Drag handle</div>
  <pre id="out">idle</pre>
  <script>
    var handle = document.getElementById("handle");
    var out = document.getElementById("out");
    var dragging = false;
    handle.addEventListener("mousedown", function (event) {
      if (event.button !== 0) return;
      dragging = true;
      handle.classList.add("is-dragging");
      out.textContent = "dragging…";
    });
    window.addEventListener("mouseup", function () {
      if (!dragging) return;
      dragging = false;
      handle.classList.remove("is-dragging");
      out.textContent = "idle";
    });
  </script>
</body>
</html>

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Element mousedown — drag start (Try It)