drag.html

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <div id="box"></div>
    <style>
        #box {
            width: 100px;
            height: 100px;
            background-color: aqua;
        }
    </style>

    <script>
        const box = document.querySelector("#box")
        box.onmousedown = function (event) {
            // 计算鼠标点的位置在哪
            let shiftx = event.clientX - box.getBoundingClientRect().left
            let shifty = event.clientY - box.getBoundingClientRect().top
            box.style.position = 'absolute'

            function move(x, y) {
                box.style.left = x + shiftx + 'px'
                box.style.top = x + shifty + 'px'
            }

            function mouseMove(event) {
                move(event.clientX, event.clientY)
            }

            // box上mousedown时，给document添加一个mouseMove，根据鼠标的移动距离，设置box的移动距离
            document.onmousemove = mouseMove
            // 在box上抬起时，清除事件
            box.onmouseup = function () {
                document.onmousemove = null
                box.onmouseup = null
            }
        };

        // 取消ondragstart事件的默认行为防止冲突
        box.ondragstart = function () {
            return false;
        };
    </script>
</body>

</html>