48.setTimeOut-ball.html

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .rect {
            position: relative;
            width: 800px;
            height: 600px;
            background-color: rgb(248, 232, 232);
            border: 1px solid black;
        }

        .ball {
            position: absolute;
            width: 50px;
            height: 50px;
            border-radius: 50%;
            background-color: rgb(245, 52, 52);
            color: white;
            font-weight: 800;
            line-height: 50px;
            text-align: center;
            border: 1px solid black;
        }
    </style>
</head>

<body>
    <div class="rect">
        <div class="ball" style="left: 0px">7</div>
    </div>
    <script>
        const rect = document.querySelector(".rect");
        const ball = document.querySelector(".ball");

        const doRun = () => {
            let left = parseInt(ball.style.left) + 3 + "px";
            ball.style.left = left;
            setTimeout(doRun, 30);
        }

        doRun();
    </script>
</body>

</html>