array.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>
<script>
    let x = [20, 30, [2, 3, 4, 5], [19, [30, 25, 35], 20], 40, 50];
    let solvearray = [];

    let a = [...x];

    while (a.length) {
        let current = a.pop();

        if (Array.isArray(current)) {

            a.push(...current);
        } else {

            solvearray.unshift(current);
        }
    }

    console.log(solvearray);


</script>

<body>

</body>

</html>