實現千位符和保留位數

    function myTho(n, s) {
        // 當不是字符串時，強制轉換爲字符串
        if (typeof n != 'string') {
            n = String(n);
            console.log(n)
        }
        let num = n.split('.');
        let zs = num[0];
        let xs = num[1] || '';
        console.log(xs);
        let arr = zs.split('').reverse();
        // arr2存放 反轉並添加 , 後的數組
        let arr2 = [];
        arr.forEach((a, i) => {
            if (i % 3 == 0 && i != 0) {
                arr2.push(',');
            }
            arr2.push(a);
        });
        // 小數部分處理
        switch (xs.length) {
            case 0:
                xs = xs + '0000'
                break;
            case 1:
                xs = xs + '000'
                break;
            case 2:
                xs = xs + '00'
                break;
            case 3:
                xs = xs + '0'
                break;
            default:
                xs = xs.substr(0, 4)
                break;
        }

        // 存放整數的數組 反轉 並以字符串形式返回， 拼接 '.' 和小數部分
        return arr2.reverse().join('') + '.' + xs;
    }