两种思路：

• 数字转字符串转数组，逆序输出（简单，可读性高）
• 数学取余构建新字符串（效率高）

Code

const reverseNumberAsStr = num => [...num.toString()]
    .reverse()
    .join('')



const reverseNumberAsStr = (num, res = '') => {
    if(num < 10) return `${res}${num}`
    res = `${res}${num % 10}`
    num = ~~(num / 10)
    return reverseNumberAsStr(num, res)
}

console.log(reverseNumberAsStr(readline()))