在JavaScript中，在单个日期前面添加零

function padZero(num) {
    return num < 10 ? '0' + num : num;
}

// 使用示例
let day = 5;
let month = 12;
let formattedDay = padZero(day); // "05"
let formattedMonth = padZero(month); // "12"

console.log(formattedDay + '/' + formattedMonth); // 输出 "05/12"

let today = new Date();
let dayOfMonth = padZero(today.getDate());
let monthOfYear = padZero(today.getMonth() + 1); // 月份是从0开始的
let year = today.getFullYear();

console.log(dayOfMonth + '/' + monthOfYear + '/' + year); // 输出格式化的日期，例如 "05/12/2023"