js 数字不足位数补0

方式一，使用padStart

如果不考虑ie，推荐https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/String/padStart#%E6%B5%8F%E8%A7%88%E5%99%A8%E5%85%BC%E5%AE%B9%E6%80%A7

'9'.padStart(2,0);// 09

方式二，使用Intl.NumberFormat

如果考虑兼容性推荐：https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Intl/NumberFormat#%E6%B5%8F%E8%A7%88%E5%99%A8%E5%85%BC%E5%AE%B9%E6%80%A7

new Intl.NumberFormat(undefined, {
    minimumIntegerDigits: 2,
    useGrouping:false
}).format(9);//09