How to convert date to string in javascript

// 格式化日期，如月、日、时、分、秒保证为2位数
function formatNumber (n) {
 n = n.toString()
 return n[1] ? n : '0' + n;
}
// 参数number为毫秒时间戳，format为需要转换成的日期格式
function formatTime (number, format) {
 let time = new Date(number)
 let newArr = []
 let formatArr = ['Y', 'M', 'D', 'h', 'm', 's']
 newArr.push(time.getFullYear())
 newArr.push(formatNumber(time.getMonth() + 1))
 newArr.push(formatNumber(time.getDate()))
 
 newArr.push(formatNumber(time.getHours()))
 newArr.push(formatNumber(time.getMinutes()))
 newArr.push(formatNumber(time.getSeconds()))
 
 for (let i in newArr) {
  format = format.replace(formatArr[i], newArr[i])
 }
 return format;
}

var a=new Date() ; 
//传入格式为数字类型
formatTime(a.getTime(),'Y-M-D h:m:s')
//临时调用如下
let start = new Date();
start = start.getFullYear() + '.' + (start.getMonth()) + '.' + start.getDay();