使用正则匹配

let string = '20220101
let result = string.match(/(\d{4})(\d{2})(\d{2})/).filter((item,index) => index > 0).join('-')
console.log(result) // 2022-01-01

把他封装一下

changeTime(time){
	return time.match(/(\d{4})(\d{2})(\d{2})/)
			   .filter((item,index) => index > 0)
			   .join('-');
}
//之后调用this.changeTime()即可