js 昨天，前天等某天日期

 // 根据今天 返回 某天，比如 昨天，参数num=1 返回日期为 2020-7-8
function fff(num){
    
			let today = new Date()
			let oneDay = new Date(today)
			oneDay.setDate(oneDay.getDate() - num)
			let myDate = new Date( oneDay );
			let result = myDate.getFullYear()+'-'+(myDate.getMonth()+1)+'-'+myDate.getDate() ;
			return result
		}
fff(num=1)//昨天日期