<div class="calendar-container">
 <div class="calendar-header">
 <div class="left btn"><</div>
 <div class="year"></div>
 <div class="right btn">></div>
 </div>
 <div class="calendar-body">
 <div class="week-row">
 <div class="week box">日</div>
 <div class="week box">一</div>
 <div class="week box">二</div>
 <div class="week box">三</div>
 <div class="week box">四</div>
 <div class="week box">五</div>
 <div class="week box">六</div>
 </div>
 <div class="day-rows">
 <!--日期的渲染的地方-->
 </div> 
 </div>
</div>

.calendar-container{
 width: calc(31px*7 + 1px);
 }
.calendar-header{
 display: flex; 
 justify-content: space-between;
 }
.year{
 text-align: center; 
 line-height: 30px;
 }
.btn{
 width: 30px; 
 height: 30px; 
 text-align: center; 
 line-height: 30px; 
 cursor: pointer;
 }
.calendar-body{
 border-right: 1px solid #9e9e9e; 
 border-bottom: 1px solid #9e9e9e;
 }
.week-row, 
.day-rows, 
.day-row{
 overflow: hidden;
 }
.box{
 float: left; 
 width: 30px; 
 height: 30px; 
 border-top: 1px solid #9e9e9e; 
 border-left: 1px solid #9e9e9e; 
 text-align: center; 
 line-height: 30px;
 }
.week{
 background: #00bcd4;
 }
.day{
 background: #ffeb3b;
 }
.curday{ 
 background: #ff5722;
 }

// 獲取今天日期
let curTime = new Date(),
 curYear = curTime.getFullYear(),
 curMonth = curTime.getMonth(),
 curDate = curTime.getDate();
 console.log(curTime, curYear, curMonth, curDate)
 // 判斷平年還是閏年
 function isLeapYear(year){ 
 return (year%400 === 0) || ((year%4 === 0) && (year%100 !== 0))
}
function render(curYear, curMonth){ 
document.querySelector('.year').innerHTML = `${curYear}年${curMonth + 1}月`; 
// 判斷今年是平年還是閏年，并確定今年的每個月有多少天 
let daysInMonth = [31, isLeapYear(curYear) ? 29 : 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]; 
// 確定今天日期所在月的第一天是星期幾 
let firstDayInMonth = new Date(curYear, curMonth, 1),
 firstDayWeek = firstDayInMonth.getDay(); 
 // 根據當前月的天數和當前月第一天星期幾來確定當前月的行數 
 let calendarRows = Math.ceil((firstDayWeek + daysInMonth[curMonth])/7); 
 // 將每一行的日期放入到rows數組中 
 let rows = []; 
 // 外循環渲染日歷的每一行 
 for(let i = 0; i < calendarRows; i++){
 rows[i] = `<p class="day-row">`; 
 // 內循環渲染日歷的每一天 
 for(let j = 0; j < 7; j++){ 
 // 內外循環構成了一個calendarRows*7的表格，為當前月的每個表格設置idx索引； 
 // 利用idx索引與當前月第一天星期幾來確定當前月的日期 
 let idx = i*7 + j,
 date = idx - firstDayWeek + 1; 
 // 過濾掉無效日期、渲染有效日期 
 if(date <= 0 || date > daysInMonth[curMonth]){
 rows[i] += `<p class="day box"></p>`
 }else if(date === curDate){
 rows[i] += `<p class="day box curday">${date}</p>`
 }else{
 rows[i] += `<p class="day box">${date}</p>`
 }
 }
 rows[i] += `</p>`
 } let dateStr = rows.join(''); 
 document.querySelector('.day-rows').innerHTML = dateStr;
}
// 首次調用render函數
render(curYear, curMonth);
let leftBtn = document.querySelector('.left'),
 rightBtn = document.querySelector('.right');
 // 向左切換月份
leftBtn.addEventListener('click', function(){
 curMonth--; 
 if(curMonth < 0){
 curYear -= 1;
 curMonth = 11;
 }
 render(curYear, curMonth);
})
// 向右切換月份
rightBtn.addEventListener('click', function(){
 curMonth++; 
 if(curMonth > 11){
 curYear += 1;
 curMonth = 0;
 }
 render(curYear, curMonth);
})