js to achieve simple calendar effect

## css module <style type="text/css">
  *{
   margin: 0;
   padding: 0;
  }
  .date{
   width: 300px;
   height: 220px;
   border: 1px solid #000;
   margin: 100px auto;
  }
  .title{
   width: 200px;
   display: flex;
   font-size: 12px;
   margin: auto;
   text-align: center;
   justify-content: space-around;
   align-items: center;
  }
  .year{
   margin: 0 40px;
   display: flex;
   flex-direction: column;
  }
  #week{
   border-top: 1px solid #000;
   border-bottom: 1px solid #000;
   margin: auto;
   list-style-type: none;
   display: flex;
  }
  #week li{
   display: inline-block;
   text-align: center;
   flex:1;
  }
  #ul{
   list-style-type: none;
   margin-top: 5px;
  }
  #ul li {
   display: inline-block;
   width: 40px;
   height: 21px;
   text-align: center;
   border: 1px solid #fff;
  }
  .current{
   color:red;
  }
  #ul li:hover{
   border: 1px solid red;
  }
  #prev,#next{
   cursor: pointer;
  }
 </style>

##html
<div class="date">
  <div class="title">
   <span id="prev">&lt;Previous month</span>
   <div class="year">
    <span id="year">2021</span>
    <span id="month">May</span>
   </div>
   <span id="next">Next month&gt;</span>
  </div>
  <!-- Use ul to make a calendar -->
  <ul id="week">
   <li>Day</li>
   <li>One</li>
   <li>Two</li>
   <li>Three</li>
   <li>Four</li>
   <li>Five</li>
   <li>Six</li>
  </ul>
  <ul id="ul">
   
  </ul>
</div>

## js code <script type="text/javascript">
  // Date object, convenient for switching months, so set it as global let date = new Date();
  // Click to switch the month event document.getElementById('prev').addEventListener('click',function(){
   date.setMonth(date.getMonth()-1);
   add();
  })
  document.getElementById('next').addEventListener('click',function(){
   date.setMonth(date.getMonth()+1);
   add();
  })
  add();
  
  //Function to create calendar function add(){
   // Current year let cYear = date.getFullYear();
   // Current month let cMonth = date.getMonth()+1;
   // Get the current date let cDay = date.getDate();
   
   // Write the year and month document.getElementById('year').innerHTML = cYear;
   document.getElementById('month').innerHTML = cMonth+'月';
   
   
   let days = new Date(cYear,cMonth,-1);
   // The number of days in the current month let n = days.getDate()+1;
   // What day of the week is the first day of each month let week = new Date(cYear,cMonth-1,1).getDay();
   let html = '';
   //Write to dom
   for(let i=0;i<week;i++){
    html+=`<li></li>`
   }
   for(let i=1;i<=n;i++){
    if(i==cDay){
     html+=`<li class="current">${i}</li>`
    }else{
     html+=`<li>${i}</li>`
    }
   }
   // Insert once document.getElementById('ul').innerHTML = html
  }
</script>