<div id="calc">
      <div id="val"></div>
      <div id="box-button">
        <div class="btn" data-type="1">1</div>
        <div class="btn" data-type="2">2</div>
        <div class="btn" data-type="3">3</div>

        <div class="btn" data-type="4">4</div>
        <div class="btn" data-type="5">5</div>
        <div class="btn" data-type="6">6</div>

        <div class="btn" data-type="7">7</div>
        <div class="btn" data-type="8">8</div>
        <div class="btn" data-type="9">9</div>

        <div class="btn" data-type="0">0</div>
        <div class="btn" data-type=".">.</div>
        <div class="btn" data-type="=">=</div>
      </div>  
      <div class="znack">
        <div class="btn" data-type="+">+</div>
        <div class="btn" data-type="-">-</div>
        <div class="btn" data-type="*">*</div>    
        <div class="btn" data-type="/">/</div>   
        <div class="btn" data-type="clear">clear</div>   
      </div> 
    </div>

#calc {
        width: 190px;
        margin: 100px auto;
        text-align: center;
      }
      #box-button {
        display: inline-block;
        width: 140px;
        vertical-align: top;
      }
      #txt {
        width: 600px;
        margin: 100px auto;
      }
      #val {
        display: inline-block;
        width: 100%;
        height: 40px;
        padding: 10px;
        overflow: hidden;
        line-height: 20px;
        text-align: right;
        border: 2px solid #ccc;
        box-sizing: border-box;
      }
      .znack {
        display: inline-block;
        width: 42px;
      }
      .btn {
        display: inline-block;
        width: 40px;
        height: 40px;
        line-height: 40px;
        border: 1px solid #ccc;
        font-size: 14px;
        text-align: center;
      }
      .btn:hover {
        cursor: pointer;
        color: #fff;
        background-color: #f00;
      }

      window.onload = function(){
        let itemSumm = document.getElementById('val');
        let valOtvet = '';
        
        let funClick = (event) => {
          let valElementClick = event.target.getAttribute('data-type');

          if(valElementClick === '='){
            valOtvet = eval(valOtvet);
          } else if(valElementClick === 'clear'){
            valOtvet = '';
          } else {
            valOtvet += valElementClick;
          }
          itemSumm.innerHTML = valOtvet;
        }

        const items = document.getElementsByClassName('btn');
        const btnLenght = items.length;
        let i = 0;
        for(;i<btnLenght;i++){
          let item = items[i];
          item.addEventListener('click',funClick);
        }
      }