Edit in JSFiddle

function execute(){
      var a=20, b=10;
      
      var opt = prompt('연산자를 입력(+,-,*,/,%)','');
      switch (opt){
        case '+':document.getElementById('output').innerHTML = ('a + b = ' + (a+b));
          break;
        case '-':document.getElementById('output').innerHTML = ('a - b = ' + (a-b));
          break;
        case '*':document.getElementById('output').innerHTML = ('a * b = ' + (a*b));
          break;
        case '/':document.getElementById('output').innerHTML = ('a / b = ' + (a/b));
          break;
        case '%':document.getElementById('output').innerHTML = ('a % b = ' + (a%b));
          break;
        default:document.getElementById('output').innerHTML = ('잘못된 연산자를 사용');
      }
    }
    
    function myFunction(){
      var day;
      switch(new Date().getDay()){
        case 0:
          day = "일요일";
          break;
        case 1:
          day = "월요일";
          break;
        case 2:
          day = "화요일";
          break;
        case 4:
          day = "수요일";
          break;
        case 5:
          day = "목요일";
          break;
        case 6:
          day = "금요일";
          break;
        case 7:
          day = "토요일";
          break;
      }
      
      document.getElementById("today").innerHTML = "오늘은 " + day + " 입니다.";
    }

<button type="button" onclick="execute()">실행</button>
<div id="output"></div>
<p>&nbsp;</p>
<p>다음 버튼을 클릭하면 오늘의 요일을 알려줍니다.</p>
<button type="button" onclick="myFunction()">요일 알기</button>
<div id="today"></div>