app.js

(function(){
    let screen = document.querySelector('.screen');
    let buttons = document.querySelectorAll('.btn');
    let clear = document.querySelector('.btn-clear');
    let equal =document.querySelector('.btn-equal');
     

    buttons.forEach(function(button){
        button.addEventListener('click',function(e){
            let value = e.target.dataset.num;
            screen.value += value;
        })
    });

      equal.addEventListener('click',function(e){
        if (screen.value==''){
            screen.value="";
        }
        else {
            let ans = eval(screen.value);
            screen.value=ans;
        }
      })

      clear.addEventListener('click',function(e){
        screen.value='';
      })

})();