index.js

var screen=document.getElementById('screen');
var btn=document.getElementsByClassName('btn');
for(item of btn)
{
     item.addEventListener('click',function(e)
     {
          btnText=e.target.innerHTML;
          screen.value+=btnText;
     });
}
function backspace()
{
     screen.value=screen.value.substr(0,screen.value.length-1);
}
function factorial()
{
     var fact=1;
     var ans=screen.value;
     for(i=1;i<=screen.value;i++)
     {
          fact=fact*ans;
          ans--;
     }
     screen.value=fact;
}
function sin()
{
     screen.value=Math.sin(screen.value);
}
function pi()
{
     screen.value+=3.14;
}     
function cos()
{
     screen.value=Math.cos(screen.value);
}
function log()
{
     screen.value=Math.log(screen.value);
}
function tan()
{
     screen.value=Math.tan(screen.value);
}
function sqrt()
{
     screen.value=Math.sqrt(screen.value);
}
function f1()
{
     screen.value+=2.71828182;
}
function pow()
{
     screen.value=Math.pow(screen.value,2);
}
function radian()
{
     var radian=screen.value;
     var ans=(radian*180)/3.14;
     screen.value=ans;
}
function degree()
{
     var degree=screen.value;
     var ans=degree*180/3.14;
     screen.value=ans;
}