script.js

var buttons = document.getElementsByClassName("button");
var display = document.getElementById('display');

var operand1 = 0;
var operand2 = null;
var operator = null;

function isOperator(value){
    return value == "+" || value == "-" || value == "*" || value == "/";
}

for(var i = 0; i<buttons.length; i++)
{
    buttons[i].addEventListener('click', function(){
        var value = this.getAttribute('data-value');
        var text = display.textContent.trim();

        if(isOperator(value)){
            operator = value;
            operand1 = parseFloat(text);
            display.textContent = '';
        }
        else if(value == "AC"){
            display.textContent = "";
        }
        else if(value == "DEL")
        {
            value=text;
            value = value.substring(0,value.length-1);
            display.textContent = value;
        }
        else if(value == ".")
        {
            if(text.length && !text.includes('.')){
                display.textContent = text + '.';
            }
        }
        else if(value == "%")
        {
            operand1 = parseFloat(text);
            operand1 = operand1/100;
            display.textContent = operand1;
        }
        else if(value == "=")
        {
            operand2 = parseFloat(text);
            var result = eval(operand1 + operator +operand2);
            if(result){
                display.textContent = result;
                operand1 = result;
                operand2 = null;
                operator = null;
            }
        }
        else{
            display.textContent += value;
        }
    });
}