index.js

function getHistory() {
    return document.getElementById("history-value").innerText;
}
function printHistory(num) {
    document.getElementById("history-value").innerText=num;
}
function getOutput() {
    return document.getElementById("output-value").innerText;
}
function printOutput(num) {
    if (num == "") {
        document.getElementById("output-value").innerText=num;
    }
    else{
    document.getElementById("output-value").innerText =getFormattedNumber(num);
    } 
}
function getFormattedNumber(num) {
    if (num =="-") {
        return "";        
    }
    var n = Number(num);
    var value = n.toLocaleString("en");
    return value;
}
function reverseNumber(num){
    return Number(num.replace(/,/g,''));
}

var operator = document.getElementsByClassName("operator");
for (var i = 0; i < operator.length; i++) {
    operator[i].addEventListener('click', function () {
       if (this.id =="clear") {
          printHistory("");
          printOutput(""); 
       } 
       if (this.id =="delete"){
           var output =reverseNumber(getOutput()).toString();
           if(output){
               output= output.substr(0, output.length-1);
               printOutput(output);
           }
        }
        else {
            var output = getOutput();
            var history = getHistory();
            if (output !== "") {
                output=reverseNumber(output);
                history = history+output;
                if (this.id =="=") {
                    var result = eval(history);
                    printOutput(result);
                    printHistory("");
                    
                } else{
                    history=history+this.id;
                    printHistory(history);
                    printOutput("");
                }
                
            }
        }  
    });
}

var number = document.getElementsByClassName("number") ;
for (var i = 0; i < number.length; i++) {
   number[i].addEventListener('click', function(){
        var output = reverseNumber(getOutput());
        if (output !== NaN) {
            output=output+this.id;
            printOutput(output);            
        };  
    } ) ;  
}