script.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 reverseNumberFormat(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("");
        }
        else if(this.id == "backspace"){
            var output = reverseNumberFormat(getOutput()).toString();
            if(output){
                output = output.substr(0 , output.length-1);
                printOutput(output);
            }
        }
        else{
            var output = getOutput();
            var history = getHistory();
            if(output == "" && history != ""){
                if(isNaN(history[history.length-1])){
                    history = history.substr(0 , history.length-1);
                }
            }
            if( output != "" || history != ""){
                output = output == "" ?
                output:reverseNumberFormat(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 = reverseNumberFormat(getOutput());
        if(output != NaN){
            output = output + this.id ;
            printOutput(output);
        }     
    });
}