calculator.js

function NumberHandler(e){
    document.getElementById('result').value += e.currentTarget.value;
    if(e.currentTarget.value === "+" || e.currentTarget.value === "-"){
        Multicalculation();
        clickBtn = e.currentTarget.value;
    }
    else if(e.currentTarget.value === "="){
        secondNumber = document.getElementById('result').value;
        secondNumber = secondNumber.substring(0, secondNumber.length - 1);
        document.getElementById('result').value = null;
        if(clickBtn === "+"){
            alert(`${firstNumber}` + "+" + `${secondNumber}` + "=" + (parseInt(firstNumber) + parseInt(secondNumber)))
            clickBtn = null;
        }
        else if(clickBtn === "-"){
            alert(`${firstNumber}` + "-" + `${secondNumber}` + "=" + (parseInt(firstNumber) - parseInt(secondNumber))) 
            clickBtn = null;
        }
    }
}


function Multicalculation(){
    result = document.getElementById('result').value
    if(firstNumber == null)
      firstNumber = document.getElementById('result').value;

    else if(clickBtn == "+")
        firstNumber = parseInt(firstNumber) + parseInt(result);
    
    else
        firstNumber = parseInt(firstNumber) - parseInt(result.substring(0, result.length -1));
    
    document.getElementById('result').value = null;
}


let btn1 = document.getElementById('btnNumber1');
let btn2 = document.getElementById('btnNumber2');
let btn3 = document.getElementById('btnNumber3');
let btn4 = document.getElementById('btnNumber4');
let btn5 = document.getElementById('btnNumber5');
let btn6 = document.getElementById('btnNumber6');
let btn7 = document.getElementById('btnNumber7');
let btn8 = document.getElementById('btnNumber8');
let btn9 = document.getElementById('btnNumber9');
let btn0 = document.getElementById('btnNumber0');
let btnsum = document.getElementById('btnSumption');
let btnmin = document.getElementById('btnMines');
let btnequl = document.getElementById('btnEqual');
let firstNumber = null;
let secondNumber = 0;
let clickBtn = null;


btn1.onclick = NumberHandler;
btn2.onclick = NumberHandler;
btn3.onclick = NumberHandler;
btn4.onclick = NumberHandler;
btn5.onclick = NumberHandler;
btn6.onclick = NumberHandler;
btn7.onclick = NumberHandler;
btn8.onclick = NumberHandler;
btn9.onclick = NumberHandler;
btn0.onclick = NumberHandler;
btnsum.onclick = NumberHandler;
btnmin.onclick = NumberHandler;
btnequl.onclick = NumberHandler;