script.js

let runningTotal = 0;
let buffer = '0';
let previousOperator;

const screen = document.querySelector(".screen");

function buttonClick(value) {
    if (isNaN(value)) {
        handleSymbol(value);
    } else {
        handleNumber(value);
    }
    screen.innerText = buffer;
}

function handleSymbol(symbol) {
    switch(symbol) {
        case 'C':
            buffer = 0;
            runningTotal = 0;
            break;

        case '=':
            if(previousOperator === null) {
                return
            }
            flushOperation(parseInt(buffer));
            previousOperator = null;
            buffer = runningTotal;
            runningTotal = 0;
            break;

        case '←':
            if(buffer.length === 1) {
                    buffer = '0';
            } else {
                buffer = buffer.substring(0, buffer.length - 1);
            }
            break;

        case '+':
        case '−':
        case '×':
        case '÷':
            handleMath(symbol);
            break
    }
}

function handleMath(symbol) {
    if(buffer === '0') {
        return;
    }

    const intBuffer = parseInt(buffer);

    if (runningTotal === 0) {
        runningTotal = intBuffer;
    } else {
        flushOperation(intBuffer);
    }
    
    previousOperator = symbol;
    buffer = '0';
}

function flushOperation(intBuffer) {
    if(previousOperator === '+') {
        runningTotal += intBuffer;
    } else if(previousOperator === '−') {
        runningTotal -= intBuffer;
    } else if(previousOperator === '×') {
        runningTotal *= intBuffer;
    } else if(previousOperator === '÷') {
        if (intBuffer === 0) {
            runningTotal = "Error";
        } else {
        runningTotal /= intBuffer;
        }
    }
    
}

function handleNumber(numberString) {
    if(buffer === '0') {
        buffer = numberString;
    } else {
        buffer += numberString;
    }
}

function init() {
    document.querySelector('.calc-buttons').addEventListener('click', function(event){
        buttonClick(event.target.innerText);
    })
}

init();