first.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 === '÷') {
        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();