main.js

const displayPrevNum = document.querySelector('#prev-number');
const displayCurrentNum = document.querySelector('#current-number');

const numberButtons = document.querySelectorAll('.number');
const operatorButtons = document.querySelectorAll('.operator');
const equalsButton = document.querySelector('#equals');
const clearButton = document.querySelector('#clear-button');
const deleteButton = document.querySelector('#delete-button');
const decimalButton = document.querySelector('#decimal-button');

const opAdd = '+';
const opSubtract = '-';
const opMultiply = '*';
const opDivide = '/';

let currentNum = '';
let previousNum = '';
let currentOperator = '';
let decimal;
let calResult;

function add(a, b) {
    return Math.round((a + b) * 100000) / 100000;
}

function subtract(a, b) {
    return Math.round((a - b) * 100000) / 100000;
}

function multiply(a, b) {
    return Math.round((a * b) * 100000) / 100000;
}

function divide(a, b) {
    return Math.round((a / b) * 100000) / 100000;
}

function operate(a, operator, b) {
    let result;

    if (a == 0 && operator == opDivide && b == 0) {
        currentNum = '';
        previousNum = '';
        return alert('Foolish mortal, you thought thou could commit such crimes?');
    }

    switch(operator) {
        case opAdd:
            result = add(Number(a), Number(b));
            break;

        case opSubtract:
            result = subtract(Number(a), Number(b));
            break;

        case opMultiply:
            result = multiply(Number(a), Number(b));
            break;

        case opDivide:
            result = divide(Number(a), Number(b));
            break;
        default:
            console.log(`Error! Expected valid operator, but instead got " ${operator} "`);
    }

    return result;

}

operatorButtons.forEach((item) => {
    item.addEventListener('click', (e) => handleOp(e.target.textContent));
});
numberButtons.forEach((item) => {
    item.addEventListener('click', (e) => handleNumber(e.target.textContent));
});

equalsButton.addEventListener('click', () => handleEquals());
clearButton.addEventListener('click', () => clearCal());
deleteButton.addEventListener('click', () => deleteLastAction());
decimalButton.addEventListener('click', () => addDecimal());
document.addEventListener('keydown', (e) => handleKey(e));

function handleNumber(num) {
    if (currentNum.length <= 10) {
        currentNum += num;
        displayCurrentNum.textContent = currentNum;
    }
}

function handleOp(operator) {
    if (!currentNum) return;

    if (currentNum && previousNum) {
        handleEquals();
    }

    currentOperator = operator;
    previousNum = currentNum;
    currentNum = '';
    decimal = false;

    displayPrevNum.textContent = previousNum + ` ${operator}`; 
    displayCurrentNum.textContent = currentNum;
}

function handleEquals() {
    if (!currentNum || !previousNum) return;

    // Fix delete button not working!

    calResult = operate(previousNum, currentOperator, currentNum);

    previousNum = '';
    currentNum = calResult;
    decimal = false;

    displayPrevNum.textContent = previousNum;
    displayCurrentNum.textContent = currentNum;
}

function clearCal() {

    currentNum = '';
    previousNum = '';
    currentOperator = '';
    decimal = false;

    displayPrevNum.textContent = previousNum;
    displayCurrentNum.textContent = currentNum;
}

function deleteLastAction() {
    currentNum = currentNum.toString();
    currentNum = currentNum.substring(0, currentNum.length - 1);
    displayCurrentNum.textContent = currentNum;
    if (!currentNum.includes('.')) decimal = false;
}

function addDecimal() {
    if (decimal == true) return;

    currentNum += '.'
    displayCurrentNum.textContent = currentNum;
    decimal = true;
}

function handleKey(event) {
    event.preventDefault();

    switch (event.key) {
        case '1':
        case '2':
        case '3':
        case '4':
        case '5':
        case '6':
        case '7':
        case '8':
        case '9':
        case '0':
            handleNumber(event.key);
            break;
        case '+':
        case '-':
        case '*':
        case '/':
            handleOp(event.key);
            break;
        case 'x':
            handleOp(opMultiply);
            break;
        case '.':
            addDecimal();
            break;
        case '=':
        case 'Enter':
            handleEquals();
            break;
        case 'Backspace':
        case 'Delete':
            deleteLastAction();
            break;
    }

}