script.js

var result = document.querySelector(".result");

const ac = document.querySelector(".AC");
const de = document.querySelector(".DE");
const point = document.querySelector(".point");
const division = document.querySelector(".division");

const seven = document.querySelector(".seven");
const eight = document.querySelector(".eight");
const nine = document.querySelector(".nine");
const multi = document.querySelector(".multi");

const four = document.querySelector(".four");
const five = document.querySelector(".five");
const six = document.querySelector(".six");
const sub = document.querySelector(".sub");

const one = document.querySelector(".one");
const two = document.querySelector(".two");
const three = document.querySelector(".three");
const plus = document.querySelector(".plus");

const dbl_zero = document.querySelector(".dbl_zero");
const zero = document.querySelector(".zero");
const equals = document.querySelector(".equals");

const keyToButtonMap = {
    '0': zero,
    '1': one,
    '2': two,
    '3': three,
    '4': four,
    '5': five,
    '6': six,
    '7': seven,
    '8': eight,
    '9': nine,
    '+': plus,
    '-': sub,
    '*': multi,
    '/': division,
    ',': point,
    '.': point,
    'Enter': equals
};

// Keyboard input support.
window.addEventListener('keydown', function (e) {
    if (result.innerText === "Error") {
        result.innerText = "";
    }

    if (e.key === 'Backspace') {
        e.preventDefault();
        removeLastChar();
    } else if (e.key === 'Enter') {
        e.preventDefault();
        evaluateResult();
    } else if (keyToButtonMap.hasOwnProperty(e.key)) {
        e.preventDefault();
        keyToButtonMap[e.key].click();
    }
});

// Attach click handlers to the calculator buttons.
document.querySelectorAll(".btn, .AC, .DE, .equals").forEach(button => {
    button.addEventListener("click", (e) => {
        let lastChar = result.innerText.slice(-1);
        let value = e.currentTarget.textContent;

        if (result.innerText === "Error") {
            result.innerText = "";
        } 
        // Prevent consecutive operators.
        else if (['+', '-', '*', '/', '.'].includes(value) && ['+', '-', '*', '/', '.'].includes(lastChar)) {
            return; 
        }
        else if (value === "=") {
            evaluateResult();
        } 
        else if (value === "AC") {
            result.innerText = "";
        } 
        else if (value === "DE") {
            removeLastChar();
        } 
        else {
            result.innerText += value;
        }
    });
});

function removeLastChar() {
    result.innerText = result.innerText.slice(0, -1);
}

function evaluateResult() {
    let operation = result.innerText.replace(/,/g, '.');

    if (!operation) {
        return;
    }

    try {
        const resultValue = new Function('return ' + operation)();
        result.innerText = resultValue.toString().replace(/\./g, ',');
    } catch (e) {
        result.innerText = 'Error';
        console.error('Invalid calculation: ', e);
    }
}