cal.js

let currentNumber = '';
let previousNumber = '';
let operation = undefined;

function appendNumber(number) {
    if (number === '0' && currentNumber === '0') return;
    currentNumber += number;
    updateDisplay();
}

function updateDisplay() {
    document.getElementById('display').innerText = currentNumber;
}

function clearDisplay() {
    currentNumber = '';
    previousNumber = '';
    operation = undefined;
    updateDisplay();
}

function chooseOperation(op) {
    if (currentNumber === '') return;
    if (previousNumber !== '') {
        compute();
    }
    operation = op;
    previousNumber = currentNumber;
    currentNumber = '';
}

function compute() {
    let computation;
    const prev = parseFloat(previousNumber);
    const curr = parseFloat(currentNumber);

    if (isNaN(prev) || isNaN(curr)) return;

    switch (operation) {
        case '+':
            computation = prev + curr;
            break;
        case '-':
            computation = prev - curr;
            break;
        case '*':
            computation = prev * curr;
            break;
        case '/':
            computation = prev / curr;
            break;
        default:
            return;
    }

    currentNumber = computation;
    operation = undefined;
    previousNumber = '';
    updateDisplay();
}