04. Numpad Calculator Exercise: DOM

function solve() {
    let buttons = document.getElementsByTagName('button');
    let operator = '';
    let result = 0;
    for (let i = 0; i < buttons.length; i++) {
        buttons[i].addEventListener('click', () => {
            if (buttons[i].value !== '=') {
                if (buttons[i].value === '+' || buttons[i].value === '-' ||
                    buttons[i].value === '*' || buttons[i].value === '/') {
                    operator = ` ${String(buttons[i].value)} `;
                    document.getElementById('expressionOutput').textContent += operator;
                } else if (buttons[i].value === 'Clear') {
                    document.getElementById('expressionOutput').textContent = '';
                    document.getElementById('resultOutput').textContent = '';
                } else {
                    document.getElementById('expressionOutput').textContent += String(buttons[i].value);
                }
            } else {
                let [operand1, operand2] = document.getElementById('expressionOutput').textContent.split(operator).map(Number);
                if (operand1 && operand2) {
                    if (operator === ' + ') { result = operand1 + operand2 }
                    if (operator === ' - ') { result = operand1 - operand2 }
                    if (operator === ' * ') { result = operand1 * operand2 }
                    if (operator === ' / ') { result = operand1 / operand2 }
                    document.getElementById('resultOutput').textContent = result;
                } else {
                    document.getElementById('resultOutput').textContent = NaN;
                }
            }
        });
    }
}