script.js

const myButton = document.getElementById('my_button');
const result   = document.getElementById('result');

myButton.addEventListener('click', () => {

    let firstValue = document.getElementById('number1').value;
    let operator   = document.getElementById('operand').value;
    let secondValue = document.getElementById('number2').value;

    firstValue = parseInt(firstValue);
    secondValue = parseInt(secondValue);

    switch (operator) {
        case '+':
            result.innerHTML = firstValue + secondValue;
            break;
        case '-':
            result.innerHTML = firstValue - secondValue;
            break;
        case '/':
            result.innerHTML = firstValue / secondValue;
            break;
        case '*':
            result.innerHTML = firstValue * secondValue;
            break;
        default:
            result.innerHTML = 'Invalid operator'
    }
})