The Code

// entry point AKA Controller
function getValues() {
    let userString = document.getElementById('userString').value;

    let reversedString = reverseAString(userString);

    displayString(reversedString);
}

// logic function
// reverse a string
function reverseAString(userString) {
    let reversedString = ''

    for (let i = (userString.length - 1); i >= 0; i--) {
        reversedString += userString[i];
    }

    return reversedString;
}

// view function
function displayString(reversedString) {
    document.getElementById('results').innerHTML = reversedString;
    document.getElementById('alert').classList.remove('invisible')
}