Function returning value 'NaN'

by Kishore Sahasranaman

HTML

<form>
    <label for="numDonut">Enter how many donuts you want:</label>
    <input type="text" id="numDonut">
    <div id="totalDonuts"></div>
    <div id="submit" onclick="callDonut();">SUBMIT</div>
</form>
</body>

JavaScript

window.getTotalDonuts = function(donutString) {
    var initialDonutCount = parseInt(donutString);
    var finalDonuts = 0;
    if (donutString.indexOf('dozen') != -1) {
        finalDonuts = initialDonutCount * 12;
    } else {
        finalDonuts = initialDonutCount;
    }
    return finalDonuts || 0;
}

window.callDonut = function() {
    //Get string number of donuts entered
    var numDonutString = document.getElementById('numDonut').value;
    document.getElementById('totalDonuts').textContent = getTotalDonuts(numDonutString);
}