Logic in if/else (javascript)
<p>Have a small if/else problem in javascript. i don't see the logic...</p>

<p>The function should check if two dates are present and give alerts if it's not present. If everything is OK, it should say so...</p>

<pre><code>function update_booking() {
    //from_date = The date of arrival
    //to_date = The date of departure

    var alert = ""; //reset alerts

    //get variables from booking form input
    var from_date = new Date(document.getElementById('from').value);
    var to_date = new Date(document.getElementById('to').value);

    //if arrival and departure date is present
    if(from_date &amp;&amp; to_date) {
        var alert = "Everything is OK";
    }

    //if one or two dates are missing
    else {
        //if arrival and departure dates are missing
        if(from_date == 'undefined' || to_date == 'undefined'){
            var alert = "Arrival date and departure date are missing";  
        }

        //if from_date is missing update with value from to_date
        if(from_date == 'undefined') {
            var alert = "Arrival date are missing";
        }

        //if to_date is missing update with value from from_date
        if(from_date == 'undefined') {
            var alert = "Departure date are missing";
        }
    } //end else if one or more date(s) missing

    //write alerts
    document.getElementById('alert').innerHTML = alert;
}
</code></pre>
