JSFiddle - React, Tailwind, and code Playground

by David Thomas

HTML

<form action="#" method="post">
    <select name="departure" size="1">
        <option value="1">NY</option>
        <option value="2">FL</option>
        <option value="3">LA</option>
    </select>
    <select name="arrival" size="1">
        <option value="1">NY</option>
        <option value="2">FL</option>
        <option value="3">LA</option>
    </select>
</form>

JavaScript

function deDupe(el) {
    var opt = el.selectedIndex,
        other = el.name == 'departure' ? document.getElementsByName('arrival')[0] : document.getElementsByName('departure')[0],
        options = other.getElementsByTagName('option');
    for (var i = 0, len = options.length; i < len; i++) {
        options[i].disabled = i == opt ? true : false;
    }
}

var departure = document.getElementsByName('departure')[0],
    arrival = document.getElementsByName('arrival')[0];

deDupe(document.getElementsByName('departure')[0]);

departure.onchange = function () {
    deDupe(departure);
};
arrival.onchange = function () {
    deDupe(arrival);
};