Loading External GeoJSON (Promises): Add data to map

<!DOCTYPE html>
<html>

<head>
    <meta charset="utf-8" />
    <title>A simple map</title>
    <meta name="viewport" content="initial-scale=1,maximum-scale=1,user-scalable=no" />
    <link rel="stylesheet" href="https://unpkg.com/leaflet@1.0.3/dist/leaflet.css" />
    <style>
        body {
            margin: 0;
            padding: 0;
        }

#map {
            position: absolute;
            top: 0;
            bottom: 0;
            width: 100%;
        }
    </style>
</head>

<body>
    <div id="map"></div>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>
    <script src="https://unpkg.com/leaflet@1.0.3/dist/leaflet.js"></script>
    <script>
        var counties = $.ajax({
            url: "https://gist.githubusercontent.com/maptastik/df8e483d5ac1c6cae3dc4a7c02ea9039/raw/9cd46849bddcfa90aab240772a12275408d6d8d0/kyCounties.geojson",
            dataType: "json",
            success: console.log("County data successfully loaded."),
            error: function(xhr) {
                alert(xhr.statusText)
            }
        })
        $.when(counties).done(function() {
            var map = L.map('map')
                .setView([37.857507, -85.632935], 7);

</html>

https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js

https://unpkg.com/leaflet@1.0.3/dist/leaflet.js