test5.html

<html>
<head>
<title>Boundary explorer</title>
<link rel="stylesheet" href="https://unpkg.com/leaflet@1.0.1/dist/leaflet.css" />
<script src="https://unpkg.com/leaflet@1.0.1/dist/leaflet.js"></script>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
</head>

<body>

<div id="map" style="height:100%; width:100%"></div>

<script>

var tiles = L.tileLayer('http://{s}.basemaps.cartocdn.com/light_all/{z}/{x}/{y}.png', {
  attribution: '&copy; <a href="http://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors, &copy; <a href="https://carto.com/attributions">CARTO</a>'
});

function getQueryVariable(variable)
{
       var query = window.location.search.substring(1);
       var vars = query.split("&");
       for (var i=0;i<vars.length;i++) {
               var pair = vars[i].split("=");
               if(pair[0] == variable){return pair[1];}
       }
       return(false);
}

var areacode = getQueryVariable("code");
var url = "http://statistics.data.gov.uk/boundaries/" + areacode + ".json";

function onEachFeature(feature, layer) {
	var popupContent = feature.properties.LAD15NM + "<br />" + feature.properties.LAD15CD;
	layer.bindPopup(popupContent);
}

// load GeoJSON from an external file
var boundary = $.getJSON(url, function(data){
	//add GeoJSON layer to the map once the file is loaded
	var boundary = L.geoJson(data, {onEachFeature: onEachFeature});
	var map = L.map('map').fitBounds(boundary.getBounds());
	tiles.addTo(map);
//	boundary.bindPopup(feature.properties.LAD15NM);
	boundary.addTo(map);
	boundary.openPopup();
	});

</script>

</body>
</html>