Accessing JS Variables with C#

I've got a variable which is set in a final script tag on page load, as this is required due to accessing the DOM (using a 3rd party library). I want to access my variable, however I get "undefined" when accessing it via C# (ASP Web Application).

How can I access this?

If I run the following from C#:

ClientScript.RegisterStartupScript(GetType(), "hwa", "hello();", true);

I get the following error: https://i.stack.imgur.com/veiLB.png

My hello function is:

function hello() {
 alert(mapData);
}

This mapData variable is set in a script tag at the end of the body, my original code was to call a JS function AFTER the site loaded, however it comes up as undefined via C# yet shows up if I use the Web Console.

Full Script:

function createMap() {
mapData = L.map('map', {
    center: [20.0, 5.0],
    minZoom: 2,
    zoom: 5
});

L.mapData = mapData;

L.tileLayer('http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', {
    attribution: '&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a>',
    subdomains: ['a', 'b', 'c']
}).addTo(L.mapData);
}

function createMarker(long, lat, type) {
    alert(L.mapData);
    L.marker([long, lat], { icon: L.icon({ iconUrl: 'Content/img/' + type + '.png', iconSize: [50, 60], iconAnchor: [22, 94], popupAnchor: [-3, -76], }) }).addTo(L.mapData);
}

function hello() {
    alert(mapData);
}

Hopefully this clarifies.

Thanks.