How can I access a variable from within a function ouside of that function?

Question

I am using javascript to get the exchange rates of currencies. I have the following function set up to do so:

var latest;

function convert(from, to){
    var requestURL = 'https://api.exchangerate.host/convert?from='+ from.toUpperCase()+'&to='+to.toUpperCase();
    var request = new XMLHttpRequest();
    request.open('GET', requestURL);
    request.responseType = 'json';
    request.send();
        request.onload = function() {
        var response = request.response;
        console.log(response);
        response = JSON.parse(JSON.stringify(response));
        latest = response.result;
        console.log(latest);
        }
    console.log(latest);
}

The variable latest is assigned the conversion rate. However,I cannot then access it outside of the function to do anything with it, as it is "null" or undeclared outside the subfunction. I have declared latest outside of the function, as has been recommended on other similar topics I've found on here. I would appreciate any help!