javascript function returning undefined in variable

Question

I've made a function in JS, when i call the function directly it gives the right results but when i put it in a variable it returns undefined. Could someone tell me what i'm doing wrong?

function getGebruikersnaam() {
    var test = "replace me";
    fetch("restservices/gegevens", {
        method: 'GET',
        headers: {
            'Authorization': 'Bearer ' + window.sessionStorage.getItem("myJWT")
        }
    }).then(response => response.json()).then(function(myJson) {
        console.log(myJson.gebruikersnaam);
        test = myJson.gebruikersnaam;
        return test;
    });
}

getGebruikersnaam();
var name = getGebruikersnaam();
console.log("variable name: " + name);