Código para conseguir a longitude e a latitude de um usuário utilizando javascript

geolocation.js

// var x = document.getElementById("demo");

function getLocation() {
    if (navigator.geolocation) {
        navigator.geolocation.getCurrentPosition(showPosition);

    } else { 
        x.innerHTML = "Geolocation is not supported by this browser.";
    }
}

function showPosition(position) {

    var longitude = position.coords.longitude;
    document.getElementById('id_longitude').value =  longitude;

    latitude = position.coords.latitude;
    document.getElementById('id_latitude').value =  latitude;
}