if ("geolocation" in navigator) {
  /* geolocation is available */
}

navigator.geolocation.getCurrentPosition( whereAmI );

function whereAmI( position ) {
    var lat = position.coords.latitude;
    var lon = position.coords.longitude;
    console.log("You are at "+lat+" , "+lon);
}

var id = navigator.geolocation.watchPosition( whereAmI );

function whereAmI( position ) {
    var lat = position.coords.latitude;
    var lon = position.coords.longitude;
    console.log("You are at "+lat+" , "+lon);

    navigator.geolocation.clearWatch(id); // cancel monitor
}