Compruebe si existe la tecla LocalStorage
if (localStorage.getItem("username") === null) {
//...
}
Blue Bear
if (localStorage.getItem("username") === null) {
//...
}//If the given key does not exist in the list associated with the object then this method must return null.
if (localStorage.getItem("infiniteScrollEnabled") === null) {
//...
}if("user" in localStorage){
alert('yes');
} else {
alert('no');
}