Nativescript with Firebase : Works login in offline mode - firebase

Question

I use nativescript-plugin-firebase to connect with Firebase. It does work when create new user / login / CRUD operations when app is online mode.
When the app is offline mode, Login authentication / Create new user is not working rest of the above mentioned things are working.
My code base:
app.component.ts
ngOnInit(): void {
firebase.init({
persist: true,
storageBucket: 'gs://**************/',
onAuthStateChanged: (data: any) => {
console.log(JSON.stringify(data))
if (data.loggedIn) {
BackendService.token = data.user.uid;
}
else {
BackendService.token = "";
}
}
}).then(
function (instance) {
console.log("firebase.init done");
},
function (error) {
console.log("firebase.init error: " + error);
}
);
firebase.keepInSync(
"/users", // which path in your Firebase needs to be kept in sync?
true // set to false to disable this feature again
).then(
function () {
console.log("firebase.keepInSync is ON for /users");
},
function (error) {
console.log("firebase.keepInSync error: " + error);
}
);
in service file
login(user: User) {
return firebase.login({
type: firebase.LoginType.PASSWORD,
email: user.email,
password: user.password
}).then((result: any) => {
BackendService.token = result.uid;
return JSON.stringify(result);
}, (errorMessage: any) => {
// alert(errorMessage);
alert('Email address / Password is wrong!!!');
});
}
Anyone help me how login will work in when offline mode.