initializeApp(firebaseConfig) ➡ This function return the firebase App object
A Firebase App is a container-like object that stores common configuration and shares authentication across Firebase services. After you initialize a Firebase App object in your code, you can add and start using Firebase services.
for sign up a new user we use the method createUserWithEmailAndPassword
for Sign in an existing user we use the method signInWithEmailAndPassword
onAuthStateChanged(auth , () => {}) this function run each time user auth change