added account linking/email existence check function

src/lib/firebase/authentication/accountLinking.ts

@@ -0,0 +1,28 @@
+import { db, auth } from '../firebaseConfig';
+import { getAuth, linkWithPopup, GoogleAuthProvider } from "firebase/auth";
+
+// Example:
+// 1. Existing Email/PWD account: ([email protected])
+// 2. An user wants to link a new sign-in google account ([email protected])
+async function linkingAccount(): Promise<void> {
+    const auth = getAuth();
+    const user = auth.currentUser;
+
+    if (user) {
+        // User is signed in
+        const provider = new GoogleAuthProvider();
+        try {
+            await linkWithPopup(user, provider);
+            console.log("Accounts successfully linked");
+        } catch (error) {
+            console.log("Error occured while linking your account", error);
+        }
+    } else {
+        // No user is signed in.
+        alert("Sign-in needed");
+        // redirect to the login page
+        window.location.href = 'route/to/login';
+    }
+}
+
+export { linkingAccount };

src/lib/firebase/authentication/googleAuthentication.ts

@@ -1,5 +1,4 @@
 import { auth } from '../firebaseConfig';
-import { FirebaseError } from 'firebase/app';
 import { signInWithPopup, signOut, GoogleAuthProvider } from 'firebase/auth';
 
 async function verifyGoogleToken(googleAccessToken: string | undefined): Promise<boolean> {
@@ -31,10 +30,8 @@ async function signInWithGoogle(): Promise<void> {
         } else {
             console.log("Please sign in again");
         }
-    } catch (error: unknown) {
-        if ((error as FirebaseError).code === 'auth/account-exists-with-different-credential') {
-            console.log("Already existing email address");
-        }
+    } catch (error) {
+        console.log("Unexpected error while signing in");
     };
 };
 

src/lib/firebase/util/userVerification.ts

@@ -0,0 +1,17 @@
+import { db } from '../firebaseConfig';
+import { collection, query, where, getDocs } from "firebase/firestore";
+
+async function checkEmailExists(email: string): Promise<boolean> {
+    const ref = collection(db, "users");
+    const q = query(ref, where("email", "==", email));
+    const querySnapshot = await getDocs(q);
+    if (!querySnapshot.empty) {
+        console.log("Email already exists in the database");
+        return true;
+    } else {
+        console.log("Email does not exist. You can register or sign-in with this email");
+        return false;
+    }
+}
+
+export { checkEmailExists };